EGG AND CHICK FATES OF COMMON TERNS IN AREAS OF HIGH AND LOW NEST DENSITY

Abstract

Common Tern (Sterna hirundo) nests were both clumped (high density) and solitary (low density) at Windermere Basin, Lake Ontario in 1992. Our objective was to measure egg fate and chick survival up to 14–16 d of pairs nesting in these two dispersion patterns. We first measured the distance to nearest neighbor for 100 randomly selected clutches, and defined low-density nests as those at a distance greater than 2 SD (m) away from the nearest clutch. Two sites contained nests at high density (HD1 & HD2), while low density nests (LD) were dispersed throughout the study site. We randomly selected 20, three-egg clutches in each of the three groups. Nearest neighbor distances of HD1 (1.08 ± 0.71 m) and HD2 (2.09 ± 1.03 m) clutches were similar and significantly different from nearest neighbor distances of LD clutches (21.9 ± 37.3 m). There were no differences in clutch initiation dates or C-egg volumes among the 60 clutches of the three study groups. LD and HD1 clutches experienced heavy egg and subsequent chick loss, while losses of eggs and chicks at HD2 were negligible. The evidence for type of predator was largely circumstantial, but for eggs that were destroyed and chicks that disappeared, the loss patterns suggested primary predation by Ruddy Turnstones (Arenaria interpres) and Black-crowned Night-Herons (Nycticorax nycticorax), respectively. The locations and timing of egg and chick losses indicates that opportunistic behavior of these avian predators nullifies any predatory avoidance advantages of high-density nesting once the predator(s) identify the food resource.