Sexual Size Dimorphism and Survival of Male and Female Blackbirds (Icteridae)
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SEXUAL SIZE DIMORPHISM AND SURVIVAL OF MALE AND FEMALE BLACKBIRDS (ICTERIDAE) WILLIAM A. SEARCY AND KEN YASUKAWA 1 Rockefeller University Field Research Center, Tyrrel Road, Millbrook, New York 12545 USA ABSTRACT.---In a between-speciescomparison of icterids, male survival decreasesrelative to female survival as the degreeof sexual size dimorphism increases.This result is consistentwith the hypothesisthat male size is limited by survival selectionin icterids;the degreeof size dimor- phism is known to correlate with the degree of polygyny in icterids, however, so the decreased relative survival of males in dimorphicspecies may be causedby somecorrelate of polygynyother than large size. Survival estimatesbased on recordsof recoveryof dead birds showpositive correlations between male size and male survival and between female size and female survival, but survival estimates based on recaptures of live birds fail to show such relationships. Received 30 September 1980, accepted 21 February 1981. DARWIN (1871) noted that "amongst birds there often exists a close relation be- tween polygamy and the development of strongly-marked sexual differences." Sexual size dimorphism is one of the most noticeable and easily quantifiable of such sexual differences.Since Darwin's time, a correlationbetween polygyny and the degreeof size dimorphism has been shown in many groups of birds (Selander 1972). This correlation has been particularly well demonstratedfor the blackbird speciesof the new world family Icteridae (Selander 1958, 1972; Orians 1961). The correlation between polygyny and size dimorphism implies that size dimorphism evolved largely in responseto sexual selection,because variability in mating successis greater, and thus sexual selectionis stronger, in polygynousspecies than in monogamousones. If males have evolved larger size than females in responseto sexual selection, then it is reasonableto ask what stabilizing forcesprevent further increasesin size. A logical hypothesisis that increased size dimorphism entails a cost of decreased male survival (Selander 1965). Thus, size would stabilize at an optimum where the cost of decreasedsurvival matches the benefit of increased mating success.One method of testingthis hypothesisis to comparethe survival of large and small males in dimorphic species.This test has been applied to several speciesof icterids, with mixed results.Lower survival of large males has been shown in Brown-headed' Cowbirds (Molothrus ater) (Johnsonet al. 1980) and Common Grackles (Quiscalus quiscula) (Baker and Fox 1979). On the other hand, studies of Red-winged Black- birds (Agelaius phoeniceus) and Yellow-headed Blackbirds (Xanthocephalusxan- thocephalus)have failed to demonstratelower survival of large males (Searcy 1979a, 1979b; Johnsonet al. 1980). A secondmethod of testingthe hypothesisthat increasedsize dimorphismlowers malesurvival is to comparemale and femalesurvival in dimorphicspecies. In thehigh- ly dimorphic Great-tailed Grackle (Quiscalusmexicanus), males have been shown to experiencelower survival than females during the winter (Selander 1965). A more complete test can be made by comparing male and female survival in a number of specieswith varying degreesof sizedimorphism. The stabilizingselection hypothesis predicts that the ratio of male size to female size should be negatively correlated Presentaddress: Department of Biology,Beloit College,Beloit, Wisconsin53511 USA. 457 The Auk 98:457-465. July 1981 458 SEARCY•.ND YAStJK•tW•t [Auk, Vol. 98 with the ratio of male survival to female survival. An analysis of size dimorphism and survival in severalspecies of grousesupports this prediction. The ratio of num- bers of adult males to numbersof adult femalesdeclines as the ratio of male weight to female weight increasesamong grouse species (Wittenberger 1978). In the present paper, we provide a further test of this prediction by comparing male and female survival and degree of size dimorphism in a number of speciesof icterids. Although we predict that increasedsize dimorphism entails a cost of decreased male survival, there is some apparently conflictingevidence. Interspecificcompar- isons within many taxonomic groups have shown that survival rate increasesas body size increases.For example, maximum lifespansincrease with body weight in interspecificcomparisons of passerines,nonpasserines (S. Linstedt, cited in Calder 1974), and mammals (Gunther and Guerra 1955). If this relationshipwere to hold within speciesas well as betweenspecies, then one would expect that male survival would increaserelative to female survival as males becomelarger relative to females. The relationshipbetween size and survival need not be the same within