7-he Condor 88~446-455 0 The Cooper Ornithological Society 1986
BODY SIZE, NEST PREDATION, AND REPRODUCTIVE PATTERNS IN BROWN THRASHERS AND OTHER MIMIDS’
MICHAELT. MURPHY~AND ROBERTC. FLEISCHER~ Department of Systematicsand Ecology,Museum of Natural History, Universityof Kansas, Lawrence,KS 66045
Abstract. We describethe breeding biology of Brown Thrashers (Toxostomarufum) in Kansas, and combine this with data from other temperate-zone breeding Mimidae to characterizerepro- ductive patterns in this group. Brown Thrashers produced clutchesof 3 to 6 eggs,but clutchesof 4 predominated. Most pairs raised 2 broods per year. Incubation required between 13 and 14 days, and hatching was usually asynchronous.Though sample size was small, asynchronyappeared to increase in frequency towards the end of the breeding season.Nestlings grew rapidly, and in 10 days or less most pre-fledging growth was completed. Young fledged normally at 11 days of age at 65% of adult weight, but with the tarsi near adult size. Nestlings starved in 27% of nests, but predators were responsiblefor most nest failures. Overall nest successwas 43%. Brown Thrashers are typical of other temperate-zone mimids. Modal clutch sizes are of either 3 or 4 eggsand all speciesare multi-brooded. Mimids from the southwesternUnited States and Mexico lay normally 3 egg clutches, but elsewhere 4 eggsare most common. Incubation length and nestling growth rate vary significantlywith adult weight, but on average, incubation is 3 days shorter and nestlings grow 36% faster than predicted. Relative incubation length and relative fledging weight both declined significantly with adult weight, whereas daily nest mortality rate increased significantly with adult size. Although our data are consistentwith the hypothesis that heavy nest predation has favored rapid nestling growth and completion of development outside of the nest, rapid growth may also function in brood reduction. Present data are insufficient to exclude conclusively either factor in the evolution of rapid development in mimids.
Key words: Broodreduction; growth; hatching asynchrony; Mimidae; nestpredation; Toxostoma.
INTRODUCTION either exploiting unpredictable food supplies, Variability of growth rates and hatching pat- or sufferinghigh rates ofnest predation. Hatch- terns in altricial nestlings have been related ing asynchrony, however, possibly occurs for chiefly to features of their food supply and the other reasons(Richter 1982, Clark and Wilson frequency of nest loss to predators. Growth 1985, Hussell 1985, Mead and Morton 1985). rates determine peak nestling energy demand In this report we describe the breeding bi- (O’Connor 1977, Ricklefs 1984) and time spent ology of Brown Thrashers (Mimidae: Toxos- in the nest, thereby influencing both the par- toma n&m) in eastern Kansas, including the ent’s ability to eliminate starvingyoung through first data on nestling growth. Aspects of their brood reduction (O’Connor 1977), and the reproductive biology have been documented probability that predators will locate and de- in a portion of their range (Erwin 1935), but stroy nestsbefore fledging(Lack 1968, Ricklefs only fragmentary information existsfor Brown 1969a, 1984). Hatching asynchrony results in Thrashers breeding west of the Mississippi size differences among young which has tra- River (Gabrielson 19 12, Johnston 1958). In ditionally been viewed as an adaptation to fa- conjunction with data on hatching and growth cilitate brood reduction (Lack 1954, Ricklefs patterns, and sourcesof nestling mortality in 1965, Howe 1976, Richter 1984). It may also other temperate-zonebreeding mimids, we also shorten exposure time for nest contents, and describe and attempt to identify the selective give the earliest hatching young growth ad- basis for breeding patterns in this group. vantages to increase their probabilities of es- One possible contributor to variability in capinga predation attempt on the nest (Hussell reproduction is body size (Ricklefs 1968, Rahn 1972, Clark and Wilson 198 1). Existing theory et al. 1975, Blueweisset al. 1978, Western and thus predicts the evolution of hatching asyn- Ssemakula 1982, Czlder 1984). Comparative chrony and rapid nestling growth in species breeding studiesmust therefore control for dif- ferencesin size. Comparisonsof allometric (i.e., size-dependent) relations of specific taxa to ’ ’ Received22 November 1985.Final acceptance3 March “average,” empirically derived allometric re- 1986. lations are in fact preferable to single species * Department of Life Sciences,Indiana StateUniversity, comparisons because they are less subject to Terre Haute, IN 47809. 3Hawaiian Evolutionary Biology Program, University error. Our results suggestthat reproductive of Hawaii, 3050 Maile Way, 3 10 Gilmore, Honolulu, HI patterns in mimids exhibit size dependence, 96822. but that a combination of ecological pressures
