A relationship between prey density and territory size in non-breeding Eastern Curlews Numenius madagascariensis

Two hypotheses have been proposed to explain the variation in territory size observed within populations. Hypothesis I assumes that individuals can assess the local food density; territory size is then determined by the amount of food contained within the territory (Norman & Jones 1984, McFarland 1986). Under Hypothesis II, an animal defends as large an area as possible and the size of the territory is regulated by intruder pressure from the other animals in the population (Myers et al. 1979, Norton et al. 1982). These hypotheses need not be mutually exclusive. When territories are used only for foraging, as in nonbreeding shorebirds, areas with greater prey densities, providing higher intake rates, are likely to attract more competitors (Goss-Custard et al. 1984). These areas therefore will be more costly to defend (Myers et al. 1979). Thus, both hypotheses predict a decrease in territory size with increasing food density (Hixon 1980). To distinguish which of these factors proximally controls territory size under field conditions, where manipulations may be impossible, both need to be measured simultaneously (Myers et al. 1979, Tripp & Collazo 1997). If the pressure from intruders is partially responsible for regulating territory size, a trade-off in time allocation between foraging and defence should exist (Ydenberg & Krebs 1987). This implies that, to make defence viable, intruder pressure should be related positively to intake rate and negatively to territory size (Carpenter 1987). If no clear relationship between territory size/intake rate and intruder pressure exists, it suggests that the cost of defence is not related to territory size and that the density of the food resource is the major operational force in the system (Hixon et al. 1983). We investigated the relationships between territory size, prey density and intruder pressure in the Eastern Curlew Numenius madagascariensis foraging on the callianassid shrimp Trypaea australiensis during the non-breeding season. This study differed from previous non-breeding shorebird territoriality studies in that the intake rate achieved by birds maintaining territories of different sizes was estimated to establish whether it co-varied with the intruder pressure and density of prey within a territory.