Models and reality: Time-energy trade-offs in pectoral sandpiper (Calidrismelanotos) migration

We used a combination of modeling and field studies to determine the spring migration strategy of Pectoral Sandpipers (Calidris melanotos). We develop ed a dynamic programming model to predict patterns that should be detected along the migratory route if Pectoral Sandpipers use a strategy of early ar rival at the breeding grounds (time minimization) or arrival at the breedin g grounds with excess energy reserves (energy maximization). The prediction s were then compared to data collected at stopover sites in the mid-contine nt of North America and at the breeding grounds in Alaska over a 5-yr perio d (1992-1996). During spring migration to their Arctic breeding grounds, Pectoral Sandpipe rs stop periodically to feed. The length-of-stay at such stopovers, for bot h time minimizers and energy maximizers, was predicted to vary inversely wi th date and body fat, and to vary directly with invertebrate abundance. We observed that: (1) length-of-stay was negatively correlated with capture da te in Missouri and Nebraska, but not in Texas; (2) length-of-stay was not c orrelated with body fat at any site; and (3) length-of-stay was positively related to invertebrate abundance at the Nebraska and Missouri sites. As th e population moves northward in the spring, three regional patterns are dia gnostic of migration strategy. Length-of-stay was predicted to be bimodal ( energy maximizer) or constant (time minimizer) with respect to latitude, bu t neither pattern was observed. The migration window, or period of time dur ing which spring migrants occur, was predicted to decrease with increasing latitude for time minimizers, a pattern that was seen for both males and fe males. Body fat was predicted to increase with latitude for energy maximize rs, a pattern that was seen for females but not males. The evidence suggests that males and females differ in their spring migrati on strategies. Both sexes attempt to arrive in the Arctic as early as possi ble after ice breakup in the spring. Additionally, females gain significant ly higher fat loads than males (up to 60% body fat for females) during migr ation, and these energy reserves may later enhance female reproductive succ ess. However, females gained large fat loads only during 1993 and 1995, whi ch had above normal spring precipitation along the migration route. We beli eve that the correlation between female body fat and precipitation reflects an abundance of high-quality stopover habitat during wet springs. This vie w is supported by model sensitivity analyses showing that the spacing and q uality of stopover habitat can strongly influence observed migration patter ns. Our results suggest the need to focus additional research on the landsc ape-level features of the flyway through which shorebirds migrate.