Cross-seasonal factors affecting breeding investment by female Pacific Dunlins

Myers proposed the Migration Distance Hypothesis (that the costs of long-distance migration force a compensating reduction in breeding investment) to explain parental care differences between species and sexes in shorebirds. This thesis proposes that seasonally increasing predation danger during migration is the main cost of extending breeding investment, and examines whether danger and other cross-seasonal factors predict reproductive patterns of Pacific Dunlins (Calidris alpina pacifica) and other shorebird species breeding in Alaska. The Predation Danger Hypothesis assumes that moult and migration schedules of shorebirds evolved to minimize exposure to raptors, especially migratory Peregrine Falcons (Falco peregrinus). The hypothesis predicts that these scheduling considerations affect parental care because they either require shorebirds to depart breeding areas early and in advance of falcon migration and moult on non-breeding areas prior to the migratory arrival of falcons; or to remain on or near northern breeding areas to moult, and migrate after falcon passage. Following breeding, Pacific Dunlins linger in Alaska until October, arriving on wintering areas after peak falcon passage. I found that breeding Pacific Dunlins renested and double-brooded extensively. Female Pacific Dunlins invested more time to reproduction and remained longer on the breeding grounds than female Western Sandpipers (Calidris mauri), their sympatrically-breeding, ecologically-similar, and early-migrating congener. The breeding seasons of both species were initiated on almost the same day, but the breeding investment of female Pacific Dunlins exceeded that of female Western Sandpipers by 7.2 days, and the seasonal decline in parental care duration of Western Sandpipers was steeper than in Pacific Dunlins. As predicted, Western Sandpipers but not Pacific Dunlins departed sooner in years with early falcon southward migration (related to snowmelt timing), and they gave up more breeding opportunity to do so. Stable isotope analyses showed that Dunlins and other sandpipers are primarily income rather than capital breeders and I found little support for the Differential Parental Capacity Hypothesis (that female shorebirds truncate care due to higher investment in egg production). These findings support the Predation Danger Hypothesis and highlight how predation danger can have far-reaching impacts on the life histories of potential prey species.