The Condor98~259-27 I 0 The CooperOrnithological Society 1996
DIFFERENTIAL REPRODUCTIVE SUCCESS OF BROWN-HEADED COWBIRDS WITH NORTHERN CARDINALS AND THREE OTHER HOSTS
DAVID M. SCOTT AND ROBERT E. LEMONY Department of Zoology, Universityof WesternOntario London, Ontario, N6A 5B7, Canada
Abstract. To understandlow productionof Brown-headedCowbirds (Molothrus ater ater) by a largehost, Northern Cardinal (Cardinalis cardinalis), we comparedthat production with thatof threesmall hosts: Song Sparrow (Melospiza melodia), Chipping Sparrow (Spizella passerina),and Yellow Warbler (Dendroicapetechia). Cowbirdswere present in only 11 of 63 broodsfledged by cardinalsbut in 62 of 93 broods fledged by other hosts (P < 0.001). Notably, neither the frequency of parasitized nests nor the number of cowbird eggsper parasitized nest varied significantlyamong the hosts. Unusual features of cardinals as hosts accountedfor the low production of cowbirds by cardinals.First, cardinal eggswere about 50% larger than cowbird eggs.Many cowbird eggs, as well as cardinal eggs,disappeared from nests, which remained active. Second, the incu- bation period of cardinals was short, only about 10 h longer than for cowbird eggs.Third, cardinal clutcheswere small (mode = 3 eggs).Many cowbird eggswere laid after incubation had begun. These features combined to produce great differencesin body massesof young cardinals and cowbirds, even when cowbirds hatched first. Differences were accentuated when cowbirds hatched after cardinals. Competition in mixed broods often resulted in underweight cowbirds, which usually died before or soon after fledging. Cowbirds reared without cardinal nestmatesgrew well and usually fledgedand survived well. Finally, inter- brood intervals were much longer for cardinals than for Song and Chipping Sparrows. Cardinalshad the lowest number of successfulbroods per host-pair. Cowbirds thrived when reared by cardinals in broods with only one or no cardinal nestmates.We suggestthat host brood-reduction enables cowbirds to exploit large hosts, particularly when the host-incu- bation period is short. Key words: Cowbird reproductivesuccess; cowbird/host laying synchrony;nesting mor- tality; Brown-headedCowbird Molothrus ater ater; Northern kardinhl Cardinaliscaminalis; Chipping Sparrow Spizella passerina;Song Sparrow Melodia melospiza;Yellow Warbler Dendroicapetechia; Gray Catbird Dumetella carolinensis.
INTRODUCTION (Southern 1958) may rear many cowbirds (unless Much has been written about the effect of the otherwise stated, cowbird refers to M. ater ater). Brown-headed Cowbird (Molothrus ater), a brood Most hosts are smaller and lay smaller eggs parasite, upon the successof its hosts, but rather than cowbirds. In nests of tolerant hosts, young less about the details of its reproductive success cowbirds usually thrive, often to the detriment with different hosts.Passerine species vary great- of their smaller nestmates (Mayfield 1965, Gra- ly in their tolerance of parasitism (Rothstein ham 1989). There are, however, tolerant large 1975). For example, Gray Catbirds (Dumetefla hostswhose eggs are appreciably larger than cow- carolinensis) are intolerant, although often par- bird eggs,including Eastern Meadowlarks (Stur- asitized (Scott 1977), and rarely rear cowbirds nella magna), Red-winged Blackbirds (Agelaius (Friedmann and Kiff 1985). In contrast, tolerant phoeniceus), and Northern Cardinals (Cardinalis speciessuch as Red-eyed Vireos (Vireo olivaceus) cardinalis) (hereafter, cardinal). Nestling cow- birds might be expected to do poorly in com- petition with larger foster-siblings, but, appar- ently, this does not always occur. Nestling cow- birds grow well togetherwith nestling Red-winged LReceived 25 August 1995. Accepted 26 January Blackbirds (Weatherhead 1989, Ortega and Cruz 1996. 2 PresentAddress: Department of Biology,McGill 199 1). On the contrary, we have noted that cow- University, 1205 Dr. Penfield Avenue, Montreal, Que- birds often did not survive well, either as nest- bec, H3A 1Bl , Canada. lings or fledglings, when reared with young car-
PW 260 DAVID M. SCOTT AND ROBERT E. LEMON
TABLE 1. Number of successfulbroods including cowbirds from clutchesbegun by three hosts” during the cowbird laying season.Percentages in brackets.
Number of broods Total broods NOIE Otte TWO Per pair
Northern Cardinal 71 21 (30) 37 (52) 13 (18) 0.89 Song Sparrow 42 7 (17) 25 (60) 10 (24) 1.07 Chipping Sparrow 27 1 (4) 18 (17) 8 (30) 1.26 G = 10.4; df = 4; P < 0.05
* Yellow Warbler omitted becauseof small sample;no warblerwas known to be successfullydouble-brooded.
