HABITAT-USE PATTERNS AMONG MIGRANT AND RESIDENT LANDBIRDS OF CONTRASTING DIETARY HABITS In A SOUTHERN MEXICAN WETLAND

the distribution of resident and migratory bird abundance across mangrove and marsh patches can produce dynamic and guild-specific habitat-use patterns. Using 32 consecutive months of point counts in a coastal landscape in Oaxaca, Mexico, we studied the annual cycle, non-breeding season (October to March) and breeding season (May to August) of terrestrial birds. For broad trophic guilds of plant-consumers and insectivores, we disclosed seasonal dynamics and assessed the spread of non-breeding season abundance across migratory habit categories and habitat types. to test for habitat-use patterns derived from hypotheses concerning specific trophic resources, we focused on subsets within trophic guilds: frugivores (plant-consumers that mainly eat fruit) and invertebrate gleaners (insectivores that mainly eat terrestrial leaf and stem invertebrates). Drawing on a resource-centred dietary requirement hypothesis, we predicted lower abundance of migrant frugivores in mangrove, where palatable fruit availability was expected to be low and resident frugivores probably exploited invertebrates to feed nestlings. Based on the competition-oriented nest predation hypothesis, we anticipated a higher proportion of migrant invertebrate gleaners in marsh. we recorded migrant birds in every month of the year. For 20 plant-consumer species, the non-breeding season density was highest among residents in mangrove (> 5 individuals per hectare). For 46 insectivore species, densities were > 5 and > 9 individuals per hectare for migrants and residents, respectively. Comparable average abundances in trophic guilds suggested both migrant and resident species were equally important. For 14 frugivore species, a higher proportion of migrants in marsh (0.90) than mangrove (0.03) resulted from reciprocal between-habitat changes in migrant and resident abundances, as predicted by the dietary requirement hypothesis. For 22 invertebrate gleaners, proportions of migrants in mangrove (0.45) and marsh (0.55) were comparable, as were abundances. Contrary to the nest predation hypothesis, any between-habitat variation in invertebrate gleaner nesting activity did not markedly influence non-breeding season abundance.