Cor Dijkstra

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Avian eggs contain substantial amounts of maternal androgens, and several studies have indicated that these are beneficial for the chick. Nevertheless, there is a large and systematic variation in maternal hormone concentrations both within and between clutches. If maternal androgens also involve costs, this might explain why not all mothers put high levels(More)
In theory, birds should control the sex ratio of the offspring they produce. In practice, we have very limited evidence to support this idea because of our difficulty in sexing nestling birds. In addition, extinction is facing an increasing number of birds. Our ability to help includes captive breeding which, again, is difficult if male and female adults(More)
Knowledge of the mechanism underlying the expression of melanin-based sex-traits may help us to understand their signalling function. Potential sources of inter-individual variation are the total amount of melanins produced but also how biochemical precursors are allocated into the eumelanin and phaeomelanin pigments responsible for black and reddish-brown(More)
Developmental stressors often have long-term fitness consequences, but linking offspring traits to fitness prospects has remained a challenge. Telomere length predicts mortality in adult birds, and may provide a link between developmental conditions and fitness prospects. Here, we examine the effects of manipulated brood size on growth, telomere dynamics(More)
Avian eggs contain considerable amounts of maternal yolk androgens, which have been shown to beneficially influence the physiology and behaviour of the chick. As androgens may suppress immune functions, they may also entail costs for the chick. This is particularly relevant for colonial species, such as the black-headed gull (Larus ridibundus), in which the(More)
The good genes hypothesis of sexual selection postulates that ornamentation signals superior genetic quality to potential mates. Support for this hypothesis comes from studies on male ornamentation only, while it remains to be shown that female ornamentation may signal genetic quality as well. Female barn owls (Tyto alba) display more black spots on their(More)
We investigated in the black-headed gull whether female deposition of antioxidants and immunoglobulins (enhancing early immune function), and testosterone (suppressing immune function and increasing early competitive skills) correlate suggesting that evolution has favoured the mutual adjustment of different pathways for maternal effects. We also took egg(More)
Maternal hormones are known to be present in avian eggs and can have beneficial effects on chick development. Recently, differences in avian yolk steroid concentrations between the sexes have been demonstrated, and in this context steroids have been proposed to be part of the avian sex-determining mechanism. In our study, we show that it is very unlikely(More)
Food consumption was measured in six female and seven male hand-raised marsh harrier (Circus aeruginosus) nestlings. Females consumed on average 4,321 g and males consumed 3,571 g of food during the nestling stage from 0 to 36 d. Total consumption until 56 d was 6,960 g and 5,822 g for females and males, respectively. On the basis of Fisher's sex ratio(More)
Female birds of several species have control over the production of daughters and sons. However, most studies failed to find a relationship between egg size and sex. This is intriguing as adjustment of egg size would constitute a powerful tool for the female to meet different resource demands of the sexes, particularly in size dimorphic species. Our results(More)