The seasonal decline in egg-volume on the basis of the individual changes in 35 known-age female Common Terns Sterna hirundo was studied with data available for 3-6 years. Interindividual differences were significant for both laying dates and egg-volume. Thus, to examine whether the decline was an effect of an assortment of females with the timing of breeding according to their individual egg-volume, the individual values of laying date were correlated with egg-volume. Correlation was not significant, suggesting that seasonal decline in egg-volume was not due to interindividual differences. Instead, egg- volume showed a gradual decrease with the progress of the season at intraindividual level i.e. the later an individual female laid in the season the smaller was the egg-volume. Moreover, the seasonal decline in egg-volume was not due to a simultaneous increase in egg-volume with an advancement of the timing of breeding with age, since the rate of seasonal decrease in egg- volume was similar for young and old birds. Individual average of laying date accounted for 22% of the individual average of fledgling production whereas egg-volume was not significantly correlated with fledging production. Thus, it is suggested that seasonal decline in egg-volume is a non-adaptive consequence of physiological effect of condition on both timing of breeding and egg-volume.
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.