Can Patterns of Energetic Condition Explain Differences in the Productivity of Arctic and Common Terns at Petit Manan Island, Maine?

In Maine, Arctic terns (Sterna paradiseae) are listed as threatened species and Common terns (S. hirundo) are listed as a species of concern. For the past century the two species responded differently to conservation efforts. The basis for these differences remains unclear. Identifying the physiological factors that limit the two species from full recovery is critical for the conservation of their populations. To better understand the basis for different recovery responses between the two species, I compared the physiological condition of the birds throughout key periods of the prebreeding and breeding stages at Petit Manan Island (PMI), Maine. I examined the condition of the terns when they first arrived at the island and throughout the incubation and chick rearing stages to ultimately link this with tern productivity patterns. The ability of individual terns to successfully fledge young depends on their physical condition before and throughout the breeding season. To determine how well an individual is meeting its energetic demands, I examined baseline levels of corticosterone (the major energy-regulating steroid), and plasma metabolites (indicators of the processes of putting on or using fat), ultimately linking these with breeding success. Measurements of parental effort were monitored to document individual activity related to clutch productivity. The species differences in condition related to parental effort may explain their different population recovery rates. Arctic terns arrived in 2006 in better condition than in 2007. In comparison, Common terns arrived in significantly better condition in 2007. In 2007, both species had higher levels of corticosterone, triglycerides and glycerol, suggesting that while both were foraging and recovering fat reserves they were also burning fat during the prebreeding period. Arctic tern condition declined from the prebreeding to the breeding period by six percent, while Common tern condition declined by seventeen percent. In general, Common terns initiated breeding with more body reserves to invest towards breeding than Arctic terns. Throughout the study, Arctics burned more fat (elevated glycerol) than Commons. While both species increased triglyceride levels as the season progressed, neither species were able to significantly improve their condition. Common terns up-regulated moderate levels of corticosterone, perhaps as a mechanism to increase foraging activity to support prey delivery to young without incurring the hormone’s cost to their own health. Overall, Arctic terns showed a greater decline in condition from the prebreeding to the breeding period, resulting in lower annual productivity. Common terns either maintained their body condition throughout the prebreeding and breeding stages, or were able to significantly improve prebreeding condition. Common terns fledged more young than Arctic terns. The inability of Arctic terns to maintain energy reserves at a critical threshold from the prebreeding to the breeding period may have had a greater affect on their breeding success. This study illustrated that resource managers working to restore tern populations in the Gulf of Maine need to take into account the impact of events (food availability and weather conditions) during the wintering and migration periods as well as on the breeding grounds to understand variation in productivity