Foraging ecology and habitat use of wading birds and shorebirds in the mangrove ecosystem of the Caeté Bay, Northeast Pará, Brazil

Abstract

Only little is known of migratory shorebirds in the tropics. The objective of this thesis was to describe the environment, the food availability, the bird community, the relationship between birds and environment and the avian consumption at tropical tidal flats. The study was conducted at tidal flats in the Northeast of Brazil.The study area comprised a low content of organic material and high fluctuations of sediment conditions and salinity. The benthic diversity, abundance and biomass was very low and variable compared to other tropical areas.Over 90% of the observed birds were migratory shorebirds, most residential birds were wading birds. Their spatial distribution could not be linked to environmental variables, however, birds had preferences for some sites and microhabitats.The harvestable fraction of the prey, the items accessible and ingestible, was very low, especially for small birds relying on small prey. The avian diets included a large variety of prey items and the attempt to find a relationship between the spatial distribution of the birds and their prey failed, presumably due to the resulting complex relationship.The avian community was split into residential wading birds and predominantly migratory shorebirds. Also, less distinct niches appeared within the shorebird community, however, most birds occupied broad niches. Birds not able to forage on large prey items had to feed opportunistically, a restriction on few prey items probably would have led to food shortages.Most species did not spend the entire ebb tide at the study area, but probably used also other, more profitable areas to obtain food. This might be crucial, since some birds cannot gain enough energy with the observed intake rates. Also, the food stock would have been depleted in a few weeks, if the birds would have foraged exclusively in the study area. However, an exclosure experiment did not show an impact, another indicator that the birds might forage elsewhere.