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House Sparrow

A new study has shown that male sparrows are able to judge if a spouse is prone to infidelity and will provide less food for his brood if his partner is unfaithful.

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

Biologists believe that male birds are unfaithful to ensure that they father as many chicks as possible, while females are unfaithful with males of better ‘genetic quality’ that are fitter and therefore could produce stronger offspring.

However, cheating comes with a cost and 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 theirs and so decide to feed less. But an alternative explanation has been put forward 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 partners to cheat.

The study, which has been published in The American Naturalist, followed the entire sparrow population of Lundy, an unspoilt island that lies 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 study also found that females may also change their behaviour when paired with a less lazy male, cheating less with a more attentive father.

The research showed that males cannot actually identify whether all the chicks in their nest are theirs or not, and instead base their decisions on how much to feed on who their female partner is.

"If chicks were switched into a nest where the female was faithful, then the father at that nest kept up his hard work providing for 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. Although some sparrow ‘divorces’ occurred, most changes of a life partner were due to a death.

The team DNA genotyped every sparrow, allowing them to build up precise family trees and find out which females were most unfaithful and who their cheating males were.

Lundy is a unique natural laboratory because it is almost a closed ecosystem with very few birds leaving or arriving from the mainland. In the entire 12 years of the study, only four birds immigrated to Lundy, most likely carried by boat.

Dr. Schroeder will continue 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 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 producing the strongest offspring.

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