Nest predation risk and deposition of yolk steroids in a cavity-nesting songbird: an experimental test.

Maternal hormones can shape offspring development and increase survival when predation risk is elevated. In songbirds, yolk androgens influence offspring growth and begging behaviors, which can help mitigate offspring predation risk in the nest. Other steroids may also be important for responding to nest predation risk, but non-androgen steroids have been poorly studied. We used a nest predator playback experiment and liquid chromatography with tandem mass spectrometry (LC-MS-MS) to assess whether nest predation risk influences deposition of 10 yolk steroids. We found no clear evidence that yolk androgen deposition changed when perception of nest predation risk was experimentally increased. However, elevated nest predation risk led to decreased yolk progesterone deposition. Overall, our results suggest yolk progesterone may be more important than yolk androgens in responses to offspring predation risk and highlight new avenues for research.

Does Vitamin E Supplementation Enhance Growth Benefits of Breeding Helpers at No Oxidative Costs?

During early postnatal development, biomolecules are particularly exposed to the detrimental actions of unneutralized reactive oxygen species. These prooxidant molecules have been claimed to mediate the trade-off between growth and somatic maintenance. Vitamin E is a key exogenous antioxidant that plays an important role in protecting biological membranes against oxidative damage. However, evidence of the effect of vitamin E supplementation during early life on growth and oxidative status in wild populations is equivocal. We tested the effect of supplementing western bluebird nestlings (Sialia mexicana) with vitamin E on growth rate, antioxidant capacity, and oxidative damage to lipids. During the period of accelerated growth (5-8 d), bill growth rate was 21% higher in supplemented nestlings from nests with breeding helpers than in supplemented nestlings from unassisted nests. Vitamin E also boosted tarsus growth rate during the period of slow growth (11-18 d), and this effect was independent of the presence of breeding helpers. Differences in body size and mass, oxidative damage to lipids, and antioxidant capacity were not evident between supplemented and control nestlings at 18 d. Therefore, we conclude that vitamin E promoted faster bill and tarsus growth, but this transient effect disappeared as soon as the supplementation ceased. Our experimental study also supports the idea that tocopherols are rapidly metabolized, since we failed to detect any evident increase of vitamin E in supplemented nestlings at age 18 d. These results provide partial support for the hypothesis that growth rate is constrained by its costs in terms of increased susceptibility to oxidative stress.