New research shows that variation in mating behaviours, parental care and differences in ornamentation of the sexes in bird species is driven by demographics rather than vice versa.

An international team of researchers from the UK, China, Germany and Hungary looked at 261 species of birds from 69 avian families, running statistical models to investigate the relationship between demographics, adult sex ratio (ASR), breeding behaviours and parental cooperation.

They found that bird species with an adult population skewed towards one sex was caused primarily by demographic factors. Male and female chicks are usually born in equal numbers but don’t survive equally as juveniles or adults, and often don’t mature at the same speed. So in some species one sex is more likely to die before they can reproduce, resulting in a skewed adult sex ratio.

Their findings are published in Nature Communications.

Professor Tamás Székely, from the Milner Centre for Evolution and Department of Life Sciences at the University of Bath, said: “We’ve previously shown that the sex ratio – the balance of males to females in a population – plays a crucial role in how birds choose mates, how promiscuous they are, and how mothers and fathers share the load in bringing up their offspring.

“However until now we haven’t known if these behaviours are driven by demography or whether they themselves cause sex differences in survival, reinforcing an imbalance between the sexes.

“For example, does parenting or breeding behaviour in species with a skewed sex ratio increase mortality in one sex, maintaining the bias?

“In many bird species, only the females incubate the eggs and by doing so, they are vulnerable to a plethora of predators. Concomitantly, a gaudy male may find mates sooner than a dull one – however, predators may well spot and kill a gaudy male with fancy plumes more likely than a dull non-ornamented male.”

Chicken or egg question

Dr Zitan Song, Professor at Nanjing Forestry University in China commented: “Separating what is cause and what is consequence was challenging – a bit like asking whether the egg or the chicken came first – so we used sophisticated statistical tools in the analyses of data from contemporary bird populations.

“The results were nevertheless convincing: unbalanced sex ratio was the cause, rather than the consequence, of sex differences in breeding behaviour and parenting.”

Professor Székely said: “Sex ratios matter because when one sex outnumbers the other, the rarer sex is more likely to be promiscuous and breed with several mates. They are also more likely to leave the other parent to care for offspring whilst they find another mate.

“The level of cooperation between parents affects the likely survival of the chicks, as well as the ability of the parents to successfully breed in the future.”

The researchers also found that when one sex is more common, there is stronger sexual selection due to having to compete harder to find mates. With male-biased species, the females tended to be larger, more ornamented or more competitive. In contrast, species where there were more females, it was the males that were larger and more ornamented.

Examples of female-biased species are peafowl, with peacocks being famous for their ostentatious tail feathers, and Great Bustards, with males being around 2.5 times the size of females.

Professor Székely said: “Our results support a one-directional evolutionary pathway: demography shapes the sex ratio, which in turn shapes social behaviour.

“This means that if one sex consistently dies younger, perhaps due to fierce competition for food, increased infection risk or higher vulnerability to predators, the sex ratio bias can influence sexual selection pressures, sex roles and mating behaviour for generations.”

Case study: Great Bustard (Otis tarda)

The Great Bustard exhibits delayed male maturation (5 years vs. 3 years in females), reducing the number of reproductively active males and resulting in a strongly female-biased adult sex ratio (0.33).

This skewed sex structure intensifies sexual selection on males, leading to extreme sexual size dimorphism (males are ~2.5 times larger than females, among the most pronounced in birds) and a mating system where males mate with multiple females, while females undertake all incubation and parental care.

Professor Tamás Székely writes about the effect of adult sex ratios on sexual selection in The Conversation Darwin got sexual selection backwards, research suggests