The relationship between sexual size dimorphism of wing length and protandry in songbirds

Background/Question/Methods

In migratory songbirds, males commonly arrive at the breeding grounds earlier than females (“protandry”) and older birds before younger ones. Theoretical models show that protandry can be an adaptive strategy to maximize extra-pair mating opportunities by males. Empirical and theoretical work suggests that the primary proximate cause of protandry is differential departure timing, but differential migration speed, which is influenced by wing length, could also play a role. We used a long-term migration monitoring dataset to test the hypothesis that sexual size dimorphism of wing length (WSSD) is related to protandry. We used species-specific linear mixed models of wing length and capture date to estimate yearly values of WSSD and protandry, respectively. Separate models were fit for each age class; ‘adults’ experienced at least one prior spring migration and ‘first time breeders’ are undertaking their first one at the time of capture. In the models, sex was specified as the fixed effect and random intercepts for year were included. Mean spring temperature was included as a year covariate in the capture date models because climate may influence migration timing. Phylogenetic least squares models were used to determine whether protandry depends on WSSD while controlling for phylogenetic similarity among species.

Results/Conclusions

A total of 48,317 sexually dichromatic birds across 25 years were used in the analyses, encompassing 11 species for the adult age class and 16 species for first-time breeders. We found that WSSD and protandry were significantly repeatable across years. Using mean values, we found that WSSD and protandry were positively related in both age classes. However, the relationship was stronger in the first time breeders compared to the adults. This could suggest that WSSD may be a more dominant mechanism for first time breeders to facilitate protandry compared to adults. Previous work has shown that adults choose more optimal flight conditions when they depart for migration (i.e., non-turbulent tail winds) compared to first time breeders and as a result, migration speed may not be a dominant mechanism underlying their protandry. First time breeders are inexperienced spring migrators, and so WSSD may innately aid their protandry.