14461 MIMID REPRODUCTION 441 have probably acted in concert to produce the A group of nests that survived incubation characteristic mimid pattern of rapid nestling was used to measure nestling growth. Most growth and short nest occupancy. nests were visited daily. Nestlings were iden- tified by clipping toenails at the first visit. At the first and all subsequentvisits, nestlingswere weighed to the nearest0.1 g (50 or 100 g Pesola METHODS Scale) and tarsus and eighth primary lengths BROWN THRASHERS measured to the nearest 0.1 mm.’ Adult sizes were obtained from specimensin the KUMNH Field studies were conducted from the end of from eastern Kansas. April through July, 198 1 and 1982 in mod- erately grazed pasture located 6.5 km west of the city of Lawrence, Douglas County, Kansas INTERSPECIFIC STUDIES (38’57’N and 95’19’W). Scattered shrubs and We restricted our analysisto speciesthat breed trees were found throughout the site, but hab- in temperate-zone regions.Our sample includ- itats with a closed canopy comprised lessthan ed all 10 speciesof Mimidae breeding in North 5% of the total area. Virtually all nests were America, and one South American species.Due located within an intensively studied area to varying degrees of completeness, sample measuring about 740 x 540 m (40 ha). sizes for different analysesvaried. We treated Nests were located by observing females in Arizona and southTexas populationsof Curve- transit to either existing nests or those under billed Thrashers(T. cuwirostre)separately since construction. We visited nests every 2 to 3 adult body size, clutch and eggsizes, and ’nest- days until eggswere laid, and then followed ling growth all showed distinct differences. them until fledging of young or destruction of Adult weights were taken from original the nest. Dates of clutch initiation were ob- sourceswhen given. Otherwise,we usedDunn ’s tained either by direct observation or by back- (1984) compilation, or the field records of as- dating from hatching date of clutches.Clutches sociatesto obtain weightsfor adults. Adult tar- observed during egg-layingwere considered to sus lengths were measured (nearest 0.1 .mm) be complete if successivevisits indicated no from 5 male and 5 female specimensfor each change in egg number. Heavily incubated specieswith data on growth of the tarsus(study clutches were also assumed to be complete. skinsfrom the KUMNH). We estimated mean Because eggs were always laid on successive eggweight for each speciesusing the egg mea- days during laying, and because we had no surements given in Bent (1948) and Fraga evidence for egg removal by the brood-para- (1985), and the conversion factors described sitic Brown-headed Cowbird (Molothrus ater), above for Brown Thrashers. This wasjustified we assumedthat clutchesfirst observed during by comparison of calculated eggweight to ac- incubation representedfull clutches.Nests that tual fresh egg weight for Crissal Thrashers (T. fledged at least one nestling were considered dorsale) and Chalk-browed Mockingbirds (M. successful.We corrected nest successfor ex- saturninus) given by Finch (1982) and Fraga posure time using Mayfield’s (196 1) method. (198 5), respectively. In both cases,calculated Additional clutch size data were obtained and observed weightsdiffered by lessthan 1%. from nest records at the University of Kansas Clutch size, incubation and nestling period Museum of Natural History (KUMNH, Law- lengths, weight gain and tarsus growth, and rence, Kansas). For the nest records to be sat- nest successwere taken from original literature isfactory for use, we required that successive sources.We used Bent’s (1948) summaries for visits had been made to each nest that indi- the former three variables only when data were cated no changein eggnumber, or the observer not available from field studies. noted that incubation was in progress. Only Rates of nestling weight gain and increase 38 of 98 nest records satisfied these criteria. in tarsus length were calculated for all species We combined the nest record-card informa- with data using Ricklefs’ (1967) graphical tion with our field data and grouped nestsinto method. Allometric relationships between 15-day periods beginning 15 April to test for body size (or egg size) and reproductive traits seasonalchanges in clutch size. We also mea- in mimids were described by applying least suredeggs for maximum length (L) and breadth squareslinear regressionto double logarithmic (B) in six nests in the field in 198 1 and for 2 1 transformations of each dependent variable clutches previously collected in the same re- versus body weight (or egg weight). Compar- gion and now located at the KUMNH. Volume ison of the mimid relationship to established was calculated as 0.5 l(L x B2; Hoyt 1979), allometric baselines(see below) were made by and then converted to weight by multiplying determining whether the 95% confidence in- by egg density (= 1.09; Manning 1978). tervals enclosing the mimid expression ‘in- 448 MICHAEL T. MURPHY AND ROBERT C. FLEISCHER
TABLE 1. Summary of the average weight(g), and tarsus and eighth primary lengths (mm) for nestling Brown Thrashers in eastern Kansas. Reported values are the mean, with the standard deviation and sample size in parentheses. Hatching is day 1.
