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Extramarital blue tit offspring have a head start

09 March 2011

Blue tits are notorious philanderers and the eggs of extramaritally conceived offspring are often the first to be laid, too. That means that the little bastards have a better chance of survival. This undermines the more traditional hypothesis that extramarital offspring do better because they have better genes. Adultery is apparently not aimed at improving the genetic quality of the offspring, concludes Groningen biologist Oscar Vedder. He will be awarded a PhD for his research by the University of Groningen on Friday 18 March 2011.

Birds are often socially monogamous, in the sense that a male and a female together bring up the chicks. However, extramarital offspring are known in virtually all bird species. Vedder chose blue tits for his research into the how and the why behind this behaviour. Blue tits are very numerous and also breed in nest boxes. That saves a lot of searching for nests and makes catching the parents to test their DNA much easier.

Head start

Vedder’s main discovery is that extramarital offspring are usually among the eggs that are laid first. Blue tits lay an egg every day for 11-12 days and the first eggs thus also hatch first. DNA comparison reveals that these offspring are often not from the ‘social partner’ of the blue tit hen.
With an average of 9 hours head start, these offspring have a better chance of survival. When the data is corrected for that head start, the bastards do not turn out to be genetically healthier than their ‘real’ brothers and sisters. The fact that popular blue tit Don Juans are betrayed by their wives just as often as they betray them also indicates that the adultery does not seem to be motivated by the search for the best possible DNA.

Brood parasitism

Blue tit hens are promiscuous because they want to increase the chances of fertilization as much as possible, according to Vledder. For example, they are less interested in a bit on the side if they have already laid enough eggs. It turns out that the hens have such a huge drive to lay eggs that they have also been known to lay a few in their neighbour’s nest too. This brood parasitism among blue tits has not been described before.


The assumption that the colour intensity of the azure blue crown of the male blue tits is an indication of genetic quality is incorrect, according to Vedder. It is true that the blue crown feathers – visible to the tits in the UV spectrum – can be used in conflicts between males, but only when there is little at stake. A beautiful crown is thus no guarantee of success with the females.

Curriculum Vitae

Oscar Vedder (Emmen, 1980) studied biology at the University of Groningen. He conducted his PhD research at the Animal Ecology research group at the Centre for Ecological and Evolutionary Studies (CEES) of the University of Groningen. The research was financed by grants from the European Union awarded to his supervisor, Prof. Jan Komdeur. Vedder will be awarded his PhD by the Faculty of Mathematics and Natural Sciences for a thesis entitled Reproductive strategies, intra-specific competition and sexual selection in blue tits. He currently works at the Edward Grey Institute of Field Ornithology  of the University of Oxford, where he is researching how blue tits adapt to changes in temperature.

More information

Oscar Vedder, tel. 00-44-78-35426589 or 00-44-1844-239348, e-mail: oscarvedder; o.h.vedder 

Last modified:13 March 2020 01.56 a.m.
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