Breeding Phenology and Nestling Morphology of Bald Eagles in the Upper Midwestern United States1

Abstract

Abstract We report on egg-laying and hatching dates (n = 656), and morphological characteristics (n = 642) of nestling Bald Eagles (Haliaeetus leucocephalus) from six study areas in Minnesota and Wisconsin, USA, from 2006 to 2011, 2014, and 2015. Nestling age was estimated from length of the eighth primary feather using sex-specific equations from the literature. Egg-laying advanced from south to north beginning in late January and continuing through mid-April; hatching began in late February and ended in mid-May. Mean annual hatching dates were correlated with the average dates of ice break-up each spring at two ice-monitoring sites (n = 8 yr, r2 = 0.90). We found no significant long-term trend or shift in hatching dates over the 10 yr. Male and female nestlings overlapped in weight, footpad length, bill depth, and culmen and hallux claw chord length; however, by 35 d these features had diverged sufficiently to allow estimation of the sex of nestlings due to the larger size of females (P < 0.05 all contrasts). Our models suggest that most of the nestling traits we measured continued to grow beyond the 70 d for which we had measurements, except for footpad, which reached predicted full size by about 46 d after hatching. Footpad length and bill depth were important traits for determining the sex of nestlings ≥35 d old. We provide a classification tree that uses footpad length and bill depth to estimate the sex of nestlings. Overall accuracy of this classification tree was 91%. Individuals with footpad length ≥132 mm and bill depth ≥29 mm (46% of our sample) had a 0.95 probability of being female and individuals with footpad length <132 mm and bill depth <30 mm (41% of our sample) had a 0.92 probability of being male, making our classification tree unbiased and robust for assigning sex for most nestlings ≥35 d old.