Weght TiUSUS Eighth primary
Day 1 5.5 (1.28; 19) 8.3 (0.84; 19) - 2 9.2 (1.50; 18) 10.6 (0.92; 21) 3 13.0 (2.51; 19) 12.5 (1.73; 19) 0.5 (O
TABLE 2. Summary statisticsfor reproductive traits in temperate-zone breeding Mimidae. All weights are in grams, and incubation length in days. Egg weights were taken from Bent (1948) except for Chalk-browed Mockingbirds. Egg weightswere also available for catbirds, Brown Thrashersand CrissalThrashers from field studies.These follow Bent’s data, which is always the first listed under egg weight. Under source listings, KS, TX, and AZ refer to weights and breeding data for populations specificto Kansas, Texas and Arizona. For clutch size and incubation length, means are given outside of parentheses,followed by the standard deviation and sample size in the parentheses.
Species Body weight Clutch size Egg weight Incubation length SOUPX
Dumetella carolinensis 36.2 3.3 (0.75; 22) 3.97 3.8 (0.67; 73) 13.4 (-; 30) 2 3.9 (-; 37) 12.9 (-; -) 3 4.1 (-; 314) 3.95 4 Oreoscoptesmontanus 43.3 3.5 (0.80; 38) 4.47 G.0 (1.3; 9) 5 Mimes polyglottos 48.5 3.9 (0.61; 182) 4.52 12-12.5* 6 M. saturninus 18.7 3.6 (0.76; 14) 6.70 13.5 7 Toxostomarufum 68.8 3.9 (0.66; 50) 5.54 12.6 (0.71; 12) 72.2 4.1 (0.71; 59) 5.82 13.6 (0.55; 5) hKs) T. longirostre 69.9 3.4 (0.80; 7) 5.95 14.0 (0.60; 3) 10 T. curvirostre 84.5 3.8 (0.50; 67) 5.85 14.0 (1.3; 18) 10 (TX) 19.4 3.0 (0.58; 56) 6.27 14 11 W) 2.8 (0.56; 15) 12 W) T. dorsale 62.1 3.0 (-; 14) 5.45, 5.5 14 13,11 T. bendirei 62.2 usually 3 5.19 - 11 T. lecontii 61.9 3.1 (0.68; 22) 5.95 14-20 14,ll T. redivivum 86.4 usually 3 1.52 14 11
* -Incubation period underestimated by I day (see text). Sources: I-Zmmerman 1963; 2-Nickel1 1965; 3-Johnson and Best 1980; 4-Crowell and Rothstein 1981; 5-Reynolds 1981; h-Laskey 1962; 7- Fraga 1985 and Mason 1985; S-Erwin 1935; 9-this study; IO-Fischer 1981; 11-Bent 1948; 12-Ricklefs 1965; 13-Finch 1982; l4-Sheppard 1970.
The two extreme examples of weight gain nificantly related (r = -0.340, y1= 12, P >> are presented in Figure 1. Initial brood size 0.05), but regional variation in clutch size ap- was four young in both nests. In the early- pears to exist. Thrashers from the southwest- seasonnest, all nestlingshatched on the same ern U.S. and Mexico (Toxostoma plus Oreo- day, grew rapidly, approached an asymptotic scoptes)lay fewer eggs per clutch (R = 3.12, weight by about day 10 and fledged on day 11 SD = 0.263, IZ = 4) than mimids from else- (Fig. 1). Their average rate of weight gain was where on the North American continent (K = K = 0.632 (tlGgo= 7.0 days). Nestlings in the 3.77, SD = 0.195, n = 5). The difference is second nest hatched asynchronously towards significant(t = 4.2, df = 7, P < 0.01). Irrespec- the end of the breeding season. Weight gain tive of location, all speciesare double- or oc- was slower (K = 0.396, tlGgo= 11.1 days), and casionally triple-brooded. by days 7 and 8 the third and fourth hatched Eggweight wasa direct function of body size. nestlings,respectively, died apparently of star- Adult body weight accounted for 87% of the vation. The fourth hatched nestling died de- total variation in eggweight (P < 0.00 1; Table spitebeing larger than the remaining birds (Fig. 3). The 95% confidence limits completely en- 1). Nestlings starved in 3 of 11 nests (27.3%), closed the average regression line describing all during the latter half of the nesting season. the relationship between adult weight and egg Overall, 10.8% of nestlingsstarved, and in all weight in passerines(Fig. 2) indicating no sig- casesthey were the last hatched nestling(s)in nificant deviation from the average passerine each nest. relationship. Incubation length was less de- Of 24 nests,43.7% fledged at least one nest- pendent on body size. In general, incubation ling (instantaneous mortality rate = 0.029 1, length in birds correlates directly with adult with a 28-day nest period). Daily mortality body weight (Rahn and Ar 1974) and eggweight rate, M (Ricklefs 1969b), was 2.96%/day. Er- (Rahn et al. 1975, Western and Ssemakula win (1935) did not correct nest successfor ex- 1982). The averagelength of incubation varied posure time, but detected a similar probability only between 13 and 15 days in mimids (Table of success(37.5%). Nest predation accounted 2), and was the longest in the Sage Thrashers for 54% of nest failures in Kansas, followed by (Oreoscoptesmontanus), despite it being the weather (27%) and desertion (9%). The cause secondsmallest species. Incubation length was for four nest failures was undetermined. not significantly correlated with either adult body weight (r = 0.163) or egg weight (r = INTERSPECIFIC COMPARISONS 0.216). Body size and reproduction.All mimids have After excluding Sage Thrashers, however, modal clutch sizesof either 3 or 4 eggs(Table significant relationships were found to exist 2). Clutch size and body weight were not sig- between incubation length and both adult 450 MICHAEL T. MURPHY AND ROBERT C. FLEISCHER
