Clutch size of the Reed Bunting Emberiza schoeniclus

ERKKI HAUKIOJA Department of Zoology, University of Turku

1. Introduction ...... 101 other parameters of clutch size distribu- 2. Materials and methods ...... 102 tion? Generally it is accepted that the 3. Clutch size ...... 103 clutch size of a population is adapted 3.1 . Variation within population . . . . 103 to time and place. The factors which 3.2. Differences between populations 107 have caused the clutch size to evolve to 4. Productivity of different clutch and the figures found nowadays at a certain brood sizes ...... 109 place are, on the other hand, a matter . Early 4.1 broods ...... 109 of much controversy. Before it is possible 4.2. Late broods ...... 115 to solve such problems of general inter- 4.3 . Production of young in relation to the age of parents ...... 116 est, much material must be gathered and 5. On the genetic basis of the clutch size 118 analysed. 5.1. The probable nature of heredity 118 An exact knowledge of the trends of the 5.2. The ways of inheritance ...... 119 clutch size of a species is one piece of evidence needed 5.3. Heritability of clutch size ...... 121 for determining the 6. On the determination of range, mode, importance of the clutch size. Very and distribution of the clutch size . . 121 thorough investigations have been con- 7. Discussion ...... 129 ducted on the clutch size of certain Acknowledgements ...... 131 species which nest in holes ; the Great Summary ...... 131 Tit Parus major (KLUYVER 1951, PER- Selostus : Pajusirkun pesyekoko ...... 132 R I NS 1965) and the Pied Flycatcher References ...... 133 Ficedula hypoleuca (v. HAARTMAN 1967a, BERNDT & WINKEL 1967) have been studied most thoroughly. Some 1 . Introduction general trends on the subject of the clutch size have been presented by LACK The clutch size of different species of (1947, 1948a) . birds varies considerably . For instance, In this paper I will present the trends the Razorbill Alca torda lays clutches of of the clutch size in a Reed Bunting one egg and the Partridge Perdix perdix population studied by me in south- up to twenty eggs (LACK 1947) . For a western Finland. certain species of bird the number of The other side of the problem of eggs laid in a clutch is less variable, clutch size is formed by the heritability although the range may still be rather of the clutch size of which there are large . Within a species the mode and extremely few data for wild birds. Dis- range often vary according to time and cussion of these facts, partly on the place. basis of the material presented in this What factors determine the range and paper, is also given.

ORNIS FENNICA Vol . 47 :101-135 . 1970 102 ORNIs FENNICA Vol. 47, 1970

2. Materials and methods

The field work of this study has been con- ducted near the town of Pori at the mouth of the Kokemdki-River (ca. 61°32' N, 21°44' E) in south-western Finland. The main study area, to which the study of a colour-ringed popula- tion of Reed Buntings has been concentrated, is in two parts and comprises the island of Välisanta (A in Fig. 1) and the northern part of the island of Hevosluoto (B in Fig. 1). The island of Välisanta is about 25 hectares in size. The vegetation of this island is de- scribed in my earlier paper (HAUKIOJA, 1968). The northern part of Hevosluoto is about 20 hectares in area. The vegetation of this area consists largely of willow thickets (the dom- inating species being Salix phylicifolia) and marsh (the predominating species are Carex aquatilis and Comarum palustre) . The study area mentioned later in this paper means the main study area plus the island of Täiluoto (C in Fig. 1) and the northern part of the island of Hanhiluoto (D in Fig. 1). The vegetation of these part-areas is much like that of Hevosluoto as presented above. Besides these areas, some field work has also been done in other parts of the delta of the Kokemdki-River . The field work was conducted during the years 1966-1969. The number of working days at the study area was 93 in 1966 and over one hundred in other years during the period when Reed Buntings, which are migr- FIG . 1 . Study area at the mouth of Kokemdki- atory birds in this part of their distribution River, south-western Finland. A + B = main area, are present. The migration times are study area, A + B + C + D = study area. given in an earlier paper of mine (HAUKIOJA Breeding sites of the Reed Bunting are striped. 1969a) . For the present paper the most im- (Tutkimusalue Kokemäenjoen suistossa. A + portant period of field work is the time when B = piitutkimusalue, A + B + C + D = tut- nests are to be found, that is from the first kimusalue. Pajusirkun pesimäpaikat viivoitet- half of May to the end of July. During this tu.) time I have worked in the area practically daily. The most frequently examined part of main study area, 79 from the rest of the study the study area is the main study area. The area, 23 from other parts of the Kokemdki- rest of the study area has been visited less River delta, and 8 from other parts of the frequently, about twice per week. town of Pori. The annual numbers of nests The search for nests, which are regularly found have been as follows: situated under a small willow bush or in a Year Nests tussock, has been quite systematic in the main 1966 52 study area. Because more than 90 % of breed- 1967 73 ing birds have been individually colour-ringed 1968 106 in most years, and the exact number and 1969 92 location of pairs has thus been known, it has Nests found have generally been examined been possible to concentrate the search for daily or every second day in the main study nests to those places where a pair had been area; in the rest of the study area the nests located earlier. Roughly 3/4 of nests of the were inspected often enough to ring broods main study area, in which eggs were assumed at a suitable age. The broods were ringed, if to be laid, were found. In the rest of the possible, at the age of five or six days, in other study area, the proportion of nests found was cases they were ringed at the age of nine days, much smaller. i.e. at the age when they would have left the 323 nests of Reed Buntings were found up nest. Nestlings ringed amounted to a little to the year 1969 ; 213 of them were from the over 1000 individuals.

E. Haukioja: Clutch size of the Reed Bunting 103

Because special care has been taken to ex- the clutch has not been included in the plain the postfledging survival of nestlings in material . relation to the brood size, an intensive netting Another criterion for a full clutch has been programme has been conducted in the area that the young have been found in the nest after the end of June. Netting has been con- after a shorter period than the minimum in- ducted on nearly every day in late summer (July cubation period (12 days) . In such cases it is and August) if weather conditions have not probable that incubation had already begun been unfavourable . In September netting has when the nest was inspected for the first time been less intensive. In the years 1966 and and the clutch was then full. 1967 I netted in the study area only. In later years this part of the programme was the same In both above but, in addition, netting was conducted in the arguments it has been lower parts of the delta by Messrs. Pertti supposed that single eggs are normally Kalinainen and Raimo Hakila. In 1968 and not lost during the incubation period. 1969 the distances between the most remote In those nests I have visited at least netting places was about six kilometers. twice during the incubation For every nest a card noting every visit to period, the the nest was made and for the present analysis loss of a single egg was verified only an edge-notched card was made for every nest . once. This source of error is therefore The edge-notched cards give the following negligible. information for each nest : the year, number and location of the nest, the day the first egg was laid and the day the first young were . Variation hatched (if these were not known exactly, 3 .1 within population they were calculated from the hatching day or, after determining the age of nestlings by The following tabulation gives the num- weighing, from this date), the number of eggs, bers of different clutch sizes of Reed of young hatched and of young ringed, the Buntings found in the study area: number of young leaving the nest, the weights of nestlings on the ringing day, the identity Clutch size 6 5 of parents (if ringed), the rings used for 4 3 nestlings and for every young bird caught later, Number of clutches 68 105 21 9 the sex, catching place(s) and catching date(s) . The information about the breeding biology of the Reed Bunting given in this paper is The mean (n = 203) is 5.14 ± 0.05. from unpublished material of mine unless This value, however, says very little otherwise specified . because there are variations of several kinds in the clutch size. 3. Clutch size The most distinct variation within a population is the change in average I have considered a clutch as full and clutch size within a season. The mean thus acceptable for the material if at clutch size in ten-day periods is given least one of the following two conditions in Fig. 2 . According to it the mean clutch has been satisfied: size during May and the first third of June is about the same, 5.3, but after The number of eggs has not changed be- the middle of June it falls sharply and tween two visits when the interval has been more than a day. This has been the most is less than four eggs per clutch on an common criterion for accepting a clutch as average in July. completed. The criterion rests on the fact that Changes of this kind in the mean eggs are laid, as in general in small passerines clutch size are typical for (DAVIS 1955), on successive days; in the case some species of the Reed Bunting in the early morning. of birds which lay more than one clutch Among several hundred possible cases, I have per season (SNOW 1955, NEWTON 1964, only twice found that the interval between v. HAARTMAN 1967a) . The decrease in two successive eggs has been two days . Both the mean clutch size during a season of these cases occurred in unusually cold is periods in May and, consequently, they do not probably adaptive (see later) . One fac- invalidate the argument. If there has been any tor affecting this is probably that the doubt that the nest was deserted when found, young of the first broods are mostly fed 104 ORNIs FENNICA Vol. 47, 1970

FIG . 2. Mean clutch size of the Reed Bunting in ten-day periods at Pori in 1966-1969 . Range is the mean ± standard error. Sample sizes given below. (Pajusirkun kes- kimääräinen pesyekoko vuosina 1966-69 Poris- sa kymmenpäivdjaksoi- na. Jana = keskiarvo ± keskivirhe. Aineiston ko- ko alla.) on caterpillars but those of the last probable that the clutch size of an indi- broods on adult insects (Diptera, Lepi- vidual varies less in different years than doptera etc.) whose nutritional value is that of the population in general. It has probably less. It may also be important been generally regarded that the male of in order for the female to survive not a pair of birds does not determine the to produce so many eggs just before size of the clutch laid in the nest of the moulting; the weights of females feeding pair. This can be solved by a single test, late broods are rather low (HAUKIOJA viz. by seeing whether the clutches of 1969a) . This may be an indication of the same female in successive years in stress . the same phase of breeding vary less than Because the mean clutch size varies those of the same male. The mean according to the season, it is not reason- difference in the clutches of the same able to make any comparisons between male laid in May in different years all clutches but only between clutches (largely by different females) has been laid during the phase when the mean 0 .9 (n = 11) . The same difference in clutch size is about the same (see Fig. the clutches laid by the same female has 2) . For this early period I have used been 0 .4 (n = 21) . The difference be- those clutches laid in May. The justi- tween the sexes is statistically significant fication for using a fixed date as the (p<0.05, one-tailed Mann-Whitney U- cessation day of "early clutches" is dis- test) . So it is obvious that the clutch cussed later. size is determined by the female of the Before studying the change in the pair and that it is determined, at least mean clutch size given in Fig. 2, it is partially, on a genetic basis . The above reasonable to study how stable or un- test also excludes the possibility that stable the clutches of an individual are females lay clutches of a certain size and thus to try to find out whether because they live in a certain territory . the clutch size is environmentally or Exceptionally, however, an individual genetically determined. female may lay clutches which vary con- If the clutch size of the Reed Bunting siderably, e.g. a female which laid six is genetically determined at all, it is eggs in May, 1967, laid three eggs during E. Haukioja: Clutch size of the Reed Bunting 105

