Portland State University PDXScholar Biology Faculty Publications and Presentations Biology 11-1986 Body Size, Nest Predation, and Reproductive Patterns in Brown Thrashers and Other Mimids Michael T. Murphy Portland State University, [email protected] Robert C. Fleischer Follow this and additional works at: https://pdxscholar.library.pdx.edu/bio_fac Part of the Biology Commons, and the Ornithology Commons Let us know how access to this document benefits ou.y Citation Details Murphy, M. T., & Fleischer, R. C. (1986). Body size, nest predation, and reproductive patterns in Brown Thrashers and other mimids. Condor, 446-455. This Article is brought to you for free and open access. It has been accepted for inclusion in Biology Faculty Publications and Presentations by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. The Condor88:446-455 ? The CooperOrnithological Society 1986 BODY SIZE, NEST PREDATION, AND REPRODUCTIVEPATTERNS IN BROWN THRASHERS AND OTHER MIMIDS' MICHAELT. MURPHY2 AND ROBERTC. FLEISCHER3 Departmentof Systematicsand Ecology,Museum of NaturalHistory, Universityof Kansas,Lawrence, KS 66045 Abstract. We describethe breedingbiology of BrownThrashers (Toxostoma rufum) in Kansas, and combine this with data from other temperate-zonebreeding Mimidae to characterizerepro- ductive patternsin this group.Brown Thrashers produced clutches of 3 to 6 eggs, but clutchesof 4 predominated.Most pairsraised 2 broodsper year.Incubation required between 13 and 14 days, and hatchingwas usually asynchronous.Though sample size was small, asynchronyappeared to increasein frequencytowards the end of the breedingseason. Nestlings grew rapidly,and in 10 days or less most pre-fledginggrowth was completed. Young fledgednormally at 11 days of age at 65% of aduli weight, but with the tarsi near adult size. Nestlings starved in 27% of nests, but predatorswere responsiblefor most nest failures.Overall nest success was 43%. BrownThrashers are typical of other temperate-zonemimids. Modal clutch sizes are of either 3 or 4 eggs and all species are multi-brooded.Mimids from the southwesternUnited States and Mexico lay normally 3 egg clutches, but elsewhere4 eggs are most common. Incubationlength and nestlinggrowth 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 fledgingweight both declined significantlywith adult weight, whereas daily nest mortality rate increasedsignificantly with adult size. Althoughour data are consistent with the hypothesisthat heavy nest predationhas favored rapid nestlinggrowth and completion of development outside of the nest, rapid growth may also function in brood reduction.Present data are insufficientto exclude conclusivelyeither factor in the evolution of rapid developmentin mimids. Key words: Broodreduction; growth; hatching asynchrony; Mimidae; nest predation; Toxostoma. INTRODUCTION either exploiting unpredictablefood supplies, Variability of growth rates and hatching pat- or sufferinghigh ratesof nest predation.Hatch- terns in altricial nestlings have been related ing asynchrony,however, possibly occurs for chiefly to featuresof their food supply and the other reasons(Richter 1982, Clarkand 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'Connor1977, Ricklefs 1984) and time spent ology of Brown Thrashers(Mimidae: Toxos- in the nest, thereby influencingboth the par- toma rufum)in eastern Kansas, including the ent'sability to eliminatestarving young through first data on nestling growth. Aspects of their brood reduction (O'Connor 1977), and the reproductivebiology have been documented probabilitythat predatorswill locate and de- in a portion of their range (Erwin 1935), but stroynests beforefledging (Lack 1968, Ricklefs only fragmentaryinformation exists for Brown 1969a, 1984). Hatching asynchronyresults in Thrashers breeding west of the Mississippi size differencesamong young which has tra- River (Gabrielson 1912, Johnston 1958). In ditionally been viewed as an adaptationto fa- conjunctionwith data on hatchingand growth cilitate brood reduction (Lack 1954, Ricklefs patterns, and sources of nestling mortality in 1965, Howe 1976, Richter 1984). It may also othertemperate-zone breeding 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 breedingpatterns in this group. vantages to increase their probabilities of es- One possible