Sparrows with unfaithful ‘wives’ care less for their young

Male sparrows can judge if a spouse is prone to infidelity and provide less food for their brood if they think their partner is unfaithful, a new study shows.

Sparrows form pair bonds that are normally monogamous, but many females are unfaithful to their partner and have offspring with other males.

Sparrow fledglings perching on a fenceBiologists believe that the male birds are unfaithful to ensure they father as many chicks as they can, while females are unfaithful with males of better ‘genetic quality’ – ones that are fitter and could produce stronger offspring.

However, the study involving researchers from the University of Sheffield, shows 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.

Researchers from the UK, Germany, and Australia have now revealed that males make the decision of how much to provide for their chicks based on the tendency of their partner to cheat. The study, published in The American Naturalist, followed the entire sparrow population of the island of Lundy in the Bristol Channel for 12 years.

Lead researcher Dr Julia Schroeder of the Department of Life Sciences at Imperial College London said: “Males changed their behaviour based on their partner. When they switched from a faithful partner to one prone to infidelity, they provided less food for their brood.”

The research showed that males cannot 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,” said Dr Schroeder. “Instead, the males may use cues from the female’s behaviour during her fertile period – for example how long she spends away from the nest.”

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.

The team 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, Dr Schroeder added.”

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

Professor Terry Burke from the University of Sheffield’s Department of Animal and Plant Sciences, who was also involved in the research, added: “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.

“This unusually extensive 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.”

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