dinals. At London, Ontario, between 1955 and not made daily but only periodically to deter- 1961, about 80% of Northern Cardinal nests mine hatching times, appropriate times for band- found during the cowbird laying seasonwere par- ing, and dates of fledging.A triple-beam balance, asitized (Scott 1963), yet few cowbirds fledged modified for field use, or Pesola scaleswere used and fewer survived for more than a few days to weigh eggsand nestlings. (Lemon 1957; Scott, unpubl.). Was this low production characteristicnot only HOST AND COWBIRD PRODUCTION of cardinals, but also of the local assemblageof species containing cardinals? To answer this We observed a total of 49 breeding pairs of car- question, we estimated the annual production of dinals from 1955 to 1961. For most pairs, the fledgling cowbirds by three other locally com- complete annual nesting history and cowbird mon accepter species:Song Sparrow (Melospiza production were determined. melodia), Chipping Sparrow (Spizella passeri- To examine cowbird production by other pas- na), and Yellow Warbler (Dendroica petechia). serines, in 1962 and 1963 we trapped and color- First, we present data on reproductive vari- banded at least one member of each of 22 pairs ables of hosts and cowbirds that affect variation of cardinals, 42 pairs of Song Sparrows, 27 pairs in parasitism. Second,annual production ofcow- of Chipping Sparrows, and 9 pairs of Yellow birds by Northern Cardinals and the three afore- Warblers. Also, we watched six pairs of Yellow mentioned species indicates that cardinals are Warblers in years after 1963. To determine the relatively poor hosts. Third, we analyze in detail number of broods and cowbirds produced by the successof cowbird eggsand nestlings in car- thesebirds we visited each pair of cardinals, Song dinal nests. Fourth, we suggestsome factors that Sparrows, and Chipping Sparrows weekly until contribute particularly to the failure of cowbirds we observed a brood (Table 1). Yellow warblers to succeedwith cardinals. Finally, we discussthe were visited less regularly. Once the presenceor conditions under which cowbirds are reared by absence of a fledgling cowbird in a brood had large hosts. been ascertained, we did not visit this host pair for another month, this being the minimum in- terval before another brood could be produced, METHODS barring overlapping nests. As our visits were not This study is based on data collected mostly be- daily, we could have missed broods or fledglings tween 1955 and 1968 from more than 700 car- that died soon after fledging. dinal nests,including about 230 parasitizednests, Our method of estimation in 1962 and 1963 that were observed on the campus of the Uni- was much less precise than in 1955 to 1961. In versity of Western Ontario, London, Ontario, the latter yearswe noted that some cowbirds died and some contiguous areas (maps in Darley et soon after fledging. Such events would have been al. 1971). Data on cardinals before 1962 were missed in 1962 and 1963. Thus, to make our basedon birds, usually color-marked, whosenests data from the two setsof years more comparable, were normally visited daily to mark cardinal and we consider only cowbirds that were seen two or cowbird eggsfor identification and to determine more days after fledging as a measure of pro- such variables as clutch size, incubation, nestling duction per host-pair. Thus, we could have un- periods, incidence of parasitism, hatching suc- derestimated, but not overestimated, the number cess,and so on, From 1963 onwards, visits were of cowbirds produced. COWBIRD REPRODUCTIVE SUCCESS 261
TABLE 2. Apparentinterval (days)between fledging groups: O-20 days and more than 24 days (Table and laying of the first eggof the next observednest or 2). brood. OTHER REPRODUCTIVE VARIABLES OF Days HOSTS AND COWBIRDS c-20 21-24 >24 Median Reproductive successof cowbirds among differ- ChippingSparrow 10 1 0 11 ent host speciesdepends upon the interplay of SongSparrow 9 0 2 14 Cardinal 6 1 15 28 many factors. First, the proportion of nests that are parasitized (incidence) and the number of cowbird eggs per parasitized nest (intensity of parasitism) set the basis of cowbird production. INTERBROOD INTERVALS AND At this point, several factors bearing on the NUMBER OF BROODS amount of interspecific competition within a nest Interspecific variation in interbrood intervals can come into play including the egg or hatchling influence the number of broods produced by dif- massesof cowbird and host, or the synchrony of ferent species during the period when they are hatching of the two species,which dependsupon vulnerable to parasitism. An interbrood interval the two incubation periods. These may overlap separatesthe date of fledging from the date of depending on the degree of synchrony of egg- laying of the first egg in the next nest. It should laying of host and cowbird, which in turn de- not be confused with the five or six days follow- pends in part upon the clutch size of the host ing nest failure that precede laying in a replace- and the day on which incubation begins. Success ment nest. of cowbirds also depends on variations in the We estimated interbrood intervals from the length of the breeding seasonof the host, and the dates of the first observations of two successive length of the interbrood interval. Finally, vari- broods of a pair or, if known, the onset of laying ation in nest size and nest site may influence in the succeedingnest. Fledgling age and by ex- survival rates of nests and their broods. trapolation the onset of laying, could usually be In Table 3 we list values for several reproduc- estimated from size or the behavior of fledglings. tive variables: egg and hatchling mass, clutch For example, fledglingswith stubby tails no lon- size, and incubation time. We provide original ger than a few millimeters, are no older than data for egg and hatchling mass and incubation about a week post-fledging. Fledglings with half- period for cowbirds and cardinals. Information grown tails are about two weeks post-fledging, on the other hostswas derived from Nice (1937) judged by the rate of growth of juvenile rectrices for Song Sparrows, from Walkinshaw (1944, of cardinals (Jarosch1976). To estimate the date 1952) for Chipping Sparrows,and Schrantz(1943) of the beginning of the clutch, we subtracted the for Yellow Warblers. Peck and James(198 7) con- approximate length of the nesting cycle, about tain modal clutch-sizesand descriptions of nests 24 days, from the estimated date of fledging. As and nest sites. these estimates were imprecise, we grouped the We used egg mass as well as hatchling mass, interbrood intervals into two discontinuous as values of the former are more likely to be
TABLE 3. Mean valuesof somerelevant variables of Brown-headedCowbirds and four hosts”;sample sizes in brackets.
Mass (9) Incubation Modal Species Adult w Hatchling time (d) clutch-size Brown-headedCowbird 40 (% 51 (8) 3.2 (10) 2.3 (9) 11.9 (12)c NorthernCardinal 43 4.9 (16) 3.5 (10) 12.3 (16) SongSparrow 22 2.3 (44) 1.8 12.6 (32) : ChippingSparrow 12 1.6 (24) 1.3 11.9 (9) 4 Yellow Warbler 9 1.5 (12) 1.3 11.3 (7) 4 sData on Brown-headedCowbirds and Northern Cardinalsfrom Scott (unpubl.),except adult cowbirdbody-masses are from Ankney and Scott (1980). Other body-massesfrom Dunning (1993). Egg and batchling-massesand incubationtime on other speciesare from Nice (1937). Schrantz (1943) and Walkinshaw(1944 and 1952). Other modal clutch-sizesare from Peckand James(1987). b Cowbird eggsfrom cardinal nests. LIncubation time basedon cowbirdeggs laid on morning of cardinal-clutchcompletion or later. 262 DAVID M. SCOTT AND ROBERT E. LEMON
TABLE 4. Numbers and percentagesof cowbird eggs laid on successivedays of the prelaying,laying, and incubationperiods of four hostsat London,Ontario.
