Cheating mates turn birds into deadbeat dads

If a male house sparrow detects that his mate is cheating, he provides less food for their brood, a new study finds.

Male birds are unfaithful to ensure they father as many chicks as they can, but females will cheat with males of better ‘genetic quality’—ones that are fitter and can produce stronger offspring.

Researchers say that cheating comes with a cost—the cheating female’s partner will provide less food for their nest of young. It has long been suspected that males know that not all the chicks in their nest are likely to be theirs, and so make a decision to provide less. But an alternative explanation is that cheating females and lazy males tend to pair up naturally.

For a new study, researchers followed the entire house sparrow population of the island of Lundy in the Bristol Channel for 12 years.

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“Males changed their behavior based on their partner,” says Julia Schroeder of the life sciences department at Imperial College London. “When they switched from a faithful partner to one prone to infidelity, they provided less food for their brood.”

Males can’t actually identify whether all the chicks in their nest are theirs or not, and instead base their feeding decision on who their female partner is.

“If chicks were switched into another nest where the female was faithful, then the male at that nest continued to feed the chicks, suggesting they have no mechanism, such as smell, to determine which chicks are theirs,” Schroeder says. “Instead, the males may use cues from the female’s behavior during her fertile period—for example how long she spends away from the nest.”

Published in The American Naturalist, the study followed 200 males and 194 females as they formed 313 unique monogamous pairs and hatched 863 broods on Lundy. Some sparrow ‘divorces’ occurred—but most changes of life partner were due to a death.

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Researchers tested the DNA of every sparrow, allowing them to build up precise family trees, and find out which females were most unfaithful and which males they cheated with. “Lundy is a unique natural laboratory because it is almost a closed system—very few birds leave the island or arrive from the mainland. In the entire 12 years only four birds immigrated to Lundy, possibly carried by boat,” Schroeder says.

Researchers are continuing to study the Lundy sparrows to uncover how and why social behaviors like monogamy arose. Being unfaithful may be a costly for females because they can only lay a limited number of eggs. It may be a hangover from when their ancestors were not monogamous, rather than a useful strategy for getting the strongest offspring.

“Many previous studies have shown that male birds reduce their care to their brood if it includes chicks sired by other males—which happens when their mate is promiscuous. This makes evolutionary sense. However, males cannot identify their own chicks and we did not know how they decided how to behave,” says Terry Burke, professor of animal and plant sciences at the University of Sheffield.

This study “has revealed that males are apparently able to tell if their mate has a tendency to be unfaithful and then invest accordingly in her offspring. This rule of thumb saves the male from needing perfect genetic information. This does raise the question of how a male knows that a female is likely to be faithful.”

Source: University of Sheffield