Studies of Black-billed Magpies Pica pica hudsonia in Idaho, USA showed greater reproductive success in early nests. Overall, offspring from early nests were larger in body size and had longer bill lengths, both characteristics associated with social dominance in the non-breeding flock. Preliminary determination of the sex of nestlings shows: 1) a disproportionate number of male nestlings in early nests; 2) both male and female nestlings are heavier in early nests than later nests; and 3) in contrast, bill lengths are greater in early males but early and late females have the same bill length. Females in our study area do not use their bills in social displays. The anatomical traits of males from early nests are associated with higher social status and thus higher eventual nesting success. This indicates that even as nestlings, Magpies acquire physical characteristics related to social status and eventual probability of breeding. We are currently unable to determine if this is due to differential parental investment throughout the nesting period, or whether it is due to differing environmental effects within early and late nesting periods.
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