Days in laying and incubationperiods Species Prelaying 1 2 3 4 5 6 and later
Northern Cardinal 20 (14) 13 (9) 44 (31) 35 (25) 17 (12) 7 (5) 6 (4) Gray Catbird 3 (12) 4 (15) 10 (38) 6 (23) 2 (8) 1 (4) 0 (0) Song Sparrow 1 (4) 2 (8) 7 (28) 6 (24) 1 (4) 2 (8) 6 (24) Yellow Warbler 40 (62) 6a (9) 9 (14) 3 (5) 2 (3) 0 (0) 5 (8) ’ ’ Includes4 eggslaid on days I or 2; in Table, two eggsassigned to eachday. accurate and more readily available in the lit- small, we added four birds that had been re- erature. Incubation times for cowbird eggswere moved from nestsin another study (Smith 1969). obtained from eggslaid in cardinal nests on the In each case,we could reasonably have predicted morning of the last cardinal egg or later. Incu- that these nestlings, becauseof their size, would bation times for cardinals were measured from have either fledged or perished in the absenceof the hour of laying of the final egg (ca. 06:30) to nest failure and our interference. Finally, we enu- its hatching. merate the frequencies of broods of different To understand cowbird reproductive success, combinations of cowbird and cardinal nestlings. the degree of synchrony in daily laying between In doing so, we assumed that the brood com- cowbird and host should be known. Cowbird position when found had not changed since eggs laid after host incubation has begun may hatching (Table 8). The sample comprised 105 hatch after host eggshave hatched.We have much parasitized nests; 75% were found before hatch- data on cowbird/cardinal synchrony in laying, ing. Some bias may have been introduced by but rather little on the other three hosts, one of using nests found containing only one or two which, the Yellow Warbler, is unusual in its re- large nestlings. In such nests(n = 8) smaller nest- sponse to cowbird laying. To make our sample lings, particularly cowbirds, might have disap- more representative of cowbird/host synchrony peared. Cardinal nestlings rarely starve to death. in our study area we use original data on cow- Only eight of 195 cardinal nestlings disappeared bird/Gray Catbird egg-laying synchrony. We from successfulnests. Values are presented as present in Table 4 the number of cowbird eggs means + SE. laid on each day of the nesting cycle. Becauseof the nest failures throughout the cycle, the data presented numerically give disproportionate RESULTS weight to the early days of the cycle, but this bias REPRODUCTIVE VARIABLES OF COWBIRDS is not great (Nolan 1978:382). AND HOSTS Laying seasons.Cowbirds begin laying at Lon- don in late April, about a week later than the SUCCESS OF COWBIRDS IN beginning of cardinal laying, about the same time CARDINAL NESTS as Song Sparrows, and about three weeks before We analyzed reproductive successof cowbirds Chipping Sparrows and Yellow Warblers, and in cardinal nestsbut could not do so for the other cease laying by mid-July. Their laying season hosts as adequate data were lacking. For cardi- encompassesthat of Yellow Warblers but ends nals we have three samples.One, comprising 148 well before those of the two sparrows and car- cowbird eggsfound on the day of laying, mea- dinals (Scott, unpubl.). suressuccess between laying and fledging (Table Number of broods.Cardinals and the two spar- 7). The second describes the fates of cowbird rows were frequently successfullydouble-brood- nestlings between hatching and fledging and ed, unlike Yellow Warblers which seemed to be compares them with cardinal nestmates (Table only single-brooded (Table 1). The number of 9). This sample includes the nestlings from the broods per pair varied significantly (P < 0.05) first sample and augments them with observa- among the double-brooded species. Cardinals tions on nestlings hatched from eggsthat, unlike produced the least number of broods per pair those in the first sample, had not been observed and Chipping Sparrows the most. on the day of laying. Because the sample was All secondbroods (n = 13) of cardinals fledged COWBIRD REPRODUCTIVE SUCCESS 263
from clutches initiated when the rate of parasit- small cardinal eggs(3.7 g). Cowbird egg-massis ism had begun to decline in early July and which about twice that of Chipping Sparrows and Yel- terminated by mid-July. Five of these broods low Warblers but only about 30% greater than were seen before August but the remainder did that of Song Sparrows. The range of egg mass of not appear until early August. Thus, the latter cowbirds probably overlaps that of large Song could only have been produced from clutches Sparrow eggs. laid when cowbirds had almost ceasedlaying. In Clutch size. Modal clutch sizesvary from three contrast, 15 of 18 secondbroods of Chipping and for cardinals, four for Chipping Sparrows and Song Sparrows were seen before August. They Yellow Warblers, to five for Song Sparrows(Ta- came from clutches in late June or early July ble 3). As these specieslay daily, the lengths of when cowbirds were still laying regularly. This laying periods vary interspecifically. Thus, the proportion (15:3) is significantly greaterthan that opportunity for cowbirds to closely synchronize (5:8) for cardinals (G = 6.8; P < 0.01). Thus their laying with their hosts is least in cardinal relatively fewer second broods of cardinals were nests and most in nests of Song Sparrows, pro- likely to be parasitized than were those of the vided that the schedule of cowbird laying does sparrows. not vary from host to host. Interbrood intervalsand nestsurvival rates. Dif- Incubation period. The incubation period of ferences in brood frequency could arise from a cowbird eggsis shorter than that of Song Spar- combination of interspecific differencesin inter- rows and cardinals and about equal to those of brood intervals or survival rates of nests. The the two smallest hosts (Table 3). The mean in- apparent interbrood interval was much shorter cubation period of cowbird eggs incubated by for Chipping Sparrows and Song Sparrows than cardinals (286.00 h + 4.54 h) was significantly for cardinals (Table 2). Significantly more inter- shorter than the 295.26 h +- 3.04 h incubation vals for sparrows (pooled values) fell between period ofcardinal eggs(one-tailed t-test, t = 1.77, zero and 20 days whereascardinal intervals usu- df = 26, P < 0.05). ally exceeded24 days (19:2 for sparrowsvs. 