Body mass change in breeding Common Terns Sterna hirundo

In 1991-1993 we studied the body mass change of adult Common Terns Sterna hirundo breeding on the German North Sea coast. In each year females' body mass increase prior to laying was very similar (about 5.4 g day(-1)). This mass increase resulted in an average body mass of about 175 g on the day of egg-laying. In 1992 and 1993 the females' body mass decreased significantly during incubation, while males' body mass was constant. In 1991, however, neither sex showed a decrease in body mass while incubating. In all years, incubating females and males were heavier than chick-rearing parents. This difference was not caused by higher mass in early incubation, because males and females were still significantly heavier in the last week of incubation than in the first week of chick-rearing. Decreasing mass during incubation and the low body masses during chick-rearing are considerd to be stress induced rather than an adaptive strategy to lower flight costs. The decrease in female body mass during incubation in 1992 and 1993 and the lower body mass of males in these years could be related to less favourable foraging conditions compared to 1991, when both males and females showed the highest average body mass values of the 3 years.