TABLE 1. Clutch sizes of individual Reed TABLE 2. Size of May clutches of Reed Bunting females in the first vs. second or Buntings in relation to the age of the female. replacement clutches in the same season. (Paju- (Toukokuisten pajusirkkupesyeiden koko suh- sirkkunaaraiden ensimmäisen/toisen tai uusin- teessa naaraan ikään.) tapesyeen koko samana kesänä .) Clutch Age of female Size of the second or size Naaraan ikä replacement clutch Size of the Pesye- Old Unknown Young first clutch Toisen tai uusinta- koko Vanha Tuntematon Nuori Ensimm. pe- pesyeen koko syeen koko 6 5 4 3 6 25 32 2 5 14 57 4 6 8 1l1 1 1 4 Q 8 - 5 - 5 5 3 3 1 1 1 4 - - - - Mean 5.5 5.2 5.0 3 - - i - Keskiarvo N 41 98 7 a cold spell in May, 1968 . It is obvious, female at a certain phase of the egg- therefore, that environmental factors laying period is the age of the fe- also affect the clutch size of the Reed male, that is if it is one year old or Bunting. older. In Table 2 the May clutches of If a nest is destroyed, the female females more than one year old and of generally lays a new clutch, at least in unknown age as well as of one-year-old the first and middle parts of the laying females are given. All age-determinations period. True second broods are probably are from individuals which were ringed laid only by females more than one year in an earlier year. Because the clutch old. The sizes of the first clutch and of size distributions are not normal, I have one laid later the same season by the tested only the proportions of clutch same female are given in Table 1 . The sizes of six and five, the most common size of the second clutch is generally clutch sizes . The difference is significant the same or lower than the size of the between females known to be old and first clutch. The only exception is the other females (which very probably also = female which was mentioned to be ex- contain some old individuals) (X2 ceptional also in regard to the clutch size 7.88) f=1, p<0.01) . of different years (see above) . Whether It is also possible to trace the rise in the second clutch in a season is the same mean clutch size between successive or lower in size compared with the first years from the tabulation below. It gives one is largely determined by the phase the clutches of females found breeding of the breeding season at which the for the first time in the main study area second clutch is laid. The fact that the in the year 1967 or later, when females clutch size does not in general rise in arriving in spring were largely colour- successive nestings is probably an in- ringed. All unringed birds, however, dication that environmental factors do were probably not one year old when not greatly affect, at least directly, the breeding for the first time in the main clutch size of an individual female, or study area. that the clutch size is genetically deter- mined, not as a certain size, but as an First year Second year upper limit which may or may not be clutch size clutch size arrived at . 6 5 A non-environmental factor greatly 6 4 0 affecting the clutch size of an individual 5 3 1

106 ORNIs FENNICA Vol. 47, 1970

TABLE 3. Numbers of completed Reed Bunting clutches found in the years 1966-1969 in Pori . Only clutches laid in May are taken into account. (Toukokuisten pajusirkkupesyeiden koot vuo- sina 1966-69 Porissa.) Clutch size Mean Pesyekoko Keskiarvo Year 6 Vuosi 5 4 3

1966 9 7 - - 5.6 16 1967 21 13 1 1 5.5 36 1968 23 26 5 2 5.3 56 1969 6 29 3 - 5.1 38

E 59 75 9 3 5.3 146

The differences in the clutch sizes of different years (X 2 = 8.82, f = 3, p < age groups might theoretically be the 0 .05) . reason for the seasonal trend in the A possible bias causing differences clutch size of the Reed Bunting. This might arise if the proportion of young would be true if the decline in the mean females in the population varies con- clutch size (Fig. 2) were due to young siderably from year to year. If this females laying their smaller clutches at were so, the breeding population of the later phases of the breeding season. year 1969 should probably contain the However, late clutches are practically greatest proportion of young females. all laid by old females, which only lay However, in this year the proportion of true second clutches, and so the above old, colour-ringed females was the largest possibility is ruled out . among females arriving during the LACK (1954a ; assumes that the lower springs of 1967-1969 when it was ex- clutch size of first year birds found e.g. pected that in the preceding year nearly in many passerines (Great Tit, KLUYVER all breeding females were colour-ringed. 1951, Pied Flycatcher v. HAARTMAN This material will not be dealt in this 1967a) is an adaptation, for females paper. breeding for the first time are on aver- Obviously environmental factors are age not able to raise as large broods as the reason for differences in the mean old females. This has been verified, clutch size between different years . The among others, for the Blackbird Turdus start of laying varies in different years. merula (SNOW 1958) and the Great In the following tabulation the mean Tit (PEAR I NS 1965) . The situation is date of clutches laid in May has been obviously the same for the Reed Bunting used as a measurement of the earliness (p. 116) . of the laying. Only nests from the main The differences in the clutch sizes of study area are taken into account because different age classes and within an in- in this area the search for nests has been dividual are thus quite small . With this most even during all phases of the breed- in mind it is assumed that the annual ing season. Day No. 1 is the first of differences in the mean clutch size in May. May are not great. These data are given 1966 22 .1 (22) in Table 3. When testing different years 1967 19±0±1 .8 (31) I have again used the proportions of 1968 13±1 .1 (39) 1969 21±1 .1 (28) clutch sizes of six and five, the most common clutch sizes . The proportions of So the only year when the mean laying these clutch sizes differ significantly in date differs considerably from others is E. Haukioja : Clutch size of the Reed bunting 10 7

TABLE 4. Mean daily temperatures in May, 3.2. Differences between populations 1966=1969, in Pori during the time 5 days before - 5 days after the mean laying date in May. (Vuosien 1966-69 keskimääräiset In addition to the intrapopulation varia- vuorokausilämpötilat Porissa 5 vrk ennen - 5 tions in the clutch size there is a marked vrk jälkeen toukokuun keskimääräisen munin- difference between different popula- tapäivän .) tions. In the following tabulation the clutch sizes of my study area (May) and Year Period , those given by GEROUDET (1962) from Vuosi ( C) Jakso Switzerland from April-May are pre- 1966 17-26.5 . 11 .7° sented. 1967 14 .-23 .5 . 9.6° 1968 8.-17 .5 . 6.3° Clutch size Mean 1969 16 .-25 .5 . 6.9° 6 5 4 3 Southern Finland 59 75 9 3 5.30 Switzerland 1968, but the mean clutch size of this 3 48 6 1 4.91 year is very near the mean value for the whole material . It is thus evident that The difference between proportions differences in the start of laying are not of six and five eggs is statistically signi- the reason for annual variations in the ficant (p< 0 .001, Fisher exact probabi- clutch size. Also the use of May clutches lity test) . as "early clutches" is thus reasonable . From northern Scandinavia I know The lowest mean clutches occurred in two places from where there are publ- the years 1969 and 1968 when the ished data on the clutch size of the temperature of the middle of May was Reed Bunting (LENNERSTEDT 1964 from also the lowest (Table 4) . It may be Ammarnäs, Swedish Lapland, and HiL- mentioned that during a cold spell in DEN 1967 from Karigasniemi, Finnish May, 1969, several females in the main Lapland) . These data are as follows: study area deserted their clutches during the egg-laying period although they were Clutch size Mea not disturbed by me and in 1968 the 7 6 5 4 only dwarf-eggs of the Reed Bunting I Karigasniemi (69° N) - 11 8 2 5 .4 ever have found were laid in two nests Ammarnäs (66'N) 1 10 - - 6.1 during the cold spell in May. Also the intervals of more than one day between The materials from Karigasniemi and the laying of successive eggs mentioned Ammarnäs differ significantly in the earlier are both from these cold periods proportions of six and five eggs (p = in May. Because the weather leads to 0.02, Fisher exact probability test) . My abnormalities in egg-laying (there are material from Pori differs significantly possibly difficulties in forming eggs) it from that of Ammarnäs (p < 0.001, seems quite natural that it may also Fisher exact probability test) but does not differ from that of Karigasniemi result in reduced clutch size. (X2 The clutch size of the population =0.79, f=1) . studied varies, according to the above From Estonia ONNO (1968) gives the treatment, with the season, the age of following clutch sizes of Reed Buntings the female, and there are, in addition, from Matsalu Bay (59° N) : differences between years . All these differences have been verified, e.g. for 6 5 4 3-1 Mean the Great Tit (KLUYVER 1951), the 18 24 6 1 5.20 Pied Flycatcher (v. HAARTMAN 1967a), and the Blackbird (SNOW 1958) . Because clutches of whole seasons have 108 ORNIs FENNICA Vol . 47, 1970 been combined in ONNO'S material, the the Reed Bunting, then it may be a comparisons with my material are best possible reason for the differences be- done by comparing the whole of my tween different areas. It is not possible material with his . There is no significant to test whether the clutch size of the difference between these two materials Reed Bunting varies according to the (X2=0.15, f=1) . habitat using my nesting material be- As considered by LACK (1947), the cause I have not much material from longer daylight hours may be the reason biotopes other than those usual in my why the clutch size tends to rise from study area. southern Europe to north. This enables An interesting aspect is still worth the parents to feed more young during discussing. For 21 broods hatched from the long days in the north. This, how- eggs laid in May (1966-1967 material ever, presupposes that food for the analysed), the mean nestling period was young is the ultimate limiting factor in 9.2 days (range 9-10 days) in my clutch size. The author (LACK 1954a, study area. These broods were not han- 1966 etc.) has produced much evidence, dled by man after the sixth day of life. indeed, to show that the amount of From Switzerland GEROUDET (1962) available food is at least on some occa- gives the following nestling periods: 10, sions the correct explanation ; what the 11-12, and 12-13 days . The differ- significance of the number of daylight ence between these and the values from hours is, however, is not known. CODY my study area is statistically significant (1966) has presented a general hypo- (F = 45 .79, f = 1, 22, p < 0,001) . thesis of clutch size according to which From England BELL (1967) mentions the clutch size is low in areas where having ringed a brood of Reed Buntings there are stable environmental condi- at the age of eight days and at the age tions. According to This hypothesis the of ten days the young were still in the clutch size is determined by how an nest. This would be quite impossible in individual bird uses its existence energy my study area; it is extremely difficult for reproduction, avoiding predators and to succeed in getting a seven-day-old for competition, and this varies in dif- brood to stay in the nest. ferent areas. LACK'S (1947) explanation The nestling period is thus longer in is a part of CODY's (1966) hypothesis. Switzerland and possibly also in Eng- The biotopes most preferred by the land where WITHERBY et al. (1958) Reed Bunting differ in various parts of give the clutch size for the Reed the species' distribution . From Neu- Bunting as 4-5 normally, occasion- siedlersee we know that the most pre- ally 6, rarely 7 in Scandinavia. If the ferred habitats of the Reed Buntings clutch size of the Reed Bunting were are reed beds (ZIMMERMANN 1944, adapted to produce as many young as KOENI G 1952) but in Finland FRITZEN possible in all areas, and this were the & TENOVUO (1957) have shown that only aim, then why are there no clutches the most preferred habitats are low of seven or eight eggs in my study area thickets of willow (Salix), which in my when it is possible to shorten the nest- study area also form the area of the ling period? A logical answer to this densest population (HAUK IOJA 1969a) . question would be that the shorter nest- It has been shown in many species that ling period per se may be very important the habitat has an effect on the mean in Northern Europe in order to avoid clutch size (in the Great Tit (KLUYVER predators or for some other reason. 1951), the Pied Flycatcher (LöHRL However, in my main study area the 1965), and the Blackbird (SNOW mean percentage of eggs producing 1958) . If the situation is the same for fledglings (nine days old) has been E. Haukioja: Clutch size of the Reed Bunting 109

