Assessing incubation behavior of least terns (Sterna antillarum) in southern New Jersey: Conservation implications

Eberly, Ilene S.
Jon Atwood, PhD
Department of Environmental Studies
Least terns (Sterna antillarum) are small, colonial beach-nesting birds that are frequently impacted by mammalian and avian predators during the breeding season. In the presence of intense predator pressure, least terns will often increase their defensive anti-predator behavior or temporarily abandon their nests, reducing incubation frequency (Thompson et. al. 1997). Reduced incubation frequency may lower nesting success by increasing exposure of eggs to predators or wind-blown sand, or allowing eggs to be exposed to high or low ambient temperatures. I examined incubation behavior of least terns in relation to predation pressure along the Cape May Peninsula in New Jersey. In addition, we explored the efficacy of small temperature data loggers, iButtons (Maxim/Dallas Semiconductor Corp., CA). From 30 May – 13 Aug 2007, iButtons were used to record nest temperature data in 46 nests at Cape May Point State Park and in 35 nests at Stone Harbor Point. Temperature data from each nest was then compared to the concurrent ambient temperature. We established a criteria for determining if a bird was incubating: if there was =2.8°C degree difference between nest temperatures and ambient temperature in a 5-min interval then a bird was concluded to be incubating (or standing over the nest) during that interval. For each nest, we calculated the ratio of “incubation intervals” to the total number of recorded intervals; this metric was defined as overall incubation frequency. Overall incubation frequency at colonies were lower at sites with intense predator activity. Finally, we demonstrate that temperature data loggers such as iButtons, are effective tools for analysis of long-term incubation behavior at least tern colonies. Because overall incubation frequency was positively associated with successful nest outcome, we suggest that this is a good metric for assessing predation pressure.