6: 15 Synchrony of laying by cowbirds and hosts. for cardinals, G = 16.9; P < 0.001). Cowbirds parasitized nests most frequently on Survival rates of nestsvaried among the hosts. day 2, followed by day 3, of the laying periods Using the Mayfield method of analysis (Mayfield of three hosts,excluding the Yellow Warbler (Ta- 1975), we determined that the daily rate of nest ble 4). With each host, more than 50% of cowbird loss for cardinals (May and June only) during eggswere laid on those two days. Clearly, with incubation and brooding was 87 losses during three hosts, the daily pattern of cowbird laying 1,355 days of observation, a rate of 0.064 nest varied little. Thus, the degree of synchrony var- losses per day. Similar determinations showed ied directly with the clutch size. That is, as car- daily rates of nest loss of 7/l 57 (0.045) for Chip- dinals had the smallestclutch-size, relatively more ping Sparrows, 15/379 (0.040) for Song Spar- cowbird eggs were laid asynchronously in car- rows, and 4/180 (0.022) for Yellow Warblers. dinal nests after incubation of the entire clutch The combined rate of loss of nests of these had begun than in the nests of the other three specieswas 26/716 (0.036), significantly lower hosts. Cowbirds, however, usually failed to syn- than that of cardinal nests (G = 7.5; P < 0.01). chronize their laying with that of Yellow War- Interspee@ variation in massesof adults, eggs, blers because of the behavior of the host. Forty and hatchlings. Cowbirds are strongly sexually of 65 cowbird eggswere laid before day 1 of the dimorphic in body mass; adult females weigh 40 Yellow Warbler laying period; most nests para- g, about 20% less than adult males (Table 3). sitized then were deserted. Of the remaining 25 Both sexes are much heavier than adult Song eggs, 15 were laid on days 1 and 2 pooled, 5 on Sparrows, Chipping Sparrows, and Yellow War- days 3 and 4, and the remainder after day 5. blers. On the other hand, female cowbirds but not males are lighter than cardinals (43 g). No- tably, adult cowbird body masses of the sexes SUCCESS OF COWBIRDS WITH bracket those of adult cardinals, but egg and DIFFERENT HOSTS hatchling massesofcowbirds (3.18 f 0.15 g; 2.34 The proportion of cardinal broods containing + 0.06 g) are about one-third less than those of cowbird fledglings was lo/44 in 1955 to 1961 cardinals (4.88 f 0.07 g; 3.55 f 0.11 g). Large (Table 5). It varied from O/7 in 1960 to 5/6 in cowbird eggs (4.0 g or more) are heavier than 1961. In each other year, only one successful 264 DAVID M. SCOTT AND ROBERT E. LEMON
TABLE 5. Proportion of successfulbroods with fledg- TABLE 7. Fate of 148 cowbird eggsin 76 cardinal ling cowbirds seenwith presumedhosts: Northern Car- nestsand 57 cowbird eggsin 20 Yellow Warbler nests”. dinal, Song Sparrow, Chipping Sparrow, and Yellow Warbler”. I” nestsof Cardi- War- Annual nals biers FEZ Causesof death P;;w&$$f cowbirds N”I”bEX per host- Buried in nest 4 20 of pairs cowbirds pair Nest abandoned before host completed laying 3 21 Northern Cardinal Removal of complete clutch (predation) 46 2 10/44 (12) 1955-1961 49 0.18 Removal of singleeggs, possibly some by 1962-1963 22 l/19 (1) 1 cowbirds 33 0 Song Sparrow Desertion after some eggsremoved 17 2 1962-1963 42 29/45 (31) 0.14 Reached hatching time, but did not hatch 9 1 Miscellaneous (laid late, storms, human Chipping Sparrow interference) 16 2 23/34 (24) 0.89 1962-1963 27 Hatched 20 3 Yellow Warbler a All nestswere found before hostsbegan laying and were followed 1962-1987 15 10/14 (10) 0.67 until fledgingor abandonment. aBased on broodsestimated t” have beeninitiated within the cowbird laying seas”” @a. 20 Apr. to ca. 20 July). Only broodswith cowbirds more than two dayspost-fledging are included. varied significantly among the hosts (Table 6). bIn brackets,numbers of cowbirds.Some broods had m”re than one cowbird. Incidence varied between 68% and 92% of Yel- low Warbler and Chipping Sparrow nests re- spectively (G = 4.4; P > 0.1). The number of brood contained cowbirds. Thus, we do not re- cowbird eggsin parasitized nests varied between gard the low proportion observed in 1962-l 963 1.55 and 2.10 eggsin Chipping Sparrow and car- as being particularly unusual. The difference in dinal nests,respectively (G = 7.0, P > 0.1). Thus, the values for 1955-1961 and 1962-1963 is not given similar amounts of parasitism in early in- significant (lo:34 vs. 1:18, G = 2.6; P < 0.05). cubation, production of cowbirds should not vary Accordingly, for further analysis, we pooled the much among the hosts. If it does mortality rates data from the two sets of years. of cowbird eggsor nestlingsvary among the hosts A pair of cardinals produced about 25% as and/or different hosts produce significantly dif- many fledgling cowbirds as each pair of the other ferent numbers of successfulbroods during the hosts. Only 11 of 63 cardinal broods had cow- cowbird laying season. birds that were seenlater than two days following fledging,in contrastto 62 broods containing cow- BASES OF COWBIRD SUCCESS OR birds in 93 broods of the other hosts (11:52 vs. FAILURE 62:31, G = 38.7; P < 0.001). Survivalfrom laying tojledging. In cardinal nests, This great difference in the production of cow- cowbird-egg mortality was about 85% (Table 7). birds between cardinals and the other hostscan- Most mortality resulted from eggremoval or nest not be attributed to interspecific differences in desertion following loss of part of a clutch. Be- incidence or intensity of parasitism, as neither cause female cowbirds commonly remove eggs
TABLE 6. Incidence and intensity of parasitism on nests of four speciesat London, Ontario between 24 April and 2 Julya.
