Do sex and habitat differences in antipredator behavior of Western Sandpipers Calidris mauri reflect cumulative or compensatory processes?

Abstract

Individuals manage their risk of predation in different ways in different situations. We studied the use of anti-predator behavior by Western Sandpipers (Calidris mauri) at Bahía Santa María, northwestern Mexico, foraging in three habitats that differed in presumed predation danger. Brackish flats are completely open, making them theoretically less dangerous for feeding sandpipers than mangroves and cattail marshes, which have closer visual horizons. Western Sandpipers are sexually dimorphic, with females about 15% longer-billed and 10% heavier than males. We previously showed that male and female sandpipers differed in their habitat choice and relative body mass in ways consistent with differential responses to predation danger (Fernández and Lank in Condor 108:547–557, 2006). Contrary to expectations, however, females were overrepresented in more dangerous habitats. Here, we examine differential usage across habitats and between the sexes of three anti-predator tactics—flock size, density within flocks, and vigilance rate—that may be used cumulatively to reinforce safety, or as trade-offs that compensate for levels of usage of each. We hypothesized, and found, that ordered differences occur among habitats, and that controlling for other factors, females were more cautious than males. For the most part, the use of these three tactics appeared to be cumulative, rather than compensatory. However, with respect to habitat use, birds appeared to compensate for the higher probability of mortality intrinsic to the use of higher-danger habitats by increasing the use of vigilance, foraging in tighter flocks, and maintaining lighter body weights (females only). Thus, both cumulative and compensatory processes operate among anti-predator tactics to determine the net level of safety and trade-off against other factors.