Fat Stores, Plumage Morphs, and Sex of Migrant White-throated Sparrows

Abstract

Abstract - A between-season comparison of lipid stores carried by migrant White-throated Sparrows, as determined by deuterium dilution and visible fat class. I compared qualitative and quantitative measures of fat stores for 144 migrant White-throated Sparrows (Zonotrichia albicolis) in fall and spring. Qualitative measures were visible fat classes and a morphometric index (100 * mass / wind chord). I calculated lipid index (fat mass/fat-free dry mass) for the quantitative measure of fat stores using the deuterium dilution technique. Spring migrants had significantly higher fat class (2.0 ± 0.1 vs. 1.0 ± 0.2), lipid index (1.06 ± 0.03 vs. 0.46 ± 0.03), and condition index (36.71 ± 0.30 vs. 34.27 ± 0.24) than fall migrants. Spring migrants also had higher lipid index values for a given fat class than did fall migrants. Spring linear regression slopes of the relationship between fat class and lipid index were steeper than fall regression slopes. Since fat classes do not consistently estimate fat storage between seasons, I advise against comparing fat classes between different seasons. Differences in areas of fat deposition or fat consumption between seasons may account for seasonal differences in lipid index values for individual fat classes. Abstract - Is wing chord a useful criterion to sex White-throated Sparrows? Determining the sex of White-throated Sparrows (Zonotrichia albicolis) outside of the breeding season can be difficult. Males tend to be larger than females and therefore wing chord length has been used to sex some individuals. However, overlap in wing chord length between males and females means that some individuals may be incorrectly sexed, while the sex of other individuals cannot be determined using wing chord criteria. I determined the sex of 159 White-throated Sparrows using molecular techniques and then examined the distribution of wing chord lengths for both sexes. In this study, all individuals with wing chord lengths ? 73.5 mm were males and all individuals with wing chord lengths ? 66.5 mm were females. Using Pyle's (1997) wing chord length criteria for sexing White-throated Sparrows (females < 69 mm, males > 72 mm), only 3% of males and 1% of females were mis-identified; however, only 42% of males and 16% of females could be safely separated from the opposite sex based on Pyle's wing chord length criteria. These data suggest that studies of White-throated Sparrows, and perhaps many other passerine species that require accurate sexing of individuals during the nonbreeding season, may necessitate the use of molecular sexing techniques. Abstract - Does lipid index differ between plumage and sex classes of migrant White-throated Sparrows? White-throated Sparrows (Zonotrichia albicollis) display a plumage and behavioral polymorphism that is genetically determined. Birds with white colored crown stripes are generally more aggressive, territorial, sing more, seek more extra-pair copulations and contribute less parental care toward offspring than birds with tan colored crown stripes. Negative assortative mating by the plumage morphs maintains this polymorphism. However, there is a higher observed frequency of white-striped (WS) male x tan-striped (TS) female pairs than TS male x WS female pairs on the breeding grounds. I proposed that differences in lipid reserves carried during fall and spring migrations might be related to the unequal frequencies of pair types observed on the breeding grounds. I looked for differences in lipid index between plumage and sex classes. In the fall, I found that TS females had lower lipid index than other plumage and sex classes for White-throated Sparrows captured at Braddock Bay on the southern shore of Lake Ontario. I also looked for evidence within each sex of unequal proportions of plumage classes in fall and spring migrants. Based on my data, TS females were significantly more common than WS females in the fall, but not in spring. Using wing chord criteria to determine sex, I also analyzed spring banding data between 1992 and 1999 from Braddock Bay Bird Observatory (Greece, NY) and found significantly more WS than TS males and significantly more TS than WS females. However, I also found that for each sex, WS birds had significantly larger wing chord lengths than TS birds. Thus, I question any within sex plumage morph frequency data that use wing chord to determine sex.