Incidence No. and percentagesof nestscontaining different number of cowbirdeggs Host % (nests) 0 eggs 1G% 2 eggs 3eggs >3 eggs x number* Northern Cardinal 82 (106) 19 (18) 37 (35) 24 (23) 14 (13) 12 (11) 2.10 eggs Song Sparrow 85 (54) 8 (15) 18 (33) 16 (30) 9 (17) 3 (6) 1.96 Chipping Sparrow 92 (25) 2 (10) 9 (45) 8 (40) 1 (5) 0 (0) 1.55 Yellow Warbler 6 (19) 6 (32) 6 (32) 3 (16) 3 (16) 1 (5) 1.92
* Most cowbird laying occ”rsin this period. b Per parasitizednest. EIncludes five parasitizednests whose exact cOntents were unknown. COWBIRD REPRODUCTIVE SUCCESS 265
TABLE 8. Speciescomposition of broodsin 105 parasitizedcardinal nests.
Broodcomposition: cardinal/cowbird ’ 2/o l/O O/l o/2 l/l l/2 2/l 212 3/I 3/Z 4/l Number of broods 19 12 13 6 14 7 17 7 6 2 2 a Thirty-one neststhat had beenparasitized did not hatchany cowbirdeggs.
(Scott et al. 1992) normal predation could not Cowbirdnestling and fledgling mortality. To always be recognized. Lossesof entire clutches provide a larger sample of nestlings for analysis, were attributed to predation. Cowbirds may have we augmented the preceding sample of 20 nest- removed some single cowbird eggson days when lings (Table 7) with recordsof 4 1 other cowbirds cowbirds were laying (there is no strongevidence hatched in nests found after laying had begun. that cardinals often eject normal cowbird eggs). There were 61 cowbird and 68 cardinal nest- Often some eggs but not the entire clutch dis- mates (Table 9). Excluding nestlings apparently appeared and the remaining eggs were aban- lost to predators,the proportion of cowbirds (27 doned. Some such incidents may have been of 43) that fledged was significantly lower than causedby cowbirds, especially at nests that were the proportion (44 of 49) of cardinals that fledged multiply parasitized. Cowbird-egg removal by (G = 9.8; P < 0.005). There were two groups of cardinals or cowbirds was important accounting nestling cowbirds: one, reared in mixed broods for about 25% of egg loss. with cardinal nestmates, and one, without any Cardinals rarely desertedparasitized nests un- cardinal nestmates,which we designateas mono- less there was obvious interference by cowbirds. specific cowbird broods, regardless of whether Cowbirds often laid before a cardinal laid the there were one or two cowbird nestlings. The two first egg of a clutch. Of 18 such nests, only two groups differed in survival rates, body masses, were abandoned without cardinal laying. In each and duration of nestling life. abandoned nest a cowbird egghad been cracked or broken. TABLE 9. Fate of 6 1 cowbirdnestlings and their 68 Mortality of cowbird eggsin Yellow Warbler cardinal nestmates,and 93 cardinal nestlingsin un- nests was about 95%. The causesof mortality, parasitizednests, observed from the day of hatching. however, differed greatly from those in cardinal nests. Most cowbird eggs(65%) were buried and/ Mixed or abandoned, usually before a first host eggwas broods COW- Cardi- laid. Accordingly predation was less important. birds nals In cardinal nests, 20 cowbird eggshatched and Nestlings 93 22 39 68 eight hatchlings fledged. Thus, reproductive suc- Fate of nestlings cessof cowbirdsfrom laying to fledgingwas 5.4%. 36 lo” 8 19 In Yellow Warbler nests, three of 57 cowbird Nest failed Disappearedor died eggs hatched; each nestling fledged. Thus, the in nest 2 1 15b 5 survival rate from laying to fledging was almost Fledged 55 llc 16 44 identical, 3/57 = 5.3%, to that in cardinal nests. Fate of fledglings Broodcomposition ofparasitized cardinal nests. Not soughtafter As a result of differential egg-mortality, the num- fledging 22 7 1 9 ber of nestlings that hatched was not only fewer Not found two days but often differed in species composition from after fledging 2 0 7 1 Survivedbeyond two the original clutch of cardinal and cowbird eggs days 31 5d 7 34 (Table 8). Thirty percent of broods lacked cow- aTwo nestmatesdisappeared one and threedays after hatching;they birds and 20% lacked cardinals. Broods of one seemto have beentaken by a predatorbefore remaining eggs were taken on the fourth day. cowbird and one or two cardinals were the most b Two nestlings,much underweight,9 and 11 g, at 4 or 5 daysof age werecollected; assumed that they would have died of starvation. common (55%) of all mixed broods. Broods of =Two nestlmgs,27 eachand about 8 days old collected;assumed that they would have wedged. four or more nestlings, which would be unusual d One cowbird wasreared with anothercowbird and a cardinal,these even for parasitized nests, formed the remaining last two weretaken by a predatoron day 9 of the nestlingperiod. Thus, the surviving cowbird becamea solitary fledgling without post-fledging mixed broods. competition. 266 DAVID M. SCOTT AND ROBERT E. LEMON
TABLE 10. Length (days) of nestling life of 70 car- rates, as do nestling Red-winged Blackbirds (Wil- dinals and 23 cowbirds reared by cardinals. liams 1940). Thereafter, we collected three nest- ling cowbirds from monospecific broods in car- Cardinalsalone Mixed broods- dinal nests and 11 nestling cowbirds from nests Two or COW- Fledged Solitary more COW birds of other hosts. These nestlings were eight or nine between nestling nestlings Cardinals birds alOne days old and gonads could be easily recognized 7 and 9 d 1 1 0 by autopsy. Males were significantly heavier than 8 and 9 d 2 5 : : : females (7 males: IS = 3 1.11 + 0.83 g; 7 females: 8andlOd 1 13 0 0 x= 26.10 f 0.84g, t = 4.26,d= 12, P< 0.005). 9andlOd 2 8 14 : 3b 9andlld 0 3 0 We do not know if sexual dimorphism affected survival of nestling cowbirds in mixed broods of 10andlld11 and 12 d 1 0: 3 7 ; cardinals. 11 and 13 d : 0 : : 1 Eflect of laying and hatching synchrony. The 0 0 1 12and13d 0 0 shorter incubation periods of cowbirds relative (1Includes a mixed brood of two cardinalsand one cowbirddescribed by Laskey(1944). to those of cardinals when coupled with syn- b Collectedon tenth day of nestinglife. chronous laying of cowbird and cardinals en- sured that these cowbird eggs hatched first. In Proportionately more cowbirds fledged from nine synchronized cases,where the dates of lay- monospecific broods than from mixed broods ing and hatching were known, each cowbird egg (11:l vs. 16:15, G=6.9; P
