Day and night feeding territoriality in Willets Catoptrophorus semipalmatus and Whimbrel Numenius phaeopus during the non-breeding season in the tropics

This study was carried out to determine whether or not, and under what conditions, Willets Catoptrophorus semipalmatus and Whimbrel Numenius phaeopus continue to defend and use their daytime feeding territories at night in a tropical environment. The study was conducted in coastal Venezuela by registering, by night and by day, the behaviour and the position of colour-marked and radio-tagged birds. Night observations were made with the use of a light intensifier.
In Willets, both territorial and non-territorial birds were observed. The use and defence of territories was observed only on sandy mud areas where Fiddler Crabs Uca cumulanta were plentiful; the soft mud sites were used only by non-territorial birds. All Whimbrel were territorial. Individuals of both species defending a given space during daytime continued to occupy and defend the same area during the night. The proportion of birds foraging at night was higher on moonlit than on moonless nights. Territorial defence involved alert postures, parallel walks, calls and the chasing of intruders. Agonistic encounters between Willets and Whimbrel and other species were rare. Willets were territorial at all states of the tide except when high tides flooded the territories. Tide had no effect on the time of feeding in Whimbrel. Moonlight was clearly the factor conditioning the occurrence of nocturnal foraging on territories by both species. Foraging strategies and the type of prey and substrata explain why the incidence of night foraging varied with moonlight in territorial Willets and Whimbrels and not in non-territorial individuals.