8 I
7-
EWp= ,340BW
6-
,613 EW,,, ’ .436BW 3 1 I I I 1 I I 2 4 6 6 10 12
NESTLING AGE (DAYS) I ’ I I I III FIGURE 1. The pattern of weight change of nestling 30 50 70 90 Brown Thrashersin an early (closedcircle) and a late nest ADULT WEIGHT (g) (open square) in Kansas in 1981. The d present on the growth curves of two nestlingsfrom the late nest indicates FIGURE 2. A log-log plot of the relationship between their weights on the day prior to their disappearancefrom eggweight (EW) and adult body weight (BW) in temperate- the nest. The late nest is displaced one day to the right to zone breeding mimids (EW,). The solid, heavy line rep- facilitate presentation. resentsthis relationship,whereas the dashedline indicates the “average” pattern between eggweight and body weight in passerinebirds (EW,; Rahn et al. 1975). The shaded region enclosesthe 95% confidence interval around the weight and eggweight (P < 0.05; Table 3). In mimid relationship. Numbers 1 through 13, respectively, both cases,the exponents differed significantly indicate Dumetella carolinensis,Oreoscoptes montanus, from zero (P < 0.05), but were also below the Mimus polyglottos,T. bendirei, T. dorsale, T. lecontii, T. exponents relating incubation length to either rufum. T. rufum (Kansas), T. longirostre,T. curvirostre (Texas), T. curvirostre(Arizona), M. saturninus,T. redi- adult weight (b = 0.200, Rahn and Ar 1974) vivum. or eggweight (b = 0.217, Rahn et al. 1975; b = 0.200, Western and Ssemakula 1982) in birds generally. In the former case, the difference The statistics of nestling growth are sum- between the exponent in mimids and other marized in Table 4. Our measure of growth birds was significant (P < 0.05). We cannot rate, t,O_go(Ricklefs 1967), is an inverse mea- explain the relatively long incubation period sure of rate. Hence, large values indicate slow of Sage Thrashers, but suspectthat environ- growth. Rate ofweight gain varied significantly mental factors (e.g., low air temperature, as in with adult body weight (Y= 0.69 l), and asymp- Murphy 1983) lengthened the incubation pe- totic nestling weight (Y = 0.691; P = 0.04, y2= riod. 9). In neither of the equations relating growth Rahn and Ar’s (1974) equation provided the rate to weight (Table 4) did the exponentsdiffer closest fit of observed to expected incubation significantly (P B 0.05) from Ricklefs’ (1968) length of the several possibleprediction equa- empirically derived exponent describing the tions (seeabove). The differencesbetween pre- relationship between growth rate and asymp- dicted and observed length averaged 2.9 days totic nestling weight in altricial birds (b = less than predicted (SD = 0.82, yt = 9). The 0.278). deviation from the predicted incubation length However, the coefficient (Y-intercept) in was significantly correlated with adult body Ricklefs’ equation was 36% higher than the weight (r = 0.909, Y = -7.7 + 5.91[log. values we obtained using either adult or WEIGHT], y1= 9, P < 0.001). This was also asymptotic weights (Fig. 3). The 95% confi- true when Sage Thrashers were excluded (r = dence limits around the regression of rate of 0.917, Y = -5.9 + 4.99[logWEIGHT], P = weight gain on asymptotic nestling weight for 0.003). Hence, large specieshad relatively the mimids (Fig. 3) did not include Ricklefs’ (1968) shortest incubation periods. allometric averagefor birds with altricial young, MIMID REPRODUCTION 451
TABLE 3. Power equationsof the form Y = aXb and statisticsdescribing variation in breeding traits in the Mimidae. The dependent variable (Y) in each regressionis given under Variable, followed by the independent variable (X) in parenthesesas follows: BW = adult weight, EW = egg weight, AW = asymptotic nestling weight, WT = rate of weight gain, and TR = rate of tarsusgrowth. Weights are all in g except for adult body weight under incubation length, which is in kg.
