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