most cowbirds (9 of 16) did not fledge until they cowbirds were part of a mixed brood. Then, cow- were at least ten-days-old in contrast to cardi- birds did not always grow well and weighed less nals, most of which (22 of 25, excluding three than cowbirds reared alone. Death of under- that fledged between 9 and 11 days) fledged be- weight cowbirds occurred in mixed broods fore that age (7:9 vs. 22:3, G = 9.3; P < 0.005). throughout nestling life, and apparently into the On the other hand, solitary cowbirds spent longer first few days after fledging. Possibly, cowbirds in the nest than cowbirds in mixed broods. All fledged prematurely when cardinals fledged, as four undisturbed solitary cowbirds remained be- solitary cowbirds fledged at an older age. Thus, tween ten and 13 days in nestsunlike many from cowbirds from mixed broods may be under- mixed broods, but the difference in ratios (4:0 weight, unable to fly well, and be at unusual risk vs. 9:7) is not significant (P = 0.15,Fisher ’s one- of predation. Woodward and Woodward (1979) tailed exact test). However, three solitary cow- noted that eight of 21 fledgling cowbirds disap- birds collected on their tenth day, if undisturbed, peared within two days of fledging. Even some could have remained longer in the nest and thus, cowbirds that hatchedbefore cardinal nest-mates increased the proportion that remained as nest- did not gain weight normally and apparently suf- lings for more than ten days. fered from intrabrood competition. Some cow- bird nestling deaths, apart from lossesto pred- DISCUSSION ators, occurred in large mixed broods, despite Cardinals, per pair, produced annually signifi- prior hatching and close synchronization of lay- cantly fewer fledgling cowbirds than any of the ing. other three hostsstudied, despitesimilar amounts of parasitism among the hosts. Therefore, car- SYNCHRONY OF COWBIRD AND HOST LAYING dinals were not ideal hosts for cowbirds. Reports on synchronization of egg laying of INTERSPECIFIC COMPETITION IN Brown-headed Cowbirds and hostsare few: Hann CARDINAL NESTS (1937), Mayfield (1960), Klaas (1975) Nolan Cardinals were suitable hosts when rearing sol- (1978), Clark and Robertson (198 l), Sealy (1992), itary cowbirds, which attained body masseschar- Wolf (1987) and Marvil and Cruz (1989). These acteristic of cowbird nestlings recorded in many studies and ours show that most cowbird eggs studies of hosts, ranging from much smaller to were laid during the prelaying and laying periods much larger than female cowbirds (Friedmann of the hosts. For most species, other than the 1929, Herrick 1935, Nice 1937, Norris 1947, Yellow Warbler, fewer than 30% of cowbird eggs Weatherhead 1989, Ortega and Cruz 1991). In were laid during the days ending with the laying those studies, body massesof about 30 g seemed of the first host egg (day 1). In Yellow Warbler typical of many cowbirds at fledgling, similar to nests, 70% of cowbird eggswere laid before any most body-masses of cowbirds reared here by warbler egg had been laid. Peak rates of depo- Song Sparrows, Chipping Sparrows, and Yellow sition in other hosts occurred on days 2 and 3 of Warblers. That is, there was no apparent differ- the laying period of the host. The daily rate of ence in body massesof fledging cowbirds reared deposition declined after day 3 of the laying pe- alone by cardinalsand thosereared by other hosts. riod, regardlessof the host’s clutch size. Thus, The sex of many nestling cowbirds was not de- for most hosts the peak days of cowbird depo- termined. Thus, the interpretation of fledgling sition occurred after the host had begun to lay body-masses is difficult becauseof the apparent and was lesslikely to desertbut before incubation existence of sexual dimorphism in body masses. by the host has begun in earnest. As a passerine But Weatherhead (1989) found no significant often begins incubation with her penultimate or sexual dimorphism in body massesof seven-day- even earlier egg (Mayfield 1960) cowbird eggs old cowbird nestlings.The nestling life of solitary laid early will undergo incubation earlier than cowbirds reared by cardinals was usually ten or cowbird eggslaid later. Thus, the advantage of more days, a period commonly reported by the an early start to incubation could offsetthe great- authors cited above. This agreeswith the dura- er probability of survival to hatching if laid later. tion of nestling life reported for many North This is particularly significant in the interaction American icterines (Bent 1958). between cowbirds and cardinals because the Cardinals were often unsuitable hosts when clutch size of cardinals is most commonly three. 268 DAVID M. SCOTT AND ROBERT E. LEMON
Cardinals begin some incubation with the first of any cowbird eggsat hatching from nests that egg (Scott and Lemon, unpubl.). Consequently, had been parasitized. cowbird eggslaid on day 3 or later began incu- Rate of brood production. Cardinals produced bation later than the first eggsofa cardinal clutch. far fewer successfulbroods with cowbirds than Cowbirds that hatched late were at a great dis- the three smaller hosts, although all hosts were advantage. None of five cowbirds, hatched from heavily parasitized. This lower production by eggslaid on days 4 or 5, fledged from otherwise cardinals arose from variation in three factors: successful broods. When cowbird laying was 1) lengths of interbrood intervals, 2) rates of nest closely synchronized with cardinals, cowbirds survival, 3) rates of survival of cowbird eggs. hatched first, by as much as 18 h. However, cow- Interbrood intervals of cardinals were about bird incubation was only about 10 hours shorter twice as long as those for the two sparrows.Yel- than cardinal incubation. Thus, there is insuffi- low Warblers apparently did not attempt more cient time for cowbird hatchlings to attain body than one successful brood. Our values agree