Variable n a b(95% CI) Y(95% CI) x P P
Egg weight (BW) 13 0.436 0.613 5.56 64.0 0.872 0.001 (0.4564.770) (5.34-5.78) Incubation (EW) 8 11.06 0.121 13.62 5.59 0.563 0.035 (0.0 18-0.224) (13.35-13.89) Incubation (BW) 8 17.26 0.087 13.62 0.065 0.605 0.025 (0.019-O. 154) (13.37-13.87) Weight gain (AW) 9 2.90 0.306 9.24 44.1 0.471 0.040 (0.0274.585) (8.63-9.90) Weight gain (BW) 9 3.29 0.253 9.24 59.3 0.477 0.040 (0.022-0.484) (8.63-9.90) Tarsus growth (BW) 6 3.52 0.268 10.30 55.3 0.63 1 0.062 (0.005-0.531) (9.48-l 1.19) Nestling period (WT) I 2.15 0.675 12.10 9.01 0.549 0.059 (0.006-1.344) (11.41-12.84) Nestling period (TR) 6 2.43 0.689 12.12 10.3 0.122 0.036 (0.139-1.239) (11.43-12.85)
Column headmgs starting with n are: sample sire, Y-intercept, regressmn c&icient (95% confidence hmits below), mean of the dependent vanable (95% confidence limits below), mean of the Independent vanable, coefiuent of determination, probability level. indicatingthat nestlingmimids grew faster than R, appeared to vary inversely with adult expected based on size. weight. A test of association between R, and Tarsus growth rate also scaledto adult body adult weight usingKendall ’s coefficient of rank weight with about the same exponent (b = correlation indicated that R, varied signifi- 0.270; Table 3). The relationship only ap- cantly with adult weight (tau = 0.6 1, P < 0.05). proached significance (r = 0.796, P = 0.062, Nestlings of large mimids (i.e., >60 g adult yt = 6). The larger coefficient for tarsusgrowth weight) thus fledged relatively lighter than indicatesrelatively slower growth compared to nestlingsof the small species(< 50 g). This was weight, despite the fact that the young fledged not true of R,, the ratio of asymptotic to adult with their tarsi at about adult size (Table 4). tarsuslength. R, was at or near 1.O in all species Presumably, this reflects the fact that the tarsi by the time of fledging (Table 4). were relatively much closer to adult size at Hatching patterns, starvation, and nest suc- hatching than was weight (e.g., Table 1). cess.Data on hatching asynchrony and the oc- Relative weights at fledging (R,) were all currence of nestling starvation were not avail- fairly low (Table 4; compare to Ricklefs 1968). able for all species(Table 5). Of the 9 species
TABLE 4. Growth rates (K, tl&, asymptotic sizes (A) and the ratios (R) of nestling size at fledgingto adult size for weight (g) and tarsus length (mm) in nestling mimids. Nestling period length is given in days. Values for weight gain are above and outside of parentheses,whereas those for tarsus growth are below and within the parentheses.
Nestling Species Adult size A K t1w R period SOUPX
Dumetella carolinensis 36.2 28.0 0.549 0.11 11 1 (24.0) (23.0) (0.468) (2) (0.96) 1 Oreoscoptesmontanus 40.1 34.1 0.543 0.85 12 2 (30.5) (33.2) (0.468) (2) (1.0) 2 Mimus polyglottos 47.7 39.1 0.452 9.7 0.82 12 3 50.0 31.5 0.492 0.75 4 (33.3) (32.2) (0.452) (Z) (0.97)* 3 M. saturninus 78.1 61.6 0.416 9.2 0.78 12 5 Toxostomalongirostre 67.7 49.9 0.443 9.9 0.73 13 3 (36.5) (35.2) (0.408) (10.8) (0.96)* 3 T. rufum 12.2 47.9 0.512 8.6 0.66 11 6 (35.0) (34.9) (0.444) (9.9) (1.0) 6 T. curvirostre 79.7 55.0 0.384 11.5 0.69 - 84.5 55.6 0.444 9.9 0.65 14 ; (34.5) (33.5) (0.344) (12.8) (0.97)* 3
* Fischer (1983) obtained lower values for R, presumably because of slight differences m our technique for measuring tarsus length. Sources: I-Zimmerman 1963; 2-Killpack 1970; 3-Fischer 1983; 4-Breitwisch et al. 1984; 5-Fraga 1985; 6-this study; 7-Ricklefs 1965. 452 MICHAEL T. MURPHY AND ROBERT C. FLEISCHER
The percentage of nests which successfully fledged young varied from 26% to 70%. To compare speciesfor differences in the rate of nest loss we calculated the average proportion (unweighted) of successful nests for each species,and converted it to average daily nest mortality rate (Ricklefs 1969b). A regression of average daily nest mortality rate (NMR) on the logarithm of adult weight yielded a signif- icant positive relationship (r = 0.727, n = 8, P = 0.04; NMR = -0.069 + O.O559[log- WEIGHT]. To our knowledge, these are the- first data for any passerinefamily demonstrat- ,306 ing a correlation between body size and repro- Rate,,, = 2.9OWeight ductive success.
