Roost use by wintering dunlin at Humboldt Bay, California : relationship to predation danger and human activity

I studied roosting dunlin (Calidris alpina pacifica) in winter at ten sites around Humboldt Bay, California to determine roost site preferences, if any, in relation to the numbers and types of predators and human activity nearby. Based on roost locations found in Colwell et al. (2003) and with the assistance of an observer, I estimated numbers of all shorebirds on the roost at the beginning and end of each one-hour observation period, which I used to obtain an index of dunlin change in abundance over each survey. I determined the proportion of the dunlin flock performing each of three behaviors (roosting, alert, and flying) every minute in order to obtain an average behavioral index over the one-hour sample period. I continuously surveyed for predators and humans over the hour to obtain a total number of each within a 200 m radius around each roost, also noting the number of attacks and predator success. Roosts varied in incidence, abundance, frequency of change in abundance, and behavior of dunlin, as well as incidence and number of predators, and amount of human activity, but roosts did not vary in average change in abundance of dunlin over a survey. There were more predators present when more dunlin were present. Dunlin left sites more often and dunlin had a higher activity levels when there were more predators present. Dunlin left sites when a greater number of peregrine falcons (Falco peregrinus) and northern harriers (Circus cyaneus) were nearby. These relationships were strongly influenced by one roost (Klopp Lake). Dunlin abundance and behavioral index were higher when merlin (Falco columbarius) abundance was higher, although one site (Coast Guard Jetty) was especially influential in this pattern. Predators had a greater influence on dunlin behavior at roosts than human activity. Disturbance might have been more accurately assessed at distances less than the buffer distance of 200 m selected, or dunlin habituated to human activity. Generally roost use was consistent between years when compared to data reported by Colwell et al. (2003). Because correlations were significant only in the presence of two sites that could be anomalies, this study indicates that roost use is influenced by factors other than predator threat and human activity. Although dunlin roosts did not appear to be limited in Humboldt Bay, patterns of shorebird use of roosts should be considered when development is proposed around Humboldt Bay.