There is increasing evidence that individuals are consistent in the timing of their daily activities, and that individual variation in temporal behavior is related to the timing of reproduction. However, it remains unclear whether observed patterns relate to the timing of the onset of activity or whether an early onset of activity extends the time that is available for foraging. This may then again facilitate reproduction. Furthermore, the timing of activity onset and offset may vary across the breeding season, which may complicate studying the above-mentioned relationships. Here, we examined in a wild population of great tits (
Abstract A central principle of life-history theory is that parents trade investment into reproduction against that in body maintenance. One physiological cost thought to be important as a modulator of such trade-off is oxidative stress. Experimental support for this hypothesis has, however, proved to be contradictory. In this study, we manipulated the nestling rearing effort of captive canaries (Serinus canaria) soon after the hatching of their nestlings using a brood-size manipulation to test whether an increase in nestling rearing effort translates into an increase in oxidative damage, an increase in ceruloplasmin (which is upregulated in response to oxidative damage) and a decrease in thiol antioxidants. We also compared the blood oxidative stress level of reproducing birds to that of non-reproducing birds, a crucial aspect that most studies have invariably failed to include in tests of the oxidative cost of reproduction. As compared to non-breeding canaries and pre-manipulation values, plasma oxidative damage (reactive oxygen metabolites and protein carbonyls) decreased in breeding canaries irrespective of sex and brood size. In contrast, oxidative damage did not change in non-breeding birds over the experiment. Ceruloplasmin activity in plasma and both non-protein and protein thiols in red blood cells did not change throughout the experiment in both treatment groups. Our results suggest that reproduction may result in decreased rather than increased blood oxidative stress. Our results may explain some of the inconsistencies that have been so far reported in experimental tests of the oxidative cost of reproduction hypothesis.
Experiments were performed using captive starlings to examine whether Starling song functions primarily in intersexual communication. Male Starlings showed a significant increase in the time spent at the nestbox, as well as in the total time spent singing, after the introduction of a female into the aviary. After the introduction of a male, only the time spent at the nestbox increased significantly. Wing-waving (a visual display associated with singing) and singing in the nest box occurred only after introduction of a female. Our results strongly suggest that Starling song functions largely in male-female interactions. The primary function of wing-waving seems to be mate attraction.
