Ecological Factors Underlying the Nonbreeding Distribution of Western Sandpipers

Avian species in which males and females migrate to different nonbreeding areas provide candidate systems to study ecological factors underlying distribution patterns. Western Sandpipers (Calidris mauri) are such 'differential migrants'. They breed mainly in Alaska and overwinter along the American Pacific and Caribbean coastlines. In this thesis, I document an increasing proportion of females at more southerly latitudes. I review existing explanatory hypotheses for differential migration, propose two novel hypotheses, and test these with data collected at four latitudes. According to the feeding niche hypothesis, intertidal invertebrates are buried more deeply towards the south, possibly due to higher ambient temperature andlor desiccation. Longer bills enable probing (foraging on buried prey) to greater depths. Females have disproportionately long bills, and therefore can exploit a feeding niche at greater vertical depth. Bill length residuals, corrected for tarsus length, were predicted to increase towards the south. This was only found in males. No clear change of feeding mode with latitude was detected. Females probed more than males at all locations, even though aspects of the ultrastructure of female bills did not indicate greater specialisation for probing. At the one site where the relationship was measured, both sexes probed more with increasing sediment temperature. According to the predation danger hypothesis, predator escape ability of males and females, indexed by wingloading, differs consistently across latitudes. Escape ability is generally reduced with higher wingloading. Individuals with poorer escape ability were therefore predicted to prefer southern sites, where less fat is required as insurance against environmental variability. Wingloading was higher overall for females. At one site I compared the sex ratio of carcasses, assessed molecularly, to that of fiee-living birds, but found no evidence for any sex-bias in predator-induced mortality. Wingloading increased with latitude in both sexes, but an index of predation danger remained constant across latitudes. Within latitudes, wingloading was lower at smaller, and presumably more dangerous, sites. My results provide evidence for both hypotheses, while other hypotheses for differential migration were not supported. I suggest that both escape performance and feeding niche divergence are important factors in determining large-scale spatial distribution in Western Sandpipers.