I I I 8 I 30 40 50 60 DISCUSSION
ASYMPTOTIC WEIGHT lg) Breeding patterns of Brown Thrashers in Ten- nesseeand Kansaswere very similar. The only FIGURE 3. The rate of nestlingweight gain (t,,,,; Rick- apparent differences between Erwin’s ( 193 5) lefs 1967) plotted against asymptotic nestling weight of temperate-zone breeding mimids. The lower, heavy line study and ours were that incubation was sig- describesthe relationship in mimids (Rate,), whereasthe nificantly shorter in Tennessee,and that clutch upper line representsthe “average” pattern for birds with size declined seasonallyin Tennessee but not altricial young (Rate,; Ricklefs 1968). The shaded region in Kansas. The difference in incubation length representsthe 95% confidence region around the mimid relationship. Numbers 1 through 9, respectively, indicate seemed likely to have arisen from method- Dumetella carolinensis,Oreoscoptes montanus, Mimus ological differences in the determination of polyglottos(from Texas), M. polyglottos(from Florida), length. Whereas we counted incubation from Toxostomarufim, T. longirostre,T. curvirostre(Arizona), the end of egg-laying to hatching of the last T. curvirostre(Texas), M. saturninus. egg, Erwin probably counted to the hatching of the first egg.The difference in seasonalvari- for which information on hatchingpatterns was ation in clutch size was probably artifactual found, 7 reported asynchrony to be common, since a definite trend existed for late clutches especiallyin large clutches.Starvation was not to be smaller in Kansas. Small sample size aswell documented. For the seven specieswith rather than intrinsic population differences data, starvation occurred frequently in three, probably account for this discrepancy. was absent in two others, and in both the Gray Breeding patterns among temperate-zone Catbird (Dumetellacarolinensis) and Curve- mimids were also quite uniform. Our empha- billed Thrasher was reported to be common sis on relating reproductive traits to body size in one study yet essentially absent in another clearly showed that much of the observed in- (Table 5). When starvation wasreported, about terspecific variability in certain traits was due 25% of all nests were affected (Table 5). to differences in body size. However, our
TABLE 5. Hatching patterns, observationson nestlingstarvation, and nest successfor mimids breedingin temperate- zone regions.Asyn and Syn refer to asynchronousand synchronoushatching, respectively. Percentages under starvation refer to the percentageof nests with nestlingsthat experienced starvation. Under nest success,the numbers indicate the percentageof nests that fledged at least one nestling. Numbers in parenthesesfollowing data indicate sources.