48 % for the years 1966-1967, which can, then the Reed Bunting, which is a material has been analysed so far. This typical species of this region, would do does not deviate much from the figure likewise . I shall next examine whether given by N ICE (1957 ) for birds building clutches of a certain size are more or open nests (46.9 %) . HI LDEN (1967) less productive than others. Because the reports a still lower predation rate for clutch size of the Reed Buntings studied the Reed Bunting from Finnish Lapland ; declines in the middle of June it is not this, however, is based on a very small reasonable to combine early and late amount of material. It is, therefore, not clutches. clear whether the mere struggle for as high a production of young as possible is the only factor in operation . 4 .1 . Early broods This question relates to another im- portant question in this paper. Why do Early broods mentioned later are those the Reed Buntings of my study area in in which young have hatched not later an early phase of the laying period lay than June 15th. I have shown earlier almost exclusively clutches of five and (HAUKIOJA 1969a) that according to six? The skewness of this distribution the post-fledgling weight development (-0.879 with standard deviation of this date of hatching is a suitable crite- 0.211) differs significantly from the nor- rion for classing young Reed Buntings mal distribution (t-test, p<0.001) . Es- of my study area as early and late ones. pecially the upper portion of the clutch Nestling period - If there are any size distribution is much truncated . differences in the proportional or abso- Studying these matters is important lute productivity of different sizes of because answers to above questions are clutches, then the first possibility is needed in order to solve more general that total losses (mainly as a result of aspects of the clutch size. predation) are dependent on the clutch or brood size. The material from my study area is given in Table 5. There 4. Productivity of different clutch and are no significant differences in relation brood sizes to clutch (X 2 = 0.84, f = 2) or brood (X 2 = 0.78, f = 2) sizes. According to LACK (1947 and later) The following possible source of dif- the most common clutch size is that ference occurs if the hatching percentage which produces most surviving young is dependent on the clutch size. Table 6 for the population. According to SKUTCH gives this material and there are no (1949, 1967) this, at least in tropical significant differences in relation to areas, does not hold true and the clutch clutch size (X 2 = 0.01f = 2). size is determined, not by the upper During the nestling phase the loss of limit of parents' ability to feed their individual young from early broods of young, but by the need to maintain the Reed Buntings of my study area after population at a reasonable level . WYN- the hatching day when some young NE-EDWARDS (1962) discusses possible perish is negligible. The nestlings have mechanisms leading to this. CODY been ringed at the age of five or six (1966) has shown that these opinions days and the size of broods at this time are not necessarily alternatives but may in relation to the size at hatching is each form a part of a more general ex- given in Table 7 . Losses (largely during planation . the first day of life) are not statistically If birds of the northern temperate dependent on the brood size (X 2 = 2.81, zone produce as many offspring as they f=3) .

11 0 ORNIs FENNICA Vol. 47, 1970

TABLE 5. Total losses of Reed Bunting clutches and broods in relation to original clutch and brood size in early broods. (Tuhoutuneet pajusirkkupesyeet ja poikueet suhteessa pesyekokoon aikaisissa pesyeissä.)

Size of Clutches Broods Pesyeitä clutch/brood Poikueita Pesye-/poikue- Total losses Total losses % N % N koko Tuhoutunut Tuhoutunut

6 46 25 54 14 2 14 5 39 19 49 22 6 27 4 1-3 } 5 4 80 17 3 17

E 90 48 53 53 11 21

It is, however, possible that large proportions of those broods in which the a broods of Reed Buntings are under- lightest young weighed less than /s of nourished . If so, this would be seen the mean weight of young in the whole from the weights of nestlings . After the brood . The greater the brood size the sixth day of life it is often not possible more broods there seem to be in which to leave nestlings in the nest after such light young are found. The differ- weighing. Therefore I have used as the ence is statistically not significant ( X 2 = weights those which have been obtained 1 .97, f = 3), however. When we take in connection with the ringing of nest- into account that large broods contain lings. These weights in relation to the more young than small broods it seems brood size at the ringing age are given that only in broods of six young, in Table 8. There are no significant if any, is the proportion of light young differences between broods of different greater (right column in Table 9) . The size at least up to this age. It is still difference is, however, statistically not possible that although mean weights do significant (X 2 = 0.40, f = 3) . not differ significantly, there are dif- After the ringing age (five or six ferences in the weight of the lightest days) there are practically no losses (ex- young in relation to the brood size. cluding total losses) during the rest of Table 9 gives the numbers and percentile the nestling period (Table 10) . The

TABLE 6. Unhatched eggs in relation to dutch size in the Reed Bunting. (Pajusirkun kuoriutu- mattomat munat suhteessa pesyekokoon.)

Unhatched Clutches Eggs Unhatched Clutch size _ Ei kuor . Pesyeitä Munia Ei kuor. Pesyekoko 0 1 2 3 4-6

6 26 14 6 - 46 276 26 9.4 5 38 11 3 3 55 275 26 9.5 4 6 1 - - 9 34 3 8.8 3 1 - 1 -

71E 26 10 3 110 585 55 9.4

E. Haukioja: Clutch size of the Reed Bunting 111

TABLE 7. Size of Reed Bunting broods at the age of 5 or 6 days in relation to the number of nestlings hatched. (Pajusirkkupoikueiden koko 5--6 vrk :n idssd suhteessa poikuekokoon kuoriu- tuessa .)

Brood size at ringing Hatched Young lost Hatched Poikuekoko reng.iässä Kuor:t Poikasia tuh. Kuor : t 6 5 4 3 2 1 N %

6 19 1 1 - 126 3 2.4 5 33 9 1 215 11 5.1 4 14 3 3 80 3 3.8 3 4 1 21 2 9.5 2 3 E 442 19 4.3

TABLE 8. Weights (g) of Reed Bunting situation is thus the same as before the nestlings at the age of ringing in relation to first day of brood size. (Pajusirkun pesäpoikasten painot ringing age excluding the (g) rengastusidssd suhteessa pesyekokoon .) life. It is obvious, consequently, that in Age Ikä early broods large and small broods are Brood size Poikuekoko days days percentagewise about equally productive 5 vrk N 6 vrk N as regards the time of nest-leaving . After the fledgling period - A quite 6 10.7-+0.3 8 13.3+0.5 13 5 10.5+-0.4 17 12.6+0.4 13 different picture is obtained from Table 4 11 .2±0.6 12 13.9±0.3 10 11 which gives the numbers of young 1-3 1l.0+0.9 3 13.3+0.4 8 captured after the beginning of July in relation to the brood size at the age of ringing. There is the same tendency in all catching periods; percentually very few young have been caught from large broods and many from small ones. By

TABLE 9. The number of Reed Bunting broods and young in broods where the lightest young weighed less than 4/5 of the mean of the brood at the age of ringing and their relation to brood size. (Pajusirkkupoikueiden määrät, joissa kevein poikanen painoi alle '/, poikueen keskipainosta, suhteessa poikuekokoon .)

Brood size Young Light young Out of broods Out of young Poikuekoko Poikasia Keveitd poikasia % Poikueista % Poikasista 6 126 6 29 4.8 5 150 5 17 3 .3 4 84 3 14 3 .6 2-3 28 1 9 3.6 388 15 17 3.9

ORNIs FENNICA Vol. 47, 1970

TABLE 10. Number of Reed Bunting young proportions from different brood sizes leaving the nest in relation to the brood size at do not significantly change after the first the time of ringing. (Pesästälähtevien palu- sirkkupoikueiden koko suhteessa poikuekokoon half of July: rengastusiässä .) 1-15.7./16.7.-15 .8. X2 = 1.06, f = 3 1-15.7./16.8.-30.9. X2 = 1.31, f = 3 Brood 1-15 ./later size at Broods leaving Young .7 years Xt = 2.16, f = 3 ringing the nest Poika- Lost . . Tub. Poikue- Pesästäläht poik sia It is thus obvious that the brood size koko at the age of ringing, reng.idss,i 6 5 4 3 2 which is practically the same as that at the age of leaving 6 12---- 72 0 the nest (Table 10), affects the survival 5 16 1 - - 85 1 of young after leaving the nest. Because 4 12 - 1 52 2 the proportions between different brood 3 2 - 6 0 2 3 6 0 sizes do not change significantly after the beginning of July, it is evident that E 231 3 the effects of brood size on the survival of young occur largely up to this time. This means that the time between nest- adding together the numbers caught in leaving (at the age of about 9 days) each brood size we get the values given and independence (at the age of about in Table 11 (right) . These numbers 30 days) is the time of real crisis for differ significantly from an assumed the Reed Buntings studied. stable catching probability of each brood Because the category of "early broods" size (X2 = 15.00, f = 3, p < 0.01) . Dur- contains broods hatched up to the 15th ing single periods, the deviation from June and on the other hand the catching equal catching probability in relation to files presented in Table 11 contain re- brood size is directive for the period coveries made after the first of July, it 1-15.7 . (X2 = 6.82, f = 3) and signi- is evident that mortality dependent on ficant for the period 16.7.-15.8. (X2 = brood size has occurred nearer the time 10 .66, f = 3, p < 0.025) . Combining all of nest-leaving than the time of be- catching periods is justified because the coming independent . If it were other-

TABLE 11. Recaught Reed Buntings after the 1st July in relation to brood size at ringing. (Kontrolloidut pajusirkut heinäkuun alun jälkeen suhteessa pesyekokoon rengastusiässä.)

Recaught Kontrolloitu Brood size Young Later years Poikue- Poikasia 1-15.7. 16.7.-15.8. 16.8-30.9. in Pori koko Myöhemmin Porissa N % N % N % N %* N 6 132 7 (5.3) 5 (3.8) 5 (3 .8) 8 (7.5) 25 (18.9) 5 215 20 (9.3) 22 (10.2) 21 (9.8) 10 (7.4) 73 (34.0) 4 111 14 (12.6) 10 (9.0) 12 (10.8) 6 (6.9) 42 (37.8) 1-3 44 8 (18.2) 9 (20.5) 4 (9.1) 3 (15.0) 24 (54.5) E 502 49 (9.8) 46 (9.2) 42 (8.4) 27 (7.7) 164 (32.7) * Calculated from young ringed in 1966-1968. (Laskettu vuosina 1966-68 rengastetuista poikasista .) F. Haukioia: Clutch size of the Reed Bunting

wise, the proportions of recaught young TABLE 13. Mean distances from birth site of female and male Reed Buntings ringed as would probably have been higher in nestlings and later recaught during their first small broods from the first half of July summer after the 15th July in relation to brood onwards, because the latest broods size at ringing age (Pesäpoikasina rengastettu- among "early broods" have not be- jen, ensimmäisenä kesänä Hendkuun puolivälin jälkeen kontrolloituien koiras- ja naaraspaju- come independent at the beginning of sirkkujen keskimääräiset etäisyydet syntymä- July but some of these have been caught paikalta suhteessa pesyekokoon .) after attaining flying capacity at the age of about fifteen days . Distance from birthplace km Differences between sexes - Young Brood size Etäisyys syntymdpaikalta Reed Buntings have been sexed (for Poikuekoko 9 _ d criteria see HAUKIOJA 1969a) when xN x N caught after leaving the nest . The num- 6 0.3 3 1.0 5 bers of recaught males and females in 5 1.2 28 0.9 12 relation to the brood size at the age of 4 0.8 15 1.6 9 ringing are given in Table 12 . The tend- 1-3 0.6 7 1 .1 3 ency of large recovery percentages for small broods and vice versa is obvious for both sexes but statistically not signif- runts in nests are largely females and icant (X2 = 2.32 for males and 4 .95 for that these from broods of six soon perish females, f = 3 for both sexes) . The table after leaving the nest . That the runts indicates that the small percentile pro- may be largely females, is shown by portion of recaught birds from the broods GAVRILOV (1968) for the Spanish of six young is principally caused by the Sparrow Passer hispaniolensis . fact that not many females from these Above I have used recoveries as a broods have been recaught . This is based measurement of survival. This, of course, on a rather small amount of material is permissible only if the brood size at but on the other hand it does not seem the age of ringing does not affect the impossible that females, which are dispersal of young during their first smaller than males even during the nest- summer and later. It is not easy to verify ling period, are less efficient at begging this in a study area only a few kilometers food than larger males during critical in diameter. I have calculated the mean periods. Another possibility is that the distances between the birth place and the place where young were recaught during the time 16th July-20th Sep- TABLE 12. Female and male Reed Buntings tember the first summer (Table 13) . In recaught after 1st July in relation to brood the years 1966-1968, when catching size at the time of ringing. (Heinäkuun alun was conducted principally in the main jälkeen pyydystetyt koiras- ja naaraspajusirkut suhteessa pesyekokoon rengastusiässa.) study area, only broods ringed in this area were taken into account. In the year 1969 when there were catching Brood Recaught size Young Kontrolloitu points all over the delta of the Koke- Poika- mäki-River, all broods ringed in the Poikue- sia a 9 koko N % N % delta-area were included . There are no significant differences among the recov- 6 132 11 8.3 11 8.3 ery distances of males (F = 1 .91, f = 5 215 21 9.8 33 15.3 3, 25) or among recaught females (F = 4 111 14 12.6 18 16.2 2 1-3 44 7 15.9 9 20.5 .66, f = 3, 49) in relation to the original brood size. Therefore, to use 502 53 10.6 71 14.1 the recovery numbers as a measurement