masses that could match potential competition closely with interbrood intervals reported else- from a full brood of cardinal nestlings. Some where for cardinals, Song, and Chipping Spar- cowbirds, although hatching first, died presum- rows: 20 days for cardinals (Kinser 1973: 171), ably from starvation induced by interspecific for Song Sparrows 10 days (Nice 1937) and 11 competition. days (Smith and Roff 198 1, Table 2), and 9 days for Chipping Sparrows (Keller 1979). Scott and OTHER FACTORS AFFECTING Lemon (unpubl.), searching deliberately for car- SUITABILITY OF CARDINAL AS HOST dinal nests, determined that intervals following Loss of cowbird nestlings through starvation and 30 broods fledged before 1 July were 17 days for interspecific competition was insufficient to ac- broods of one and 32 days for broods of two or count alone for the low production of cowbirds more nestlings. by cardinals. If all nestling cowbirds that were The rate of nest survival to fledging was lower lost through starvation, either as nestlings or re- for cardinals than for each ofthe other three hosts cent fledglings,had survived, production of cow- studied, which agreeswith estimates of rates of birds would have only doubled. Apparently predation on nestsof thesespecies (Martin 1993). therefore, some major factors that did not op- Martin showed that bush or shrub-nesting spe- erate on the small hosts reduced the successof cieswere often more vulnerable to predation than cowbirds. ground-nesting species, contrary to widespread Removal of cowbird eggs.Cowbird eggsrarely belief. He found that the percentagesof nestslost disappear singly from active nests of small ac- to predators in forest and shrub/grassland hab- cepter hosts (Hann 1937, Hofslund 1957, May- itats for the hostsstudied by us were for cardinals field 1960, and Nolan 1978). We found, how- (54.7 and 53.3 in two studies), Chipping Spar- ever, that single cowbird eggsas well as cardinal rows (41.2), Song Sparrow (36.5), and Yellow eggsfrequently disappearedduring the days when Warbler (34.2). Three of these speciesare shrub cowbirds were laying in cardinal nests. We do or bush nesters but Song Sparrows usually nest not know, however, whether these eggswere be- on the ground or close to it (Peck and James ing removed by cowbirds or cardinals or some 1987, pers. obs.). Our estimate of daily mortality predator. Mayfield (1960) and Mengel and Jen- rates of cardinal nestswas about 0.064 lossesper kinson (1970) suggestedthat female cowbirds, day, almost identical with the daily mortality rate when confronted by host eggsas large or larger of0.062 reported by Filliateret al. (1994). J.N.M. than their own, may err in their selection of an Smith (pers. comm.) noted that daily nest-failure eggfor removal. Rothstein (197 1) found no ejec- rates of Song Sparrows varied annually from tion of artificial cowbird eggsby cardinals in sev- 0.015 to 0.045, similar to our estimate of 0.040. en tests but later (1975) reported one ejection in Estimates of cowbird-egg survival in the nests seven tests. Burhans (pers. comm.) found that of the four hosts did not agree completely with cardinals ejected white-painted cowbird eggsin estimates of cowbird fledglingsseen with the four seven of 15 tests. Regardlessof the cause of re- hosts. Five percent of cowbird eggssurvived to moval, these cowbird egg-lossesare a distinctive fledging in our samples of cowbird eggs from feature of the interaction between cowbirds and nests of cardinals and Yellow Warblers. Yet Yel- cardinals. This removal accountsfor the absence low Warblers produced significantly more cow- COWBIRD REPRODUCTIVE SUCCESS 269 bird fledglingsthan cardinals. It seemslikely that nals, they are good hosts (Weatherhead 1989), our sample of Yellow Warbler nests was biased. despite being slightly larger than cowbirds. Most cowbird eggs(4 1 of 57) were found in early Twenty-seven % of 94 cowbird eggs fledged, a nestsin May and usually were buried or deserted high rate of successcompared with most smaller before incubation. Clark and Robertson (198 1) hosts(Young 1963a) and cardinals. Red-winged found that Yellow Warblers did not often accept Blackbird eggsare about 20% smaller than car- cowbird eggsin early nestsbut did so later. Clear- dinal eggs(4.0 g vs. 4.9 g) and have longer in- ly, most cowbird production must come from cubation periods than cardinals (Young 1963b). late Yellow Warbler nests, of which we had an These differencesmay result in less competition inadequate sample. This sample (n = 16) of cow- in mixed broods of Red-winged Blackbirds than bird eggs, accepted by the host, produced four in mixed broodsof cardinals.Cowbirds were most fledglings,a survival rate of0.25. By comparison, successfulin cardinal nests with greatly reduced 170 cowbird eggsfound undergoing incubation cardinal broods, but no information was pre- by cardinals had a survival rate of 0.07. The sented by Weatherhead to indicate if clutch and probability of cowbird egg successwas 0.20 (40 brood reduction was also important in cowbird eggs) and 0.33 (12 eggs) in Song Sparrow and successin Red-winged Blackbird nests. Chipping Sparrow nests respectively. Song Spar- Variation in cowbird nestling mortality in re- rows rarely desert parasitized nests (Cavalcanti lation to brood size and composition, as shown 1981, Smith and Arcese 1994) and high repro- in cardinal nests, helps to explain how cowbirds ductive successof cowbird eggsis to be expected. can survive in nests of large hosts. Egg removal Values of cowbird-egg successof 0.30 and 0.52 by cowbirds reducesthe host clutch, particularly were reported by Young (1963a) and Smith and when there is multiple parasitism. Thus, intra- Arcese (1994) for Song Sparrows. Smith and brood competition will be low when the joint Arcese(1994: Table 4) studiedcowbirds that were brood has been reduced to a number that can be smaller than Song Sparrows but Young (1963a) readily fed. Furthermore, a cowbird nestling may reported on cowbirdswhich were larger than Song receive more food from a large host than