A study coordinated by the Doñana Biological Station (EBD-CSIC) and the Pyrenean Institute of Ecology (IPE-CSIC) shows that the physical attractiveness of birds does not determine their reproductive success.

One of the main weapons used by animals to ensure the transmission of their genes is the development of sexual characteristics, in the form of ornaments, that highlight their quality. In general, the most attractive individuals are also those with the best physical and genetic conditions, so one would expect them to achieve greater reproductive success. However, a study coordinated by CSIC researchers highlights that a better physique could mean more disputes and, therefore, less time to devote to raising offspring. This work, published in the Journal of Evolutionary Biology, suggests that a lower plumage colouring in birds could lead to a reproductive advantage, depending on the social context.

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Developing and maintaining high-quality sexual ornaments is no easy task for anyone. In many animals, such as birds, colourful visual cues that indicate their level of attractiveness are costly to produce and maintain. This is the case for male pied flycatchers, a small migratory bird that signals its sexual attractiveness, as well as its social status, through the colouring of its back. This colouring can range from pale brown to the irresistible sapphire black of dominant males.

However, displaying high attractiveness – with its attendant costs – may not always be beneficial in terms of offspring. For example, in a densely populated forest crowded with competitors, having an irresistibly dark plumage that also highlights the dominant character should be synonymous with success. However, the CSIC scientific study does not support this idea. This work analyses annual data on breeding density and plumage colour for nearly 2,000 males examined since 1984. Contrary to expectations, individuals with intermediate colouring turned out to be the most reproductively beneficial.

This apparent contradiction could be due to the high price to be paid for individuals showing a sign of high sexual and social status. “The most attractive males enjoy better breeding territories or better females but are at the same time the centre of all eyes, also those of their competitors. Defending their possessions can lead to physical exhaustion that reduces chick care and compromises their survival,” explains Nacho Morales-Mata, first author of the study. The greater expenditure on defence could ultimately favour males with an intermediate colouring who, despite not having the best physique, could spend less time fighting and more time and energy on raising more chicks.

In conclusion, this study suggests that, depending on the social context, it may be better to be a little less graceful, avoiding disputes with other males and thus having more time and energy to raise offspring.