Speaes Hatch pattern Nestling starvation Nest success
Dumetella carolinensis Awn (1, 4‘ Very low (3) 58 (1X 70 (4), Present (15.1%; 4)* 69 (% 44 (6) Mmus polyglottos Unknown Absent (7) 56 (7) M. saturninus Asyn (8) Present (20.0%; 8) 27 (8) Oreoscoptesmontanus Syn (9) Present (-30%; 9) 45 (9) Toxostomarufum Asyn (10) Present (27.3%; 10) 44 (lo), 37 (11) T. longirostre Syn (7) Absent (7) 26 (12) T. curvirostre Asyn (7) Absent (7) 37 (12) Asyn (13) Present (?; 13) Unknown T. dorsale Asyn (2, 14) Unknown 48 (14) T. lecontii Asyn (15) Unknown Unknown
* The figure of 15. I% refers to percentage of nestlings that starved. A higher value would occur if expressed in relation to number of nests. Sources: I -Johnson and Best 1980; 2-Bent 1948; 3-Johnson and Best 1982; 4-Kendeigh 1942,5-Slack 1976; 6-Best and Stauffer 1980; 7-Fischer 1983; S-Fraga 1985; 9-Reynolds 1981; IO-this study; I I-Erwin 1935; 12-Fischer 1981; l3-Rtcklefs 1965; l4-Finch 1982; 15--Sheppard 1970. MIMID REPRODUCTION 453 method of analysis also highlighted a number predation in mimids favors short nest occu- of unambiguous trends in the data indicating pancy and completion of growth outside of the that ecological factors have significantly influ- nest. Emphasis on leg growth and the attain- enced the evolution of life histories in the ment of functional maturity of the legsof nest- Mimidae. lings ensuresthat early fledging is possible,es- Clutch size, for example, was independent pecially in ground-foraging birds. Chalk- of body size yet did vary with geographicdis- browed Mockingbird nestlings, for example, tribution. All specieswere multibrooded, and can successfullyfledge at 9 days of age if dis- laid normally 3 to 4 eggsper clutch. Mimids turbed by predators, though nest departure of the arid southwestern U.S. and Mexico, normally occurs 3 to 5 days later (Fraga 1985). however, produced smaller clutches than Dark-eyed Juncos (Bunco hyemalis) also ex- species breeding elsewhere. This was evident perienceheavy nestpredation, and exhibit very even within Curve-billed Thrashers (Table 2). similar growth patterns (Smith and Andersen Nestling Curve-billed Thrashers from Arizona 1982; see also Austin and Ricklefs 1977). also showed the slowest relative growth rate We suspect,however, that other factors have of all speciesin the sample (Fig. 3). The small also contributed to the evolution of nestling average clutch size of these species, and the mimid growth patterns. For example, hatching heavy nestling starvation in Curve-billed asynchrony is typical of most mimids, and Thrashers from Arizona (Ricklefs 1965) but along with rapid nestlinggrowth, comprise the not Texas (Fischer 1983), suggestthat the rate essential components of the brood reduction at which offspring can be supplied with food strategy(O ’Connor 1977). This paradigm pro- is limited in desert environments. This may poses that when parents are faced with poor be due either to low habitat productivity (Ro- food suppliesthat are temporally stable, starv- senzweig 1968) or limitation of adult activity ing nestlings should be eliminated rapidly to by thermal stresses(e.g., Calder 1968, Ricklefs avoid feeding young that are destined to die and Hainsworth 1968, Austin 1976, 1978). (O’Connor 1977). Hatching asynchrony pro- The strongestpattern, however, wasthe con- duces size differences that are quickly accen- sistent trend for rapid development of both tuated by rapid growth. In this regard, rapid embryos and nestlings,and the generally short growth is critical becauseit increasesnestling periods of nest occupancy. Incubation length energy demands (Ricklefs 1984) and maxi- did not scaleclosely to body size in our sample, mizes intrabrood competition, which even- due mainly to the long incubation period of tually kills the starving offspring. Patterns of Sage Thrashers. In all species, however, in- nestling starvation that conform to the brood cubation was shorter than expected based on reduction model have been observedin Curve- either adult size or egg weight. Relative incu- billed Thrashers(Ricklefs 1965) Chalk-browed bation length also decreased significantly as Mockingbirds (Fraga 1985) and Brown body size increased, hence, the large species Thrashers (this study). Gabrielson’s (19 12) ob- had the shortestrelative lengthsof incubation. servations on the distribution of food to four Nestling growth rate was dependent on body nestling Brown Thrashers also match predic- size, but nestlings nonetheless grew signifi- tions of the brood reduction model in that the cantly faster than expected. Growth patterns smallest nestling received significantly fewer were such that young fledged at only 60% to feeds than expected by chance (n = 878 feed- 80% of adult weight, but with the tarsi always ings, G = 11.7, df = 3, P < 0.01). near adult size. Relative weight at fledgingalso Thus, rapid nestling growth (and hatching decreased as adult body size increased, indi- asynchrony) in Brown Thrashers, and possibly catingthat the young of the large speciesfledged other mimids, may also function in brood re- at relatively earlier stagesof development than duction. Though nestling starvation is less the offspring of small species. common than nest