from Sparrows. Less is known about cowbird success a small host. It should therefore grow faster, as with Chipping Sparrows. Young (1963a) report- Wiley (1986) argued in the case of nestlings of ed a successrate of 0.17 (n = 12 eggs)of cowbirds. Shiny Cowbirds (Molothrus bonariensis)in nests As Chipping Sparrows are a widespread host, a of a large host, the Greater Antillean Grackle comprehensive study of parasitism on this spe- (Quiscalus niger). cies is needed. Much multiple parasitism and decreasein host Thus, speciesdifferences in interbrood inter- clutch and brood size occurred in parasitism of vals, rates of nest survival, and cowbird-egg suc- two even larger hosts: Brewer’s Blackbirds (Eu- cesscombined to produce differencesin the an- phagus cyanocephalus)(Furrer 1974, cited by nual frequency of successfulparasitized broods, Friedmann et al. 1977) and Eastern Meadow- and so could account for differences in cowbird larks (Elliott 1978). Egg mass is 4.6 g for the production by different hosts. former speciesand 6.1 g for the latter (Schon- wetter 1983). The incidence of parasitism was COWBIRD SUCCESS WITH OTHER 55% for Brewer’s Blackbirds and 70% for Eastern LARGE HOSTS Meadowlarks. Host-egg reduction was marked Incubation periods of large hosts may be much in Eastern Meadowlark nests. It must also have longer than those of brood parasites(Mason and been considerable in Brewer’s Blackbird nests, Rothstein 1986, Soler 1992) which could hatch as several broods comprised only cowbirds. The several days before host eggs.The parasites can successof cowbirds differed strikingly between grow well enough to offset the potential advan- these hosts.About 33% of cowbird eggsin Brew- tage of larger hatchling massesof the host. How- er’s Blackbird nestsproduced fledglings,but only ever, the incubation periods of some large hosts about 6% of cowbird eggsproduced fledglingsin may not be much longer, and parasite nestlings Eastern Meadowlark nests. Details of these stud- may experience strong competition from host ies do not permit evaluation of the basis for the young (Mason 1986, present study). differential successof cowbirds, but it seemslike- Red-winged Blackbirds are a common large ly that the difference in egg mass could be im- host of Brown-headed Cowbirds. Unlike cardi- portant. Meadowlarks, in general, seem to be 270 DAVID M. SCOTT AND ROBERT E. LEMON unsuitable hosts of cowbirds in North America LITERATURE CITED and in South America (Gochfeld 1979). ANKNEY,C. D., AND D. M. Scorr. 1980. Changesin Host-brood decreasewas clearly implicated in nutrient reserves and diet of breeding Brown- the successof other speciesof cowbirds in the headed Cowbirds. Auk 97:684-696. nests of large hosts. Bronzed Cowbirds (MO/O- BENT, A. C. 1958. Life histories of North American blackbirds, orioles, tanagers,and allies. U.S. Nat. thus aeneus)in south Texas multiply parasitized Mus. Bull. 21 l:l-547. two large hosts and removed or damaged many CARTER,M. D. 1986. The parasitic behavior of the host eggs(Carter 1986). Fourteen nestling cow- Bronzed Cowbird in south Texas. Condor 88: 1l- birds fledgedfrom seven nestsof theselarge hosts, 25. which contained only nine host nestlings. Fraga CAVALCANTI,R. B. 1981. Nest desertion: theory and testsof its adaptive significancein birds. Ph.D.diss., (1985) and Mermoz and Reboreda (1994) also McGill Univ., Montreal, Canada. showed that clutch and brood reduction of large CLARK,K. L., ANDR. J. ROBERTSON.1981. Cowbird hosts allowed fledging of Shiny Cowbirds. parasitism and evolution of antiparasitestrategies The adaptive basis for egg removal by cow- in the Yellow Warbler. Wilson Bull. 93:249-258. DARLEY, J. A., D. M. Scorr, ANDN. K. TAYLOR. 1971. birds has not been satisfactorily explained (Scott Territorial fidelity of catbirds. Can. J. Zool. 49: et al. 1992, Sealy 1992). Nevertheless, removal 1465-1478. of eggs of large hosts clearly benefits cowbirds DUNNING,J. B., JR. 1993. CRC handbook of avian and allows them to exploit large hosts. In par- body masses.CRC Press. Boca Raton. FL. ticular, more host eggswould be lost or damaged E~~rorr,~P.F. 1978. Cowbird parasitism in the Kan- sas tallgrassprairie. Auk 95161-167. if nests were multiply parasitized. FILLIATER.T. S.. R. BREITWISCH.AND P. M. NEALEN. Factors other than host-brood decrease and 1994.’ Predation on Northern Cardinal nests:does short incubation periods of cowbirds may enable choice of nest site matter? Condor 96:761-768. brood parasites to fledge along with young of FRAGA, R. M. 1985. Host-parasite interactions be- tween Chalk-browed Mockingbirds and Shiny large hosts. Recently, Ortega and Cruz (1991) Cowbirds. p. 829-844. In P. A. Buckley, M. S. experimented by adding Brown-headed Cowbird Foster, E. S. Morton, R. S. Ridgely, and F. G. eggs to normal unreduced clutches of Yellow- Buckley [eds.], Neotropical ornithology. Omi- headed Blackbirds (Xunthocephalus xanthoce- tholog,.Mono&. 36. Washington, DC. phalus), a much larger bird than a cowbird. De- FRIEDMANN.H. 1929. The cowbirds. Charles C. Tho- mas, Springfield, IL. spite the increase in clutch size and the pro- FRIEDMANN,H.,L.K.K~~ANDS.I.RO~~~~~EIN. 1977. nounced disparity in sizes of host and cowbird A further contribution to knowledge of the host nestlings, most (seven of eight) cowbirds fledged. relationsofthe parasiticcowbirds. Smithson. Con- Ortega and Cruz (1992) showed that cowbirds nib. Zool. 235:1-75. FRIEDMANN,H., AND L. F. KIFF. 1985. The parasitic had a relatively larger gape than the host males, cowbirds and their hosts. Proc. Western Found. which could enable cowbirds to compete suc- Vert. Zool. 2~226-302. cessfully with larger host-siblings. FURRER,R. K. 1974. Nest site stereotypyand optimal breeding strategy in a population of Brewer’s ACKNOWLEDGMENTS Blackbirds(Euphagus cyanocephalus). Ph.D.diss., Univ. of Ziirich. Zurich. Switzerland. Much of the data presented was collected by J. A. GOCHFELD, M. 1979. Beggingby nestlingShiny Cow- Darley, S. Geil, P. C. Smith, N. K. Taylor, and E. 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