Parental care typically involves elaborate reciprocal within-family interactions, and traits such as parental provisioning and offspring begging should therefore coevolve. There is indeed mounting evidence for a phenotypic covariation, also in the canary (Serinus canaria), our model species. Such covariation may arise due to maternal effects, rendering this relationship particularly sensitive to environmental conditions. Here, we manipulated the social environment by pairing females with either their chosen or non-chosen male. Subsequently, all clutches were cross-fostered to separate pre- and postnatal effects. We found a positive covariation between offspring begging and parental provisioning, which was, however, unaffected by mate preferences, and we found no evidence for differential allocation. In addition, there was no effect of assortative mating, which is thought to reinforce parent–offspring covariation. The fact that parent–offspring covariation is consistently observed in canaries suggests that it is biologically relevant, but it requires further studies to elucidate its sensitivity to environmental variation.
Parental investment by males is less common among birds with polygynous mating systems than in monogamous species. Here, we examined the contribution of males in feeding nestlings in the facultatively polygynous European starling, Sturnus vulgaris. 1. In nestbox colonies around Antwerp, Belgium, males and females within monogamous pairs divided the feeding duties about equally, with a slight bias towards the female, and responded similarly to changes in brood size and age of nestlings. 2. The proportion of primary females receiving male assistance in feeding nestlings was significantly smaller than the proportion of monogamous females during each of the three nestling age stages (early, mid and late) we considered. In most primary broods, the strong decrease in male assistance was not due to the male directing part of his feeding effort toward the brood of the secondary female, but was due to the fact that the male's investment in feeding primary nestlings was negatively affected by his polygynous behaviour. As prospecting females were present after hatching of the primary broods (as contrasted to other studied starling populations), most males spent time trying to attract and courting additional females instead of giving parental care to the primary brood. This suggests that males trade off the attraction of additional females against giving parental care to an existing brood. The proportion of secondary females receiving male assistance in feeding was significantly smaller than the proportion of monogamous females during the early- and mid nestling stages. Overall, secondary females received less male assistance than primary females. The amount of male help to primary and secondary broods was not related to the hatching interval between the primary and secondary brood. 3. Primary females did not suffer reduced breeding success compared to monogamous females. In secondary broods, nestling mortality (partial brood loss) was significantly higher than in both primary and monogamous broods, while average nestling weights were significantly lower. These results suggest that secondary females, as contrasted to primary females, are not able to compensate fully for the reduction in male assistance. 4. During the mid-nestling stage, when nestlings grow most rapidly, but not during the early- and late-nestling stages, polygynously mated females feeding young without male assistance significantly increased their per-caput feeding rate compared with aided polygynous females and monogamous females, and made as many feeding visits as did polygynous pairs in which the male assisted, and as monogamous pairs. The higher nestling mortality rates in polygynous broods without male help during the early and mid-nestling stages suggest that unaided females cannot compensate fully (in terms of quantity or quality of food delivered), that male starlings can improve female fledging success by assisting in feeding nestlings, and that the reduced reproductive success of secondary females is directly linked to the strongly reduced male assistance in feeding.
According to the 'fertility-announcement hypothesis', the song of paired males might function partly as a paternity guard strategy and partly to maximize their own extra-pair copulations (EPCs). A major prediction of this hypothesis is that males should sing most when the fertility of their mate reaches a temporal (both seasonal and diurnal) peak. We report some tests of this hypothesis from a study of monogamously paired male European starlings Stumus vulgaris. Mated males sang significantly more during the fertile period of their mate and most males even completely ceased singing after their mate's fertile period. During the ovulatory period mated males sang significantly more in the late morning (0900-1200 hours) following egg-laying, when most females may have reached peak diurnal fertility, than early in the morning (0600-0900 hours). For six females, we were able to determine precise laying times during their ovulatory period and we found that their mates had a significantly higher song rate within the insemination window (the first hour following egg-laying) than before egg-laying. Although male starlings sing most when their mate's fertility reaches a seasonal and diurnal peak, our observations suggest that post-pairing song in monogamous males does not function primarily to deter other males attempting EPCs, or to attract extra-pair mates. Our results rather suggest that post-pairing song in monogamous males is directed mainly towards their own female and functions to stimulate her to solicit copulations. This may be important in the context of sperm competition if frequent pair copulations result in a higher fertilization rate for the male when EPCs have occurred.
The emission of heavy metals in the environment is often accompanied by emissions of acidifying pollutants. The acidification of terrestrial ecosystems results in the leaching of calcium and consequently a lowered concentration or availability of this essential nutrient in the environment. Calcium deficiency in the diet may lead to an increased absorption of toxic metals. In this study we investigated the effect of calcium availability on the accumulation of lead and essential metals (calcium, copper, iron and zinc) in important target tissues of zebra finches (Taeniopygia guttata). When exposed to 20 ppm lead via drinking water, zebra finches excluded from an additional calcium supply (i.e. oyster shell grit) accumulated significantly higher concentrations of lead in liver, kidney, muscle, brain and bone than zebra finches with access to oyster shell grit. Levels of calcium, copper, iron and zinc were not significantly affected by either the calcium availability or the increased absorption of lead. There were no significant sex related differences, suggesting that males and females have a similar demand, metabolism and/or distribution of calcium and other metals in the non-reproductive period. This study clearly illustrates the importance of calcium supply on the bioavailability of lead.