ORNIS FENNICA Vol. 47, 1970

of survival has seemed justified. It may 4 young which is the second in order still be possible that the original brood among ringing material (28 early size results in long-distance wandering broods) and the least productive have after independence, but this kind of been small broods and broods of 6 phenomenon does not seem probable. young, the numbers of which in the It may be stressed that the values in ringing material of early broods are 17 Table 13 as such have no biological and 22 respectively . equivalents ; they only show what I If we weight the recovery numbers have observed, but I think there is no for males and females we arrive at the reason to believe that they were not following values and combined produc- comparable with each other. tivity indices : The difference in numbers of recaught is statis- Brood Relative productivity index females (71) and males (53) size d 9 Combined tically not significant. The most evident 6 0.26 0.20 0.46 cause of this uneven distribution is that 5 0.26 0 .31 0.57 females probably stray less than males 4 0.26 0.27 0.53 during their first summer. This topic 1-3 0.22 0.22 0.44 will be treated more thoroughly in a later paper. That the numbers of fe- The order of these combined values males and males are at least nearly equal is the same as that of the numbers of among young Reed Buntings is shown broods of different size ringed. by the fact that during August, 1966- In the above indices the deviation 1969, 292 a a and 305 ~ ~ were from the mean is greater for females among the sexed young Reed Buntings than for males (p = 0 .03, one tailed captured for this study . Mann-Whitney U-test) . This topic is Table 12 gave the percentages of discussed later. recoveries from broods of different size . The clutches from which the early The importance of these figures for the broods have hatched were laid in May . population is obvious only when the According to Table 3 the dominant absolute brood size is taken into account. clutch size is 5 in May broods ; clutches In the following tabulation I present of 6 eggs are also common but clutches the productivity ( = recovery) indices of 4 and 3 eggs occur much less. (recovery per cent/100 X brood size) In Table 14 I have calculated (accord- for both sexes as calculated from the ing to binomial distribution) the prob- data of Table 12 . able distribution of broods of different sizes which come from a certain clutch Brood Productivity index size (excluding total losses) at the age size ö 4 of ringing . In these calculations the 6 0.50 0.50 hatchability of eggs was regarded as 5 0.49 0.77 of 4 0 .50 0.65 90.6 % (Table 6) and the survival 1-3 0.41 0.53 young up to the age of ringing as 95.7 % (Table 7) . These values were assumed According to the tabulation the brood to be independent of the clutch size. sizes of 4, 5 and 6 young are equally In Table 14, the calculated mean pro- efficient for producing males and smaller ductivity indices for each clutch size are brood sizes are nearly as efficient. For also given. These have been calculated females the brood size 5 which also is by using both the productivity indices the most common in my material (43 of females alone and the combined in- early broods ringed) is the most prod- dices for each brood size. Both methods uctive. The second is the brood size of provide us with the result that the most

E. Haukioja: Clutch size of the Reed Bunting 115

TABLE 14. Calculated brood size distribution at the ringing age in the Reed Bunting per 100 nests and their productivities in relation to the original clutch size. (Laskettu pajusirkun poikkue- kokojakautuma rengastusiässä 100 pesää kohti tuottoisuusindekseineen suhteessa alkuperdiseen pesyekokoon.)

Brood size Relative productivity index Clutch size Poikuekoko Suhteellinen tuottoisuusindeksi Pesyekoko 6 5 4 3 2 1 9 8 9 6 43 39 15 3 - - 0.25 0.26 5 50 37 11 2 - 0.28 0.27 4 57 34 8 1 0 .25 0.25 3 66 30 4 0.22 0.22

productive clutch size is five . Six eggs, June. I have further divided these into closely followed by four eggs are the two groups: those hatched before the following in order and then come end of June and those hatched after- clutches of three . The order of the wards. This practice has been adopted combined values is exactly the same as because the earliest of the late broods that occurring naturally, and that based have mainly been laid during the phase on females alone is nearly the same . of the laying period when the mean Thus it seems evident that the clutch size clutch size is high (Fig. 2) . The later of the population studied is adapted to broods from these late clutches are from produce clutches from which the number the phase of laying period when the of offspring produced is the largest. This mean clutch size is declining. The earliest optimum clutch size for efficient repro- of the late clutches probably give rise to duction is a little 'higher than the broods of about the same size as in optimum brood size and so allows nor- "early broods" but for reasons of caution mal losses of single eggs and/or young. I have not combined these. Those clutch- es hatched after the beginning of July clearly form a group of their own. 4 .2 . Late broods In Table 15 young recovered from broods hatched during the latter half of By late broods is meant those from which June are given in relation to brood size young have hatched on or after the 16th at the age of ringing.

TABLE 15. Reed Buntings hatched during 16-30.6., ringed as nestlings and recaught after the 15th July in relation to brood size at the age of ringing. (16-30.6. kuoriutuneet, pesdpoikasina rengastetut ja heindkuun puolivälin jälkeen kontrolloidut pajusirkut suhteessa poikuekokoon rengastusiässä .)

Recaught Productivity index Brood size Young Kontrolloitu Tuottoisuusindeksi Poikuekoko Poikasia a 9 6 42 4 3 7 16.7 1 .00 5 130 13 14 27 20.8 1 .04 27 ~ 4 1 1 5 1-3 3 3 18 .5 0.56 199 21 18 39 19.6

ORNIs FENNICA Vol. 47, 1970

TABLE 16. Reed Buntings hatched in July, ringed as nestlings and recaught after the beginning of August in relation to brood size at the age of ringing. (Heinäkuussa kuoriutuneet, pesäpoika- sina rengastetut ja elokuun alun jälkeen kontrolloidut pajusirkut suhteessa poikuekokoon rengas- tusiässä.) Recaught Productivity index Brood size Young Kontrolloitu Tuottoisuusindeksi Poikuekoko Poikasia i

6 ------5 40 1 6 7 17.5 0.88 4 51 5 4 9 22.5 0.90 1-3 23 1 3 4 17.4 0.50 114 7 13 20 17.5

Table 16 gives the same information The most common brood size in later for broods hatched in July. broods has been four and, according to Because these tables are based on Table 16, this has also been the most much smaller data than those of early productive. Because the mean clutch size broods I think that there is no good decreases by more than one egg per reason to analyze these as carefully as clutch during the time when broods those of early broods. Some features hatching in July are laid, this, perhaps, may, however, be mentioned . indicates that the most productive clutch Table 15 gives much the same kind and brood size varies even during this of picture as Table 11 for early broods period. This, however, is circular rea- and it seems apparent that, in relation soning . This period should be divided to brood size, no great changes as regards into separate smaller periods but the survival have taken place. The most paucity of the material does not allow it. productive brood size seems to be five, which is also the most common. The brood size of six gives nearly the same 4 .3 . Production of young in relation to productivity, which, however, is based the age of parents on much smaller material . Those broods hatched in the latter half of June were The numbers of young caught after the nearly all fed by both parents . The beginning of July in relation to the broods can thus be compared with each brood size of those broods fed by an other. The earliest broods hatched in old female (aged on the basis of being July are generally fed by both parents ringed in an earlier year) are given in but later broods are as a rule fed by the Table 17 where also the numbers of female alone . The material in Table 16 young caught produced by other females is rather heterogeneous in this respect . (among which there are probably old Out of broods of five young, two broods individuals, too) are given . The mean were fed by the female alone ; no young percentage for the former group is 30, from these broods were recaught later. that of the latter 23. The difference is, Of the remaining six broods fed by both however, statistically not significant; the parents, five produced at least one young trend is nevertheless the same for all bird recovered later. From smaller broods common brood sizes : old females have there are young recovered from broods produced perceptually and absolutely fed either by the female or by both of more young from their first broods than the parents . young females.

E. Haukioja: Clutch size of the Reed Bunting 11 7

TABLE 17 . Reed Buntings from early broods recaught after the beginning of July in relation to brood size at ringing and the age of female. (Aikaisin kuoriutuneiden ja heinäkuun alun jälkeen kontrolloitujen pajusirkkujen määrät suhteessa naaraan ikään ja poikuekokoon rengastusiässä .)

Young ringed from Young recaught from Poikasia rengastettu Poikasia kontrolloitu Brood size Old 9 Unknown age Old Unknown age Poikuekoko Vanha Ikä tuntematon Vanha Ikä tuntematon N % N %

6 58 84 11 23 11 13 5 30 185 9 30 44 24 4 28 83 12 43 20 24 1-3 3 41 1 (33) 15 37 E 109 393 33 30 90 23

The corresponding figures for broods It is possible that the differences ob- fed by an old male or by a male of un- served between the percentile produc- known age are given in Table 18. The tivity of young and old parents is due mean numbers of recovered young are to old females laying earlier and young 34 and 21 % respectively . The difference ones later. However, this is not a plau- is not significant between groups here, sible explanation because only early either, but the trend is again the same. broods were considered, all of which My material is rather small to make it were laid in May and the difference be- possible to test the productivity indices tween the mean laying dates of old and for broods fed by two-birds known to be young females is less than a week. old, by two birds one of which was known to be old, and by two young birds. 86 young, produced by two old parents gave 22 recaught young (26 %) . 87 young, produced by one parent known to be old, gave 29 recaught young (33 %) .

TABLE 18 . Reed Buntings from early broods recaught after the beginning of July in relation to brood size at ringing and the age of male. (Aikaisin kuoriutuneiden ja heinäkuun alun jälkeen kontrolloitujen pajusirkkujen määrät suhteessa koiraan ikään ja poikuekokoon rengastusiässä.)