predation among mimids, These data, and the finding that daily nest the variable occurrenceof starvation may itself mortality rate rose significantly as adult body reflect the temporally and/or spatially variable size increased, suggeststrongly that minimi- nature of food supplies.At present, we cannot zation of the time spent in the nest exposed to identify which, if either, of these models (nest predators as vulnerable eggsor nestlingsis ex- predation vs. brood reduction) is primarily re- tremely important for mimid reproductive sponsiblefor the pattern of rapid embryo and success.In accordance,predation was the ma- nestlinggrowth that we have detected. We feel jor causeof nest failure in all the studies cited it is likely that both have contributed to the in Table 5 (see also Biedenweig 1983), except evolution of nestling mimid growth patterns. for Arizona Curve-billed Thashers (Ricklefs This conclusion, though unsatisfyingfrom the 1965). Our findingsare therefore in agreement standpoint of clean hypothesistesting, perhaps with Fischer’s (1983) proposal that heavy nest more realistically reflectsthe multiple selective 454 MICHAEL T. MURPHY AND ROBERT C. FLEISCHER pressuresimpinging on individual reproduc- BEST,L. B., AND D. F. STAUFFER.1980. Factors affecting tive success.We suspect that many ground- nestingsuccess in riparian bird communities. Condor foraging birds (which nest close to the ground 82:149-158. BIEDENWEG,D. W. 1983. Time and energy budgets of and frequently experience heavy nest preda- the Mockingbird (Mimus polyglottos) during the tion), face similar selection pressures,and ex- breeding season.Auk 100:149-160. hibit natterns similar to those found in the BLUEWEISS,L., H. Fox, V. KUDZMA, D. NAKASHIMA,R. Mimihae. PETERS,AND S. SAMS. 1978. Relationships between body size and some life history parameters.Oecologia Clearly, more data and experimental testsof (Berl.) 37~2.57-272. theseideas are necessary.Information is need- BREITWISCH,R., P. G. MERRITT,AND G. H. WHITESIDES. ed on hatching natterns. nestling growth. and 1984. Whv do Northern Mockingbirds feed fruit to rates of starvarik in all species,i% especially their nestlings?Condor 86:28 l-2f7. for mimids of the desertsof southwesternNorth CALDER,W. A. 1968. The diurnal activity of the road-__ runner, Geococcyxcalifornianus. Condor 70:84-85. America. It would be instructive also to de- CALDER,WM. A., III. 1984. Size, function and life his- termine whether food supplies, hatching pat- tory. Harvard Univ. Press, Cambridge, MA. terns, the frequency of nestling starvation, and CLARK,A. B., ANDD. S. WILSON. 1981. Avian breeding predator activity covary on either an annual adaptations: hatching asynchrony, brood reduction and nest failure. Q. Rev. Biol. 56:2_53-277. or seasonalbasis (Mead and Morton 1985, this CLARK,A. B., AND D. S. WILSON. 1985. The onset of study). Such information may allow discrim- incubation in birds. Am. Nat. 125:603-6 11. ination between the several hypotheses that CROWELL,K. L., AND S. I. ROTHSTEIN.198 1. Clutch sizes have been proposed to account for the evo- and breeding strategiesamong Bermudan and North lution of hatching asynchrony and nestling American passerines.Ibis 123~42-50. DUNN, J. B., III. 1984. Body weights of 686 speciesof growth rates in passerinebirds. North American birds. Western Bird-Banding. Monogr. No. 1. SUMMARY ERWIN,W. G. 1935. Some nesting habits of the Brown Thrasher. J. Tenn. Acad. Sci. 10:179-204. Our analysis of reproduction in Brown FINCH,D. H. 1982. Rejection of cowbird eggsby Crissal Thrashers and other temperate-zone breeding Thrashers. Auk 99~719-724. mimids leads us to conclude that (1) bodv size FISCHER,D. H. 1981. Breeding bioloav of Curve-billed \I I accounts for much of the interspecific vari- Thrashers and Long-billel Thra%ers in southern ability in certain reproductive traits, but that Texas. Condor 82:392-397. FISCHER,D. H. 1983. Growth, development, and food (2) ecological pressureshave selected for dis- habitsof nestlingmimids in south Texas. Wilson Bull. tinct patterns, including rapid development of 95197-105. embryos and nestlings. We suggestthat both FRAGA,R. M. 1985. Host-parasite interactions between frequent loss of neststo predators, and unpre- Chalk-browed Mockingbirds and shiny cowbirds. Or- nithol. Monogr. No. 36, American Ornithologists’ dictable (but stable) food supplies have fa- Union, Washington, DC. vored the evolution of the rapid develop- GABRIELSON,I. N. 1912. A study of the home life of the mental rates that characterize temperate-zone Brown Thrasher, Toxostoma rufum (Linn.). Wilson Mimidae. Hatching asynchrony is also com- Bull. 24:65-94. mon in mimids, and appears to function in HOWE, H. F. 1976. Egg-size, hatching asynchrony, sex and brood reduction in the Common Grackle. Ecol- brood reduction. Further data, and testsof these ogy57:1195-1207. hypothesesare necessary,particularly in desert HOYT, D. F. 1979. Practical methods of estimating vol- species. ume and fresh weight of bird eggs.Auk 96~73-17. HUSSELL,D.J.T. 1972. Factors affecting clutch size in arctic passerines.Ecol. Monogr. 42:3 17-364. ACKNOWLEDGMENTS HUSSELL,D.J.T. 1985. 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