Climate change is altering birds’ hormones and plumage, reducing the presence of female key sex organs and altering vital sex ratios in breeding pairs, experts have found.
Male birds are now growing bigger, hind legs are thinner and they are developing bigger breasts, making them larger and stronger, according to researchers at the University of Exeter.
Changes in sexual dimorphism – the difference between the sexes – are contributing to bird population decline and may lead to genetic changes that are toxic to birds’ ability to cope with the challenges of climate change, they warned.
“The stark changes to birds’ anatomy mean these changes also affect their ability to endure the challenges of climate change,” said Anna Morris, a plant pathologist at the university.
“When male birds turn from lean and strong to large and weak, they will spend far more time wandering the planet looking for mates, a behaviour highly damaging to their ability to achieve a healthy and stable population structure.”
The results showed that changes in body shape and sex ratios during wild-breeding, called alopecia, were occurring throughout the world.
Morris and colleagues have now analysed 86 data sets to study the effects of these effects and have identified key sex organs that have been affected.
In previously ecologically stable populations of redshank, a small white bird which is grey on the wing and blue on the breast, changes in species sex ratios are important because they could tip the balance away from males as important food-supply providers.
Male and female redshanks, male and female nightjars (Myrmuridae), male and female jews (Odontomorhynchus caudatus), male and female buff-tailed geese (Panulagus nummularis), male and female swifts (Nepenthes bengalis), male and female common nuthatches (Acanthosteus viridis), male and female redbud tree warblers (Strelitzia rhodoplanum), and male scarlet tanagers (Buxurus bellum) showed all were growing underdogs in their local populations.
A new assessment showed the effect to be more pronounced in certain species in southern Europe, such as northern redshanks and nightjars.
Redshanks, western tussocks, common swifts and nightjars are primarily migrants foraging in agricultural fields, where death rates are lower than in the wild, and at some locations they still operate in conical sex ratios.
In fact, the ditching of the long-duration courtship phase of a redshank male as a result of the female sex change is so serious the surviving sexants are no longer capable of sustaining stable populations.
Nightjars, which prefer feeding in the dry, wet and wild at night, also experienced significant reductions in rates of species sex ratios of primary significance as a result of the changes in body shape and sex ratios.
Researchers are now turning their attention to how these species are recovering in order to prevent further population declines.