Young ringed from Young recaught from Poikasia rengastettu Poikasia kontrolloitu Brood size age Old Unknown age Old Unknown a Poikuekoko Vanha a Ikäuntematona Vanha å Ikä tuntematon N % N %

6 48 84 9 19 13 16 5 50 165 22 44 31 19 4 48 63 18 37 14 22 1-3 9 35 3 33 13 37 E 155 347 52 34 71 20 1'18 ORNIs FENNICA Vol. 47, 1970

5. On the genetic basis of the clutch Rissa tridactyla (COULSON & WHITE size 1.961), and the Great Tit (KLUYVER 1951, v. HAARTMAN 1969) . In some species of birds whose clutch Although it has been found that the size variation is well known (especially clutch size of an individual is less vari- the Great Tit) there are very great able than that of the population, this environmental modifications to the clutch does not necessarily prove a genetic basis . size in addition to the possible genetic KLUYVER (1951), finding individual basis . This, above all, is the factor mak- stability in the Great Tit, regards the ing a field study of the heritability of constancy of the clutch size of an indi- the clutch size of wild birds very diffi- vidual female not necessarily as a mark cult. So it is not astonishing that very of inheritance but states that birds living little is known about this phenomenon. permanently in a certain environment In the following I shall discuss some may lay clutches of a certain size and this problems relating to the topic because might perhaps not be genetically but the genetics of clutch size is of prime environmentally determined constancy . importance in developing theories of In the Reed Bunting territorial determin- the evolution of the clutch size (see ation was ruled out as a probable mech- BIRCH & EHRLICH 1967) . anism leading to stable clutch sizes in the same female (p. 104) . As another argument against the heritability of of heredity 5 .1. The probable nature clutch size in the Great Tit KLUYVER (1963) states that females producing Several kinds of evidence have been most ringed young also produce most presented in order to show that clutch young for the subsequent population. size is genetically determined. The laying But the number of ringed young per qualities of poultry are known to be at female was the combined value from least partially determined by genetic the first and possible second brood, and factors because selection may improve thus it had very little to do with the egg-production in some strains (e.g. genetics of the clutch size (but rather et al . NORDSKOG 1967) . This, however, with the possible genetics of the number is not equivalent to the situation in wild of clutches in a season) . birds because poultry do not generally lay clutches. The second kind of evidence In his earlier analyses, LACK has treat- of genetic determination of the clutch ed the topic of clutch size evolution as size of birds is the fact that the clutches if the heritability of clutch size were of the same female in successive years selfevidently high . WAGNER (1957) has vary less than those of the whole popula- criticized this and regarded the upper tion. This has also been verified in this limit of the clutch size as the only herita- study of the Reed Bunting. The same ble factor. He stresses the great impor- kind of situation has been found e.g. in tance of environmental factors for the the Alpine Swift Apus melba (LACK & determination of the clutch size. LACK ARN 1947), the Swift Apus apus (LACK (1958) has also discussed phenotypic & LACK 1951), the Starling Sturnus vul- modifications of the clutch size espe- garis (LACK 1948b), the Velvet Scoter cially in the Great Tit. Thus it is worth Melanitta fusca (KoSKIMIES 1957), the discussing what is most probably in- Collared Flycatcher Ficedula albicollis herited . (LöHRL 1957), the Pied Flycatcher On the basis of the present investiga- (CURIO 1959), the Skylark Alauda ar- tion (p. 105) I see no reason to think vensis (DEL I US 1965), the Kittiwake that the clutch size is genetically de- E. Haukioja : Clutch size of the Reed Bunting termined in terms of the number of dish Lapland (Ammarnäs) contains a eggs. It seems more reasonable that the clutch with seven eggs. v. HAARTMAN clutch size is determined genetically by (1969) also mentions two nests with how an individual female responds to seven eggs from Finnish Lapland. certain environmental stimuli; there are The above opinion that the limit(s) thus thresholds for certain stimuli which of the clutch size and responses to certain are probably genetically different. The environmental stimuli (and not the results presented by MILLER (1960) for number of eggs laid) are determined the Zonotrichia species is an indication genetically excludes the opinion that that genetic differences exist between clutch size is a form of balanced poly- threshold values in populations of the morphism (LACK 1948b, HUXLEY 1955) . same species in their responses to photo- It also explains well small differences periodism, which is an example of much in mean clutch sizes and small or the same kind of nature. Of course, negligible differences in range when these genetically determined responses comparing different populations (see must be such that, in certain environ- v. HAARTMAN 1954, JOHNSTON 1954) . mental conditions, they produce a clutch Whether these differences in mean whose size is adapted to probable con- clutch sizes are due to differences ditions during the raising of the young. in environment or in genetically de- I think this best explains the general termined responses to these environ- constancy and at the same time makes mental stimuli, is very difficult to judge the great potential range and flexibility without experiments . However, because of an individual's clutch comprehensible . the heritability of the clutch size is "Nongenetic variation is usually adaptive probably not high (p. 121), it is evident and controlled by natural selection, since that changes in genetic factors are not genetic factors determine the amount of the first importance when clutch and the direction of the permissible sizes change with changing environ- flexibility of the phenotype" (MAYR mental conditions (introduced species, 1966,p.139) . MOREAU 1944, form an extreme case) The clutch size distribution found in (see also v. HAARTMAN 1954, and for the Reed Bunting deviates significantly an opposite opinion LACK 1947) . from a symmetric distribution . Although The above model of the genetics of the we may suppose that clutches of seven clutch size partly contains the opinions eggs would be very nonproductive (this presented by LACK (1954b) and JOHNS- is treated later, there is a very large TON (1956) . drop in the clutch size distribution of my material at the upper end - 68 5.2. The ways of inheritance nests with six eggs (in two Mays of four possible even the mode) and not a In what ways is the genetic basis of the single nest with seven eggs has been clutch size transferred to the following found. This is best explained, I think, generation? I have the impression, by supposing that there is an upper although I have not found it express- limit to the clutch size between six and ed, that the hypothesis that the de- seven eggs, which was not exceeded in termination of the clutch size is based the population I studied. The same limit on individual selection (for birds is probably also found in most popula- formulated in detail by LACK 1947 tions studied elsewhere (Switzerland, and later) rests on the following as- Estonia, Northern Lapland, see p. 107) . sumptions: the genetic factors determ- Only the material originating from Swe- ining clutch size are transferred to the ORNIS FENNICA Vol. 47, 1970

following generation by female and male such a common clutch size is almost but not alike and the most productive clutch size quite as probable. Such a situation is not impossible, and the evolution towards a certain is at least in general that which produces dominant clutch size or certain sizes in a certain most progeny (females and males) . place and at a certain time is probable through LÖHRL (1957), however, mentions that apparently not so quick as with the former the possible share of both parents in alternative. this respect is not known . Because the most productive and common clutch The ways of inheritance from both size of the Reed Buntings is that which parents are easily understood . Also there produces most females but no more are no difficulties in principle to the males than other common clutch sizes, other alternative. These possible ways two possibilities as to genetic influence are plasmatic or maternal inheritance on the clutch size remain open. These (found to affect e.g. the size of the alternatives are that the genetic factors poultry, COCK & MORTON 1963) and affecting clutch size are either inherited perhaps also inheritance through the Y- from the female only or from both chromosome of the female is not im- parents . As far as I know, practically no possible . attention has been paid to the different The supposed heritability through survival rates of both sexes in relation females or both of parents gives roughly to clutch size and the possibility that the the same order of clutch sizes as that clutch size of birds is maternally inher- found in the field in the Reed Bunting. ited. Although no final solution is likely I cannot therefore find any support to be found without experiments into for either possibility from this (and in the ways of inheritance, it is necessary positive cases it would be circular rea- to discuss some aspects which are im- soning) . Thus both ways are possible portant in explaining the results of and, as our knowledge of these facts is studies of the same kind as this. only in its infancy, it is not reasonable to favour either of them, but keep both If the clutch size of the laying female is possibilities in' mind when treating the determined by genetic factors inherited from topic. the mother bird of this female only, the laying It may be mentioned that the analyses female in question has more probably been LACK (1948 born in a clutch belonging to the most common made earlier, principally by and productive size than to some other clutch and later), of the most productive clutch size. The male of this pair may, on the other size are as well explained on the basis hand, have been born in a less superior size of clutch size inheritance through fema- of brood. However, if it does not affect the clutch size of the females of the subsequent les alone as on the basis of inheritance generation, the clutch sizes of the following through both parents. As a matter of generation will again be as well or better fact the views of this paper reveal adapted to prevailing conditions. The evolution further possible sources of error which of the most productive clutch size to the most may have arisen earlier common one by individual selection will not in treatments. only be possible but also obvious and probably E .g. if the trapping of birds is concen- comparatively quick. trated to the ringing place as in the If the clutch size of the following generation work Of PERRINS (1963) on the Great is determined by genetic material from both parents and if in some broods more females Tit, then, according to the findings of are produced than in others, these broods will DHONDT & HUBLE (1968), more males probably dominate although males are produced than females will probably be caught. in equal numbers from all brood sizes. The If proportionally more males than laying female is quite probably born in a clutch of the most common size according to results females are produced from clutches given earlier for the Reed Bunting. That the of a certain size as in the Reed male of this pair is also a descendant from Bunting, it is probably that a biassed E. Haukioja : Clutch size of the Reed Bunting

