Steroid hormones play critical organizational and activational roles during vertebrate development, impacting everything from sexual differentiation to metabolic activity. For oviparous species such as birds, these hormones are transferred from female to egg during follicle maturation, and differences in relative and absolute concentrations of the steroid hormones may reflect differences in life history, developmental, and ecological conditions. Prior work on yolk steroid hormones has focused on a handful of candidate hormones (e.g., testosterone, androstenedione, and corticosterone), but we used high-performance liquid chromatography with tandem mass spectroscopy (LC–MS–MS) to quantify 27 yolk steroids from the eggs of seven shrubland bird species (American Robin, Turdus migratorius Linnaeus, 1766; Brown-headed Cowbird, Molothrus ater (Boddaert, 1783); Brown Thrasher, Toxostoma rufum (Linnaeus, 1758); Eastern Towhee, Pipilo erythrophthalmus (Linnaeus, 1758); Field Sparrow, Spizella pusilla (A. Wilson, 1810); Gray Catbird, Dumetella carolinensis (Linnaeus, 1766); Northern Cardinal, Cardinalis cardinalis (Linnaeus, 1758)). In addition to comparing steroid profiles across species, we conducted exploratory analyses to determine how the hormones clustered using a principal component (PC) approach and if PCs were correlated with aspects of egg resources (relative egg size, proportion yolk), life-history traits (embryonic and nestling development speed), and nest-predation risk (daily survival rate (DSR)). We documented substantial interspecific variation in both absolute and proportional endocrine profiles. PCAs indicated that glucocorticoids generally clustered together (PC1), but other classes of steroids did not. PC2 and PC3 strongly covaried with egg resources, DSR, and development speed, suggesting that they reflect adaptive patterns of maternal hormone deposition.
Nesting strategy shapes territorial aggression but not testosterone: a comparative approach in female and male birds
