Behavioral, energetic, and conservation implications of foraging plasticity during migration

Abstract

Dietary plasticity is widespread and frequent in many landbird species during migration and typically involves shifts from stereotyped insectivorous diets during the breeding season to inclusion of other animal or plant matter, especially fruit. As with other forms of behavioral plasticity, flexibility in diet has probably evolved in response to environmental uncertainty, which, I argue, most landbirds encounter in terms of food resource availability during migration. The spatial and temporal uncertainty in insect availability during autumn stopover may have influenced the evolution of dietary flexibility during migration. Experiments and empirical observations from studies on Block Island, Rhode Island, off the northeastern coast of North America demonstrate that seasonal dietary shifts to fruit can strongly affect en route foraging behavior, habitat use, and migratory departure decisions. Migrants feeding on fruit use less expensive foraging behaviors, encounter more "prey" items per unit time, and perform fewer search movements than when feeding on insects. Furthermore, fruit removal experiments revealed that the presence of fruit influenced the habitats selected by frugivorous migrants such as Yellow-rumped Warblers (Dendroica coronata) during autumn. Because of seasonal frugivory, many species are selecting habitats that are different from those selected at other times of the year. Furthermore, dietary shifts also play a major role in migrant energy budgets during stopover by increasing energy intake while decreasing the energy expended during stopover foraging. Experiments with Red-eyed Vireos (Vireo olivaceus) and Catharus thrushes suggest dietary plasticity can increase energy intake and facilitate lipogenesis in species capable of assimilating novel diet types. Use of more temporally and spatially stable fruit resources may also decrease searching and handling time, and decrease energy expenditure during stopover foraging, resulting in positive net energy budgets for migrants. Aspects of migrant biology that change during the annual cycle, such as dietary shifts to fruit, must be integrated into conservation plans for landbird populations during migration. The behavioral ecology of near&c-neotropical landbirds during migration is not a simple extension of the breeding or wintering season biology. Environmental and physiological pressures , such as increased energetic demand from nocturnal flights or unpredictable variation in habitat and resource availability, may be quite different during migration than at other periods during the annual cycle. These pressures have strong implications for the evolution of migrant behavior, life history, and population dynamics (Moore 1991a). One mechanism used by migrants to cope with these pressures is the ability to be flexible in behavior during migration. Migrating landbirds demonstrate this behavioral plasticity in a variety of ways, including flexibility in habitat selection, foraging maneuvers, and social interactions. An additional strategy used by many species is dietary plasticity, particularly visible as an expansion from insecti-vory during the breeding season to a diet including large proportions of fruit during migration. This dietary plasticity can profoundly influence the behavior, energy budgets, and ultimately the conservation of migrant landbirds during stopover periods, BEHAVIORAL PLASTICITY Behavioral plasticity has received extensive theoretical and empirical attention (e.g., Klopfer and MacArthur 1960, Klopfer 1967, Levins 1968, Stephens and Charnov 1982; Greenberg 1984a,c, 1987a, 1990; Ford et al. 1990, Martin and Karr 1990). Plasticity can be viewed as more than the simple antithesis of specialization (Morse 198Oa), as has been suggested in the earlier use of the term (KIopfer and MacArthur 1960). Earlier, "plasticity" was used to define the resource or behavioral breadth of an organism , but Morse (1980a) first suggested that the concept of plasticity extends beyond the gener-alist and specialist dichotomy. He suggested that stereotypy and plasticity were the exploitation of resources under changing conditions in consistent and variable manners, respectively, and that plasticity was characterized by behavior that lacked long-term predictability in time or space. Greenberg (1990) proposed a further, functional definition: plasticity is the flexibility of organisms of the same genotype to vary in phenotype in the face of change. This definition of plasticity best suits the changing biology of nearctic-neotropical migrant landbirds, given their very diverse behavioral repertoire when experiencing spatio-temporally unpredictable environments. Environmental instability can influence the evolution of behavioral and life history strategies through random shifts in direction and magnitude of selection pressures (Alerstam and Enckell 1979, Real 1980, Thompson 1991, 53