distribution of young in relation to field data on this matter, and my ma- brood size will result. In general, it terial is also limited in this respect . As is possible that the production of males two early clutches with three eggs (out and females in relation to brood size of three possible) are included in the differs. If, for instance, many males are material (following tabulation) which produced in relation to females from shows the relation between the clutches large broods, as is the case with the of mothers and daughters, it is evidently Reed Bunting, and the clutch size is biassed in this respect . Heritability inherited by one generation from another cannot be high, however . through females alone, selection will not favour these large broods in spite of the Clutch size of fact that they may produce many young Mothers Daughters according to catching files. It is there- 6(10.5.), 6 (18.5.), 6 (5.6.) 3(27 .5.) fore highly desirable that 3 (30.5.) 605.5.) analyses of the 6(10.5.), ?5(3 .6.) 5(15.5.) same kind as in this paper dealing with 6 (25.5.) 5(22 .6.) the proportional production of females and males in relation to clutch size The dates on should be made . It is probably not which the first egg of possible to get information the clutch was laid is given in brackets. of this kind All clutches of daughters are from their from the ringing recoveries of ringing first breeding season. centres because the birds recovered are A similar table given by LÖHRL seldom reliably sexed. The best (and probably only) way is field work of the (1957) for the Collared Flycatcher same kind as possibly indicates somewhat higher heri- that described here. The tability. easiest way to get this kind of material is to obtain it from birds nesting in nest- boxes but, as will be discussed later, 6. On the determination of range, this material is not quite comparable in mode, and distribution of the clutch size all respects with that presented in this paper. Clutch size evidently possesses survival value if, the following season (for spe- cies breeding when one year old), the 5 .3 . Heritability of clutch size gene pool of the breeding population contains genes from this particular clutch It has been found in this paper that or from parents of this clutch. This some, perhaps many, environmental fac- means that not only the young but also tors can modify the clutch size of the the parents are important. As a matter Reed Bunting. The great importance of of fact, if selection is to favour a certain environmental factors was shown when, genotype, it is very important that the in different environmental conditions, parents live after the breeding attempt, the same female laid early clutches because in stable populations they, more whose size varied between six and three. probably than their descendants, are According to this large potential range in those that live the following breeding the clutch size of an individual and the season if the annual adult mortality rate probable nature of inheritance it is is less than 50 %. The situation is in assumed that the heritability of the practice not so simple because not all clutch size is not high or at least adults breed every year . This makes the that the correlation between the clutch adults still more important if the size size of females and their daughters is of the population is to remain the same. not high . It is laborious to collect Another complicating factor is that the 122 ORNIs FENNICA Vol. 47, 1970 mortality rates of adults and first-year TABLE 19. Return tendency of female Reed Buntings in relation to the brood size raised in birds are probably interdependent (see the first brood the preceding summer. (Naa- KLUYVER 1966) . raspajusirkkuien palaaminen suhteessa edellisen It is therefore important to know kesän ensimmäisen poikueen kokoon.) whether it is deleterious for the parents No.of fledglings Y -parents Returning to raise large broods. According to in the first brood weight data for the Reed Bunting Poikasia ringed following year Emoja ren- Palasi seur. HAUKIOJA 1969a) it is evident that if ensimmäisessä gastettu vuonna the brood size affects the survival of the pesyeessä parents then this would be seen in 6 10 6 females. However, in my small material 5 10 5 (Table 19) there is no indication of 4 12 8 this. It is, however, still possible that 2-3 4 0 the size of the second clutch is that E 36 19 which affects the survival of females . However, second broods are only laid by old females, which have earlier been consider different features of the clutch shown to lay large clutches in the early size distribution of the Reed Buntings phase of the breeding cycle. According studied. Only "early broods" are dealt to Table 19 this does not lead to their with. lowered survival. Thus there is no in- The absolute productivity index for dication that mortality is dependent on broods of six young is rather low in the brood size in breeding Reed Bunting Reed Bunting. So it is worth studying females . This was to be expected in what would be the productivity index view of the fact that the annual mortal- for clutches of seven eggs. I have not ity rate of adults is' probably less than made field experiments by adding extra 50 % (e.g. Table 19 shows that 19 out of eggs to nests, and there is no proof that a possible 36 breeding females returned this would give results comparable the following year and it is improbable with "natural" brood sizes (see WYNNE- that all females returned to exactly the EDWARDS 1964) . We can, however, same breeding site and were thus calculate what would be the probable recorded; this topic will be treated in brood size distribution for early clutches more detail in a later paper) . Because of seven eggs assuming that the hatch- even the largest broods are evidently ability and first-day mortality is the small enough not to diminish markedly same as for other brood sizes. The dis- the survival of the parents, it is evident tribution of broods/100 clutches of that among clutches of different sizes seven eggs are as follows : occurring naturally, selection favours those clutch sizes which produce most Young Nests surviving descendants. These are, in the 7 38 population studied, clutches of five, six, 6 39 four and three, which was the order of 5 18 4 4 their natural occurrence, too. 3 1 Now it is reasonable to study whether the clutch size distribution of the Reed The productivity index of broods with Bunting can be understood as a sign that 3-6 young would be according to it has adapted to prevailing conditions Table 14 and using both sexes 0.26. when trying to raise young as efficiently Because the recovery percentage tends as possibly. In the following I shall to decrease as brood size rises, it E. Haukioja: Clutch size of the Reed Bunting 123 is probable that broods of seven young in clutches of seven eggs than in smaller would be less productive than broods of clutches . This would cause higher pro- six young, which were already rather ductivity indices but, on the other hand, non-productive . So it is probable that the females would have to lay an extra clutches of seven eggs, which have not egg which does not seem to be a reason- been found in my study area, would be able situation. less productive than clutches occurring Thus there is no reason to believe naturally, or about as productive as the that clutches occurring naturally were most non-productive clutch sizes . So the not determined so as to produce as many ± rigid upper limit of clutch size dis- genes as possible for the gene pool of tribution occurring naturally seems to the following breeding population . have been formed to suit prevailing con- In the Reed Bunting, the truncation ditions and in order to prevent breeding of the upper portion of the clutch size attempts which were doomed to be un- distribution leads to the non-existence of successful. Three complicating points of clutches of seven. This or a large clutch view are worth mentioning . The first is size seems to be uncommon in other that old females, whose clutch size is passerines nesting in the open, too (see higher than in young females, would be v. HAARTMAN 1969) . In an arctic en- the most likely to lay clutches of seven vironment such clutches are, however, eggs . Earlier I have presented material less uncommon (H I LDEN 1967) and in which suggests that old females are more passerines nesting in holes they are not efficient in raising broods . However, the uncommon at all (v. HAARTMAN 1969) . tendency of the productivity index to be In the arctic environment, a possible low in broods of six young also holds and much discussed cause is a longer true for old females as calculated from working day for parents . It is, however, the small material presented in Table 17 : improbable that this is the sole reason, Clutch size Old Young if it is a reason at all . For species breed- ing in holes, the above explanation 6 1.38 0.78 5 1.50 1.00 cannot be the correct one. LACK (1947) 4 1.72 0.96 gives the explanation that species breed- ing in holes, where the rate of predation The absence of very large broods is is lower compared with open nesting thus understandable . species (NICE 1957), have evolved a The second complicating fact is that slower rate of development . v. HAART- all broods have been about as efficient MAN (1954) has shown that the nestling in producing males but the proportion periods are not a good measurement in of females produced in broods of six this respect because in hole-nesting young is small. Thus the production of species the young stay in the nest some young from clutches of seven eggs might days after achieving their maximum still be rather high . This, however, is weight while open-nesting species do no valid argument if the clutch size is not . RICKLEFS (1968), studying the inherited through females only, but weight curves of nestlings, came to the neither is this, of course, proof of female- conclusion that LACK's (1947) explana- bound inheritance . However, it seems tion cannot be the correct one . probable that the production of males RoYAMA (1966) has presented a rea- would not be higher in clutches of seven sonable hypothesis according to which than in smaller clutches. "the optimum working capacity" of pa- The third point is that the hatchability rent birds varies, and the bottle-neck in and/or nestling survival might be lower feeding young may fall at different 124 ORNIs FENNICA Vol. 47, 1970 phases of the nestling or fledgling to brood size would be more affected by period. The most obvious periods the clutch size in hole-nesting species of crisis are the nestling time or the than in those nesting in the open. This period just after that. In Reed Bunt- would be true if the amount of food ings, as well as possibly in other temper- were the limiting factor and the clutch ate zone passerines which leave the nest size of hole-nesting species were adapted before they are able to fly, this period to the maximum mean which parents is just after leaving the nest (see MAR- are able to raise during the nestling LER 1956) . In early broods of the Great period. The second aspect is that, if the Tit it is evidently during the nestling crisis period occurs in hole-nesting spe- period (ROYAMA 1966) . According to cies during the nestling period, the this author, the differences are based on energy demand of the brood is not pro- the fact that in hole-nesting species the portional to the number of nestlings as brood remains in the nest long after the has been mentioned earlier. If this is time when the main demand on their true, it is assumed that the upper por- energy is for growth. They stay in the tion of brood and clutch size distribu- nest also during that phase when the tion, that is clutches larger than the main demand on their energy is for mode, in hole-nesting species do not have maintenance . Before homoiothermy has such a deleterious effect on the survival developed (for passerine species, see of the whole brood as in open-nesting DAWSON & EVANS 1960, MAHER 1964) species . It is evident, then, that the and to some extent after it, the food upper portion of the clutch size distribu- requirements of the whole brood are tion would be less abrupt in hole-nesting not proportional to the size of the species than in others. The third possible brood; great broods are proportionally way of proving plausibility of the rea- easier to raise than small ones. In species soning is to study domesticated birds. breeding in open nests, the situation is principally the same, except that their I shall now study some published nests are generally less well insulated figures on the weights of passerines in and thus the heat loss of nestlings is relation to the brood size. LACK (1948b ) higher. They also leave the nest propor- found differences in weight in relation tionally earlier (adaptation for heavier to brood size in the Starling. The ma- predation, NICE 1957) . The greatest terial was, however, very small. PERRINS energy demand occurs at the phase when (1965) has assembled a lot of data on young birds leave the nest and the energy the Great Tit in this respect and there demand per individual nestling is much was a tendency for nestlings in large more than if they remained in the nest . broods to be lighter than those in small We may thus ask the following im- broods. ROYAMA'S (1966) data show the portant question: are the large clutches same trend. The same thing has also of hole-nesting species caused by the been shown for the Blue Tit Parus fact that they are able to eliminate the caeruleus (LACK et al . 1957) . v. difficult phase of feeding single young HAARTMAN (1954) has shown the same which are unable to fly and follow the kind of trend in the Pied Flycatcher. parent but nevertheless consume more SEEL (1970) reports lowered survival energy than the same number of young and low weights in House Sparrow in the nest? Passer domesticus nestlings in large If this reasoning holds true, we have broods but not in the Tree Sparrow three possible ways in which to study Passer montanus. the matter. The first is that the weight In open-nesting passerines the follow- distribution of young birds in relation ing are known to me where similar E. Haukioja : Clutch size of the Reed Bunting 1'25 weights have been taken : SCHRANTZ Because hens do not lay ordinary (1943) for the Eastern Yellow Warbler clutches, this species has to be disregard- Dendroica aestiva, LACK & SILVA ed when studying domesticated birds in (1949) for the Robin Erithacus rube- order to prove the plausibility of the cula, SNOW (1958) for the Blackbird, above reasoning . HINDS (1959) has and TYRVÄINEN (1969) for the Red- given data for domesticated Canaries wing Turdus diacus. In none of these Serinus canaria. This is an open-nesting species did the young in large broods species, and if the clutch size distribution weigh less than those in small broods, is the same as for wild birds of the same but the young in broods of the most kind, it may be assumed that the skew- common size were on some occasions ness of the clutch size distribution is heavier than those in small broods. This negative. However, the skewness of the possibly indicates that the feeding effi- clutch size distribution is clearly positive ciency of parents is at least partially ( -I- 0.334, with SD of 0.148) if only regulated by the need to feed or small clutches showing some embryonic de- broods consume very much energy. The velopment are taken into account. For former might be confusing when inter- other, "all-clear", clutches the skewness preting the above results . Another thing is 0 .393 (with SD of 0 .185) . These is that we have very little information values are more positive than in any about the importance of nestling weights. other species analysed in this paper, However, the predictions presented including hole-nesting species. earlier do not contradict the data pre- Two possible explanations are ob- sented here. vious : the first is that in domesticated In Fig. 3 I have given the clutch size Canaries, food is no limiting factor in distributions of some Finnish passerines. raising broods and thus large broods The material is mainly from v. HAART- are not deleterious but very productive MAN's paper (1969) . This figure accords and that is why the disribution is with the predictions presented earlier; positively skewed . This is what might that is, those species breeding in holes be expected on the basis of the above seem to have more symmetric clutch size reasoning . However, it is also possible, distributions than those breeding in the that Canaries are heterogeneous in re- open. In order to examine the skewness lation to their native areas and thus the of clutch size distributions more pre- positive skewness might be due to a cisely, I have computed skewness values large proportion of birds producing (by using the momentum method, SNE- clutches typical of those regions where DECOR & COCHRAN 1968) for those the clutch is small and a small propor- distributions of small and medium-sized tion of clutches from those regions passerines presented in Fig. 3 when the where the clutch size is large . However, material comprises more than 100 the small clutch size data of wild birds clutches and also for those monthly given by CHARVOZ & GEROUDET (1962) clutch size distributions which are larger do not show positive skewness and than 100 in the material concerning the material presented by HINDS Swiss passerines presented by GLUTZ (1959) is clearly one-topped . Thus (1962) . These figures are given in Table the first explanation seems the most 20 . Species which raise their young probable. almost or up to the age of flying capacity Now we can turn to our original in the nest have significantly lower question; can the skewed clutch size skewness values than those species nest- distribution of the Reed Bunting be ing in the open (p G 0.05, one-tailed regarded as a result of individual selec- Mann-Whitney U-test) . tion? Because the above predictions

126 ORNIS FENNICA V01. 47, 1970

E. Haukioja: Clutch size of the Reed Bunting 127

TABLE 20. Skewness of clutch size distributions of some passerines, H = v. HAARTMAN 1969, G = GLUTZ VON BLOTZHEIM 1962. (Eräiden varpuslintujen pesyekokojakautumien vinoudet, H = v. HAARTMAN 1969 G= GLUTZ VON BLOTZHEIM 1962) .