contributor to variability in capinga predationattempt on the nest (Hussell reproductionis body size (Ricklefs 1968, Rahn 1972, Clarkand Wilson 1981). Existingtheory et al. 1975, Blueweisset al. 1978, Westernand thus predicts the evolution of hatching asyn- Ssemakula 1982, Calder 1984). Comparative chrony and rapid nestling growth in species breedingstudies must thereforecontrol for dif- ferencesin size. Comparisonsof allometric(i.e., size-dependent) relations of specific taxa to Received22 November1985. Final acceptance 3 March "average,"empirically derived allometric re- 1986. lations are in fact preferableto single species 2 Departmentof Life Sciences,Indiana State University, comparisons because they are less subject to TerreHaute, IN 47809. error. Our results suggest that reproductive 3 Hawaiian EvolutionaryBiology Program,University of Hawaii, 3050 Maile Way, 310 Gilmore, Honolulu, HI patterns in mimids exhibit size dependence, 96822. but that a combination of ecological pressures [446] MIMID REPRODUCTION 447 have probablyacted in concert to produce the A group of nests that survived incubation characteristicmimid 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 firstand all subsequentvisits, nestlingswere weighedto 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 obtainedfrom specimensin the KUMNH Field studies were conducted from the end of from eastern Kansas. April through July, 1981 and 1982 in mod- erately grazed pasture located 6.5 km west of the city of Lawrence,Douglas County, Kansas INTERSPECIFICSTUDIES (38057'N and 95019'W).Scattered shrubs and We restrictedour analysisto species that breed trees were found throughoutthe site, but hab- in temperate-zoneregions. Our sample includ- itats with a closed canopy comprised less than ed all 10 species of Mimidaebreeding in North 5% of the total area. Virtually all nests were America,and one SouthAmerican species. Due located within an intensively studied area to varying degrees of completeness, sample measuringabout 740 x 540 m (40 ha). sizes for differentanalyses varied. We treated Nests were located by observing females in Arizonaand south Texaspopulations of Curve- transit to either existing nests or those under billedThrashers (T. curvirostre)separately since construction. We visited nests every 2 to 3 adult body size, clutch and egg sizes, and nest- days until eggs were laid, and then followed ling growth all showed distinct differences. them until fledgingof young or destructionof Adult weights were taken from original the nest. Dates of clutch initiation were ob- sourceswhen given. Otherwise,we used Durin's tained either by direct observationor by back- (1984) compilation, or the field records of as- datingfrom hatchingdate of clutches.Clutches sociates to obtain weightsfor adults. Adult tar- observed duringegg-laying were consideredto sus lengths were measured (nearest 0.1 mm) be complete if successive visits indicated no from 5 male and 5 female specimens for each change in egg number. Heavily incubated species with data on growthof the tarsus(study clutches were also assumed to be complete. skins from the KUMNH). We estimatedmean Because eggs were always laid on successive egg weight for each species using 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-headedCowbird (Molothrus ater), above for Brown Thrashers.This was justified we assumed that clutchesfirst observed during by comparison of calculatedegg weight to ac- incubationrepresented fuli clutches.Nests that tual fresh egg weight for CrissalThrashers (T. fledged at least one nestling were considered dorsale)and Chalk-browedMockingbirds (M. successful. We corrected nest success for ex- saturninus)given by Finch (1982) and Fraga posure time using Mayfield's(1961) method. (1985), respectively.In both cases, calculated Additional clutch size data were obtained and observedweights differed by less than 1%. from nest recordsat 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 recordsto be sat- nest successwere takenfrom originalliterature isfactory for use, we required that successive sources.We used Bent's (1948) summariesfor visits had been made to each nest that indi- the formerthree variables only when data were cated no changein egg number,or the observer not available from field studies. noted that incubation