and between species,however. We proposeinstead that there is an optimum size in terms of survival for each speciesand that, as this optimum increasesbetween species,sur- vival increases.Within a species,survival of individualsdecreases as their sizeeither increasesor decreasesrelative to the optimum. Thus, the more sexualselection has pushedmale size above the optimum, the lower shouldmale survival be relative to female survival. At the same time, in between-speciescomparisons, male survival should increase with male size and female survival should increase with female size. M•ATERIALS AND METHODS Survival rates were calculated from banding return data obtained from the Bird Banding Laboratory of the United States Fish and Wildlife Service. Return recordswere of two general types:(1) recaptures-- records of recaptures of previously banded birds caught alive, and (2) recoveries--records of recoveries of previously banded birds found dead. Recapture and recovery data were analyzed separately using a methodsimilar to that of Fankhauser(1967, 1971). We obtainedrecapture and recoverydata for all icteridsbanded after the postjuvenalmolt. We limited analysisto returns of birds banded before 1975. Becausereturn recordswere completethrough August 1978, a bird had a minimum of 32/5yr in which to be recapturedor recovered.In the analysisof recapture data, we used only recordsof birds recapturedat least 6 monthsafter banding, becauseit appearedthat, for all species,few banders bothered to report recapturesof birds banded in the same season.In the analysisof the recoverydata, we includedbirds recoveredwithin 6 monthsof banding. For both types of data, we calculateda survival rate for a given sex of a given specieswhenever we had at least 15 return recordsfor that species-sexclass. Recapturerecords gave an estimateof the relative proportionsof the banded populationsstill alive in each time period after banding. For recoverydata, we requestedonly recordsof birds killed by human agency,so these records gave an estimateof the relative proportionof the banded populationsthat would have continued to survive except for human interference. We obtained recovery records for all icterids that had been shot (constitutingthe great majority of recoveries),trapped, poisoned,collected as speci- mens,killed in controlor bandingoperations, killed by strikingwires or towers,or killed by beingstruck by motor vehicles,trains, or airplanes. Recapturedata were assembledfor analysisin the form of the number recapturedbetween 0.5 and 1.5 yr after banding, 1.5 and 2.5 yr after banding, etc. Recovery data were assembledin the form of the number recovered0-1 yr after banding, 1-2 yr after banding, etc. For both typesof data, annual survival rates for each species-sexclass were calculated using the formula: %survival = 100 • y. N• where N• is the number encounteredin the ith year and x is the last year a bird of a given species-sex class was encountered. July1981] SizeDimorphism and Survival 459 It is important to note that thesesurvival estimatesare not influencedby possibledifferences between males and females of the same speciesin the probability of recapture or recovery. It is undoubtedly true that, for most species,the chancethat a given female will be encounteredby a given samplingmethod is differentfrom the chancethat a given male will be encountered.For this reason,sex-ratio data are rarely trustworthy. Becauseour survival estimatesare calculatedusing only within-sex data, however, and the bias due to differential encounterability appears in both the numerator and denominator of each survival rate estimate(and thus always cancelsout), the differential vulnerability of the sexesto sampling has no effect on the survival estimates. We calculatedsurvival rates separatelyfor Bullock's Oriole (Icterus galbula bullockii) and Baltimore Oriole (Icterus galbula galbula), which have recently been lumped as subspeciesof the Northern Oriole. We felt justifiedin this separationbecause the subspeciesare separatepopulations having different sizes and survival rates. We used wing length as our measure of size because it correlates well with fat-free weight within speciesof birds (Connell et al. 1960) but is much less seasonallyvariable than live weight (e.g. Searcy 1979b). Average wing lengths used in interspecificcomparisons were obtained from Ridgway (1902). In caseswhere Ridgway gives measurementsfor more than one subspecies,we used measurementsof the nominate subspecies. Ridgway also provides two other measurementsthat have been used previouslyas indicesof size in birds: tarsus length and total length. For the 10 speciesof icterids consideredin this paper (20 species-sex classes),there are very strongcorrelations between thesetwo alternative measuresand wing length (for tarsus length vs. wing length r = 0.969, P < 0.01; for