Species Source Clutches Skewness ,SD Security of nest Laji Lähde Pesyeitä Vinous Pesän suojaisuus

Parus major April, G 534 0.214 0106 + Sturnus vulgaris H 192 0.117 0.175 + Parus caeruleus April, G 125 0.055 0.217 + Sturnus vulgaris April, G 199 0.042 0.172 + Parus major May, G 353 0.024 0.130 + Turdus merula April, G 380 -0.005 0.125 - Delichon urbica June, G 128 -O.194 0.214 + Passer montanus May, G 138 -0198 0.206 + Turdus iliacus S-Finland, H 285 -0 .247 0.144 - Saxicola rubetra H 111 -0 .281 0.229 - Phyll. trochilus S-Finland, H 1,27 -0.353 0.215 - Muscicapa striata S-Finland, H 266 -0.393 0.149 - Ficedula hypoleuca May, G 434 -0.428 0:1!17 + Ph. phoenicurus H 114 -0.492 0.180 + Turdus merula May, G 302 -0.541 0:140 - Parus major Early, H 225 -0.577 0.1162 + Sylvia atricapilla May, G 193 -0.637 0.175 - Ph. phoenicurus May, G 182 -0.649 0:180 + Fringilla coelebs H 3118 -0.691 0.137 - Lanius collurio H 108 -0.806 0.233 - Acroceph. scirpaceus June, G 135 -0.828 0.209 - Turdus pilaris S-Finland, H 292 -0.886 H 0;143 - Ficedula hypoleuca 1210 -1.091 0.070 + Turdus philomelos S-Finland, H 221 -1.196 0164 -

were shown to be valid or not at least the Reed Bunting on the basis of indi- invalid for several temperate zone pas- vidual selection . serines, individual selection for deter- A factor probably contributing to the mining the most productive clutch sizes decided abruptness of the upper limit is would probably give the frequency the shortening of the nestling period, at distribution occurring naturally in the least from Switzerland to southern Fin- Reed Bunting. It is therefore possible land, in the Reed Bunting. I think it is to explain the very abrupt upper limit best explained as a way of maximizing of the clutch size distribution found in the production of young in conditions

FIG. 3. Clutch size distributions of some Finnish passerines . The material is taken from v. HAARTMAN'S work (1969) except for the Reed Bunting, which is from this study. Size of the mode is given. Abbreviations of generic names are as follows : S. = Sylvia, E. = Emberiza, A. = Anthus, T. = Turdus, F. = Fringilla, L. = Lanius, M. = Muscicapa, C. = Carpodacus, Co. = Corvus, P. = Pica, S. = Saxicola, Ph. = Phylloscopus, Mo . = Motacilla, D. = Delichon, H. = Hirundo, Pa. = Parus, St. = Sturnus, Fi. = Ficedula, Pho . =Phoenicurus. (Eräiden suomalaisten varpuslintulaiien pesyekokojakautumat. Aineisto v. HAARTMAN'in teoksesta (1969) paitsi paju- sirkku, mikä tästä kirjoituksesta. Runsaslukuisin pesyekoko mainittu. Sukunimien lyhennykset yllä.) 128 ORNIs FENNICA Vol. 47, 1970 where it is not possible to raise a brood mer phases . When it is evident that of seven fledged young easily. This is the most productive clutches which arrived at by shortening the vulnerable females at a certain time and place lay nestling period . Shorter nestling periods are determined by individual selection, in northern populations have been noted it is a good reason to stop the breeding to exist in the Redwing (ARMSTRONG phase when the losses of parents are 1954, TYRVÄINEN 1969) and the greater than the production of young. Gambel's Sparrow Zonotrichia leuco- At this phase the most productive clutch phrys gambeli (OAKESON 1954) . size has evidently fallen to or near Also the lower limit of the clutch size three eggs in the Reed Bunting popu- distribution found in the Reed Bunting lation I have studied . is rather distinct, three eggs in the It is therefore possible to understand population I studied. The probability the present situation as the result of of finding clutches of two eggs is, selection favouring individual parents however, rather small because clutches and their young. The commonest of three eggs are not common in clutches are those which are at present my material, either. Whether there most productive and the limits of clutch is any fixed lower limit to the clutch size distribution are found where pro- size as presented by JOHNSTON (1956) ductivity becomes too low. is not easy to verify. The lower limit of The situation examined in this paper clutch size distribution can be thought is principally that of the Reed Bunting to have formed in the following way. but because other species, treated in (This has been presented in part in Fig. 3 and Table 20, showed similar another paper, HAUKIOJA & KALINAI- tendencies, the concepts of this paper NEN 1970) . The lowest clutch sizes are in most points acceptable also for which the females lay are such that other nidicolous species in the temperate breeding attempts are on an average zone. profitable for preserving genes of the Although accepting individual selec- parents for the gene pool of the fol- tion as a force leading the evolution of lowing breeding population. As the clutch size on the basis of facts presented breeding cycle continues, the most pro- in this paper, it is also reasonable to ductive clutch size decreases according discuss the possibilities of group selec- to the above treatment . On the other tion in the population studied. Finding hand the time which the parents (in the most common clutch size to be the late broods of the Reed Bunting females most productive we only show that in- alone) have left for moulting and other dividual selection is in operation ; this, premigratory phases becomes shorter. as such, does not necessarily disprove The postnuptial moult of the Reed group selection (see v. HAARTMAN Bunting lasts a little less than two 1954, p. 71) . The possibility of group months (unpublished) and the pre- selection is not ruled out but because migratory fat deposition probably a nesting success and the size of subse- week or so. Moulting may in some con- quent breeding bird populations are ditions be a dangerous period (see generally not well correlated (e.g. Tom- DAWSON 1967, HAUKIOJA 1969a) and PA 1964, LACK 1966), possibilities of when it has to be gone through quickly group selection in a species whose dis- it is probably still more dangerous . Now persal is considerable (as is probable in it is evident that selection favours such the Reed Bunting, Haukioja, in prep .), breeding attempts which are made at are small. It is, of course, possible that that period when parents and young the situation has been different in the have enough time for normal late sum- past but in speculating on these possi- E. Haukioja: Clutch size of the Reed Bunting 12 9 bilities we are on very uncertain ground mind, it is assumed that no single factor (see BIRCH & EHRLICH 1967) . can explain the whole complex pheno. menon of clutch size evolution in dif- ferent species and CODY'S (1966) work 7. Discussion is a good attempt to find possible inter- acting factors. An evident ultimate reason for limiting As far as I know, all attempts, ex- the clutch size in nidicolous birds from cluding the paper by LACK (1949), to the temperate zone has in some cases show links between the final produc- been shown to be the lowered survival tivity index and the commonness of of nestlings from big broods (e .g. LACK clutch size in nidicolous species have & LACK 1951) . Important factors in- been made with hole-nesting species. volved are probably the amount of According to the above, however, these available food and factors affecting the are not quite comparable with open- food requirements of the young and the nesting species in all respects. In these capacity of the parents to collect food analyses the non-productiveness of large (ROYAMA 1969) . Other factors are broods has not always been evident. probably in operation, too, or operate LACK (1948b) found that in the Swiss together with the former. ROYAMA Starling the brood size of five (the most (1966) has paid considerable attention common clutch size) gave as many sur- to the heat-insulation of nests, which vivors as larger clutches . We thus find, helps in lowering the maintenance cost therefore, that the commonest clutch is per nestling and is therefore a factor the smallest giving a large number of affecting the rather high clutch sizes surviving nestlings. This situation ac- found in hole-nesting species . These are cords well with views presented earlier able to build a well Insulated nest and, in this paper, especially when we take furthermore, hide it well. into consideration the low decline in Another possible group of factors clutch size frequency distribution (Fig. which may have an influence either by 3, Table 20) . In British Starlings the themselves or together with the former situation may be the same but produc- is the lowered survival rate of parents tivity may also be smaller in large broods when raising large broods. This has, so (LACK 1948b) . far as I know, not been shown to be CURIO'S (1960) data on the produc- the correct explanation in any species of tivity of the Pied Flycatcher in relation birds but the materials presented when to brood size gave no support to the studying this phenomenon (KLUYVER non-productiveness of large broods . 1963, PERKI NS 1965, LACK 1966) are LACK (1966), when analysing English difficult to interpret because dispersal data for the same species, found that of breeding birds is a very laborious and common clutch sizes produce most sur- difficult field to study. The above papers vivors but the situation was by no all consern the Great Tit which, like the means clear (see v. HAARTMAN 1967a) . Reed Bunting, has an annual adult Using Finnish material the latter author mortality rate rather less than 50 % calculated that there were no note- (HAUKIOJA 1969b) . The survival of worthy differences in survival in relation parents must therefore be important. to brood size. However, if in this ma- It is obvious that the above factors terial we consider as broods of normal (at least) are selected in such combina- size those with seven eggs (about 15 % tions that as many genes as possible are of broods) hatched from clutches com- carried to the gene pool of the following menced between May 26 and June 4, breeding population. Bearing this in the results are quite different . The 13 0 ORNIs FENNICA VOI. 47, 1970 situation seems, therefore, not to have clutch size we are on rather uncertain been explored enough in the Pied Fly- ground in comparing the productivities catcher. of different clutch sizes. The possible In the Great Tit the situation is sources of errors may be so numerous perhaps a clearer one. LACK et al. that I think that even the fact that in (1957) found the most productive some species the most common clutch clutch size was the most common in size has been shown to be the most this species as well as in the Blue Tit. productive is a rather good result. Using more material the most produc- The most thorough investigations into tive clutch size for the Great Tit was 9 the population dynamics of nidicolous (LACK 1966), which is near the com- birds have been conducted on hole- monest clutch size of the population stu- nesting species. This has allowed much died. The annual variation was, however, data to be collected, but we also need large . This is what might be expected on the same kind of data for species the basis of rather symmetrical clutch building open nests although they are size distribution and with individual more laborious to study. These species, selection in operation determining the however, form the bulk of all passerines . clutch size. It may be mentioned that the Great Tit, Based on the ideas presented earlier which is one of the most thoroughly in this paper, it is assumed that the non- investigated species in this respect, has productiveness of large broods is more the second largest clutch size of all clear-cut in open-nesting species because passerines (LACK 1968) . This naturally the after-fledging period is probably on allows many aspects to be studied very an average more critical for these than thoroughly but such an extreme species for hole-nesting species. is not necessarily a good study object LACK'S (1949) attempts to show this in all respects. for certain thrushesgave variable results Future research into the whole sub- and there was no clear tendency, if, ject of clutch size will depend to a large indeed, there was any tendency at all, degree on forthcoming knowledge of for large broods to be non-productive. the genetics of clutch size. This, how- The material presented in this paper for ever, demands experimental work or the Reed Bunting is the only one, as perhaps thorough analyses of large ma- far as I know, where it has been shown terials of colour-ringed parents and their for an open-nesting species. descendants as proposed by BIRCH & I think there is no reason to treat EHRLICH (1967) . Both methods are thoroughly those investigations based rather laborious and difficult to put into on productiveness during the nestling operation in practice . period only, although they may give certain ± valid results for hole-nesting species. In all the above cases the survivors from certain brood sizes have been treated by combining females and males together, which may be a source of error. It is also possible that the genetic factors determining the clutch size have pleiotropic effects on other features which may be favoured by selection differently from clutch size. Until we know more about the heredity of the E. Haukioja: Clutch size of the Reed Bunting

