Dietary requirements influence the structure of shorebird assemblages, and information on diet is a key to understanding why and when particular species of shorebird use an area and how competition for food might shape their geographical distributions. We describe the diet and patterns of use of food resources of four migratory shorebirds at Samborombón Bay, Argentina, one of the most important sites for shorebirds in the western hemisphere. The birds consumed items representing at least 15 taxa but only a few taxa comprised the main resources consumed. The four species of shorebird showed dietary flexibility influenced by season and tidal level. Co-occurring species showed complementary differentiation in two dimensions of their respective trophic niches (taxonomic composition, size-class). The greatest differences in taxonomic composition of the diet were between Red Knots, which fed mainly on molluscs, and the other three species, which fed mainly on polychaetes. Polychaete consumers that co-occurred during autumn (Hudsonian Godwit and White-rumped Sandpiper) focussed on different prey sizes. Alternatively, shorebirds that co-occurred during winter (Hudsonian Godwits and Two-banded Plovers) fed mainly on polychaete of similar size but showed differences in the taxonomic composition of their diets. Interspecific competition for food resources might shape the composition of the shorebird assemblage of our study area during the austral autumn and winter.

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The phenomenon of over-summering in southern non-breeding areas by borealbreeding birds is particularly prevalent among shorebirds. Despite its frequency, it is understudied compared with most other aspects of shorebird ecology. Our aim was to expand knowledge of this subject through a study of red knots Calidris canutus rufa over-summering at a site in Argentina during the austral winter. We measured the proportion of one-year-old and adult over-summerers and evaluated the roles of flightfeather condition and physiological status (through leukocite profile, physiological stress index and presence of blood parasites) as triggers for over-summering. We also explore sex-ratio, the evolution of body mass and extent of breeding plumage within each age-class. Over-summerers were 57% yearlings and 43% adults, 46% females and 54% males. Almost all yearlings exhibited incomplete molt of primary feathers and some were in active primary molt. This suggests that the condition of flight-feathers and the timing of molt are likely to be important factors selecting for deferred migration during the first year of life. Other factors, not associated with flight-feather molt, seem to trigger over-summering in adults, which had completed flight feather molt but had low fat loads and/or incomplete alternate plumage. We found no evidence of a weakened immune system, high loads of blood parasites or high stress levels that can explain this poor migratory conditioning and therefore over-summering. Our data indicates that our adult age-class comprises both young individuals postponing first breeding until they are at least two years old and sexually mature individuals with prior reproductive experience skipping a breeding opportunity. Breeding propensity and age at first breeding are both poorly known, but key demographic parameters that determine population growth. This study suggests that potentially they can be estimated from mark-recapture at non-breeding areas and this warrants further study.