Acknowledgements Second or replacement clutches are of the same size as or smaller than the first clutch I wish to express my gratitude to my teacher, (Table 1) . Year-old females lay on average Professor Paavo Voipio, for valuable advice, in- smaller clutches than older females (Table 2) . cluding positive criticism of the manuscript, and Mean clutch size differs significantly for providing pleasant working facilities . My in thanks are also due to Dr. Martti Soikkeli for different years (Table 3), probably as a result encouraging me to begin a population study of weather conditions at the time of laying and for criticism of the manuscript, Prof . (Table 4). The start of laying of the popula- Tarvo Oksala, Mr. Risto Lemmetyinen, lic . tion evidently does not affect the mean clutch phil., and Mr. Markku Haukioja for many helpful discussions, and Mr. Pertti Kali- size. nainen and Mr. Raimo Hakila for help The clutch size of the population studied in field work. Mr. Arvi Laaksonen, Mr. was significantly larger than in Switzerland Ilkka Sten, M. A. and Prof. Rauno Tenovuo have in various ways helped me. Mr. Christo- and smaller than in a population studied in pher Grapes, B. A. has checked my English . Swedish Lapland. Populations studied in My parents deserve thanks for their support Estonia and Finnish Lapland did not differ during the field work . I also want to thank significantly from the population studied . my wife, Mrs. Kaija Haukioja, B. A. for her patient attitude during my long field work The productivity of different clutch sizes periods . was studied in order to throw light upon the This paper forms part of a larger study determination of clutch size. In "early" supported financially by the Satakunta Fund broods of the Finnish Cultural Foundation, the Leo (hatched not later than the 15th of June) and Regina Weinstein Foundation and the neither the proportion of nests destroyed (Ta- Emil Aaltonen Foundation . ble 5), the hatchability of eggs (Table 6), the mortality of nestlings (Tables 7 and 10) nor the weights of young (Tables 8 and 9) were significantly correlated to the brood size. The nestling period could therefore not be shown Summary to be a critical period from the point of view of productivity . This paper deals with the clutch size deter- Young recovered after leaving the nest were mination of passerine birds by analyzing clutch used as a measurement of the productivity of size material mainly of the Reed Bunting. the brood size. Table 11 gives the numbers The field work concerning the Reed Bunt- of young recaught after the beginning of July ing has been conducted while studying a in relation to brood size. Percentagewise small colour-ringed population in south-western broods produced more recovered birds than Finland in 1966-1969 . 323 nests of Reed large ones . The same phenomenon was observed Bunting were found and over 1000 nestlings for both sexes (Table 12) . Thus the time bet- were ringed during the study. Survival of the ween leaving the nest and the beginning of young was studied by means of intensive July was shown to be a critical period for the netting . production of young. The mean clutch size for completed clutches The most efficient brood size was five, was 5.14 +- 0.05. Fig. 2 gives the change in which was also the most common . The pro- mean clutch size during summer . duction of females was very small in broods Clutches laid by an individual female are with six young. Table 14 gives the probable less variable in the same phase of the breeding brood sizes for clutches of different size and cycle than those of the whole population, the mean productivity for each clutch size. which indicates at least partial genetic deter- The order of productiveness of the clutches mination of the clutch size. Environmental was the same as that of their natural occur- factors, however, also have an effect on the rence. size of clutches laid. Young recovered and productivities for 132 ORNIs FENNICA Vol. 47, 1970 brood sizes for broods hatched after the S e 1 o s t u s : Pajusirkun pesyekoko middle of June are given in Tables 15 and 16. Parents over one year old proved to be Kirjoitus valottaa varpuslintujen pesyekoon more efficient in raising broods than younger määräytymistä pääasiassa pajusirkun pesye- parents (Tables 17 and 18) . kokoaineiston pohjalla . The following conclusions about the genetics Pajusirkkua koskevat maastotyöt on suori- of the clutch size can be made according to tettu tutkittaessa vuosina 1966-1969 väri- the nesting material presented: The clutch rengastettua populaatiota Porissa, Kokemäen- size is genetically determined more probably joen suistoalueella. Kyseisenä aikana löytyi 323 as norms of reaction to certain environmental pajusirkun pesää, joista rengastettiin yli 1000 stimuli than as mere numbers of eggs, the pesäpoikasta . Poikasten myöhempiä vaiheita clutch size inheritance can theoretically be pyrittiin selvittelemään tehokkaalla syyskesäi- passed on by both parents or by females only, sellä verkkopyynnillä. and the heritability of the clutch size is evi- Täysilukuisten pajusirkkupesyeiden muna- dently low. Imperfect knowledge about the määrä on ollut keskimäärin 5.14± 0.05 . Kes- genetics of the clutch size makes it difficult kimääräisessä pesyekoossa tapahtuu vuoden- to interpret study results. aikaisia vaihteluja (kuva 2) . Because in the Reed Bunting the commonest Saman naaraan munimat pesyeet ovat sa- clutch size proved to be the most productive, massa pesimäkierron vaiheessa eri vuosina the survival of females did not decrease kooltaan vähemmän vaihtelevia kuin koko po- although they produced large broods (Table pulaation pesyeet, mikä osoittaa pesyekoon 19), and the productivity of clutches larger olevan ainakin osittain geneettisesti määräyty- than found naturally would evidently have nyt. Kuitenkin myös ympäristötekijät vaikut- been low, the clutch size was supposed to be tavat munittavan pesyeen suuruuteen . determined by individual selection. In order Toisen tai uusintapesyeen koko on yleensä to get more knowledge about this fact data sama tai pienempi kuin ensimmäisen pesyeen concerning the clutch size of some other (taulukko 1) . Yksivuotiaat naaraat munivat passerine species were analyzed. Some predic- keskimäärin pienempiä pesyeitä kuin vanhem- tions based on what might be expected in mat naaraat (taulukko 2) . Eri vuosien välillä individual selection were made as to the on merkitseviä eroja keskimääräisessä pesye- breeding of hole- and open-nesting passerines . koossa (taulukko 3), mikä liittynee muninta- Because these predictions (more symmetrical ajan säätilaan (taulukko 4) . Populaation mu- clutch size distribution, and weight develop- ninnan keskimääräinen alkamisajankohta ke- ment more influenced by brood size in hole- väällä ei ilmeisesti vaikuta keskimääräiseen nesting species) proved to be right, deter- pesyekokoon. mination of clutch size on the basis of indi- Tutkitun populaation pesyekoko on mer- vidual selection seems to be normal for tem- kitsevästi suurempi kuin sveitsiläisessä vertailu- perate zone passerines. Thus the observations aineistossa ja merkitsevästi pienempi kuin made of the Reed Bunting would seem to eräässä Ruotsin Lapin populaatiossa. Sen si- support a more general explanation for clutch jaan Virosta ja Suomen Lapista ilmoitetut pa- size determination. jusirkun pesyekoot eivät poikenneet merkitse- It is therefore possible to explain general västi tutkitun populaation pesyekoosta. features of clutch size of hole- and open- Pesyekoon määräytymisen selvittämiseksi nesting species (the former have larger clutches tutkittiin eri kokoisten pesyeiden tuottoisuut- and more symmetrical clutch size distributions) ta. "Aikaisissa" pesyeissä (ennen 16.6. kuo- as results of individual selection. Studies of riutuneissa) eivät kokonaan tuhoutuneiden pe- both groups are needed for a further exam- sien osuus (taulukko 5), munien kuoriutumis- ination of the problem of clutch size. It is prosentti (taulukko 6), poikaskuolevuus (tau- very important to determine the genetic basis lukot 7 ja 10) eivätkä poikasten painot (tau- of the clutch size. lukot 8 ja 9) olleet merkitsevästi riippuvaisia E. Haukioja: Clutch size of the Reed Bunting 133 pesyekoosta . Pesäpoikaskausi ei täten osoit- tä tehtiin eräitä yksilöllisen valinnan vallitessa tautunut kriittiseksi poikasten selviämisen todennäköisiä ennusteita avo- ja kolopesijöiden kannalta . pesimisestä . Koska nämä ennusteet (kolopesi- Pesästälähdön jälkeen kontrolloituja poika- jöiden symmetrisempi pesyekokojakautuma ja sia käytettiin poikuekoon tuottoisuuden mit- poikuekoosta riippuvaisempi painonkehitys ) tana . Taulukossa 11 esitetään heinäkuun alun osoittautuivat suunnaltaan oikeiksi, näyttää jälkeen kontrolloidut poikaset suhteessa pesye- pesyekoon määräytyminen yksilöllisen valin- kokoon . Pienet poikueet tuottivat prosentuaa- nan perusteella lauhkean ja viileän vyöhyk- lisesti merkitsevästi enemmän kontrolloitujo keen varpuslinnuilla normaalitapaukselta . Tä- poikasia kuin suuret . Sama ilmiö havaittiin ten pajusirkulla havaitut seikat täydentävät ja molemmilla sukupuolilla (taulukko 12) . Kriit- tukevat yleisempää selitystä pesyekoon mää- tiseksi kaudeksi poikasten selviämisen kan- räytymisessä. nalta osoittautui täten pesästälähdön ja heinä- Avo- ja kolopesijöiden pesyekoon yleispiir- kuun alun välinen aika . teet (jälkimmäisillä suurempi) pesyekoko ja lä- Absoluuttisesti tuottoisin poikuekoko oli hempänä normaalijakautumaa oleva pesyeko- viisi, mikä oli myös tavallisin. Kuuden poika- kojakautuma) ovat seikkoja, jotka siis ovat sen poikueissa oli naaraiden tuotto jo hyvin ymmärrettävissä yksilöllisen valinnan perus- vähäistä . Taulukossa 14 on esitettynä eri ko- teella. Molempien ryhmien tutkiminen, mikä koisista munapesyeistä todennäköisesti tulevat avopesijöillä on yleensä työläämpää, on tar- poikuekokojakautumat sekä kunkin pesyekoon peen pesyekoko-ongelman ratkomisessa . Eri- keskimääräinen tuottavuus . Pesyeiden tuot- tyisen tärkeää on pesyekoon geneettisen puo- toisuusjärjestys oli sama kuin niiden runsaus- len selvittäminen . järjestys luonnossa . Kesäkuun puolivälin jälkeen kuoriutuneiden poikueiden kontrolloidut poikaset ja poikuei- den tuottoisuudet on esitettynä taulukoissa 15 References ja 16. Yli yksivuotiaat emot osoittautuivat tehok- ARMSTRONG, E. A. 1954. The behaviour of kaammiksi poikasten kasvattajiksi kuin yksi- birds in continuous daylight . Ibis 96: 1-30. vuotiaat (taulukot 17 ja 18) . BELL, B. D. Maastotutkimuksen perusteella 1967. Polygamy in the Reed voidaan pe- Bunting. Brit Birds 60:218-219 . syekoon genetiikasta tehdä seuraavat päätel- BERNDT, R. & W. WINKEL . 1967. Die Gelege- mät : Pesyekoko määräytynee geneettisesti to- gr6sse des Trauerschnäppers (Ficedula dennäköisimmin reaktionormeina tiettyihin hypoleuca) in Beziehung zu Ort, Zeit, ympäristöärsykkeisiin eikä varsinaisina muna- Biotop and Alter. Die Vogelwelt 88 : 97-136 . määrinä, pesyekoko voi teoriassa periytyä BIRCH, L. C. & P. E. EHRLICH jälkeläisille joko molempien vanhempien kaut- 1967. Evolution- ary history and population biology. Nature, ta tai pelkästään naaraan kautta ja pesyekoon Lond . 214:349-352 . heritabiliteetti on ilmeisesti alhainen . Puutteel- CHARVOZ, P. & P. GEROUDET 1962. Serin cini. linen tietämys pesyekoon periytymisestä vai- U. Glutz von Blotzheim (ed .), Die Brut- keuttaa tutkimustulosten tulkintaa. v6gel der Schweiz, p. 560-562 . - Aar- gauer Tagblatt, . Koska yleisin pesyekoko osoittautui paju- Aarau COCK, sirkulla tuottoisimmaksi, emojen oma selviä- A. G. & J. R. MORTON 1963. Maternal and sex-linked effects on size and con- minen ei alentunut, vaikka ne tuottivat suuria- formation in domestic fowl . Heredity 18 : kin poikueita (taulukko 19) ja lisäksi todet- 337-350 . tuja suurempien pesyeiden tuottoisuus olisi il- CODY, M. L. 1966. A general theory of clutch meisesti ollut alhainen, oletettiin pesyekoon size. Evolution 20:174-184 . määräytyvän yksilöllisen valinnan perusteella . COULSON, J. C. & E. WHITE 1961. An analysis of the Asian lisäselvitykseksi tutkittiin eräiden mui- factors influencing the clutch size of the Kittiwake . Proc. Zool. Soc. London den varpuslintulajien pesyekokotietoja . Näis- 136 :207-217 .

13 4 ORNIS FENNICA Vol. 47, 1970

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