The long-tailed tit’s nest is an architectural marvel. A pair spends up to three weeks gathering moss hair and lichen to create a cosy cocoon, held together by strands of spiders’ web. Positioned in a thorny bush or tree, facing eastwards to catch the morning sunlight, the nest expands elastically as the nestlings grow inside it. There may be up to 15 chicks, although the average is around nine.
Long-tails are honorary tits, of a different family to that of our ‘true’ Irish species, the ‘blue’ ‘great’ and ‘coal’. The lifestyle is tit-like but, when it comes to raising chicks, long-tails do things differently.
If a nesting attempt fails, the aspiring parents don’t just skive off to lick their wounds; instead, they help their neighbours with nest-building and bring food to the others’ chicks. “It is better to light a candle”, the disappointed birds believe, “than to curse the darkness”.
Nor is this behaviour entirely altruistic. The neighbours’ youngsters are often their cousins, so helpers are furthering the survival of their own genes. The assistance is appreciated and new bonds are formed with the families being helped.
When flocks of long-tails begin roaming the countryside in the autumn, they will contain not only adults and their fledglings, but also kind neighbours who helped out.
There is a downside, however. In January, when it’s take-your-partners time again, the potential suitors on offer will tend to be flock members, many of them relatives. Breeding too close to one’s kin isn’t recommended; it can lead to unhealthy offspring. The ‘incest taboo’ deters humans from ‘consanguinity’.
The pharaohs and in-bred European royalty ignored that to their cost.
Birds and wild mammals solve the incest problem by ‘sex-biased dispersal’, in which male and female youngsters move on differently, males remaining closer to their natal origins than females. The separation ensures that, at mating time, would-be breeders don’t accidentally encounter their kin.
Young long-tailed males and females, however, remain close to each other and to relatives who may have helped raise them. So, do incestuous unions occur and how do long-tailed tits avoid this?
Amy Leedale and a team at the University of Sheffield have studied long-tailed tit behaviour and analysed the birds’ DNA. Comparing the relatedness of pair members, she found that there was “a reduction in fitness in inbred individuals”. Incest “is a major source of inbreeding depression”, she says. It affects hatching success and inbred individuals tend to be less fit.
Children learn to recognise the faces of their relatives and the sounds of their voices. Can young long-tailed tits do likewise?
Leedale recorded the calls of paired males and females. She found that tits prefer to choose partners whose calls differed from those of their blood relatives. The birds, it seems, recognise their siblings, and other close family members, by the sounds they make and avoid mating with them.
This ability, the researchers conclude, “offers a probable recognition mechanism” for the selecting of mates.
Being able to ‘discriminate between kin and non-kin’ is useful in other ways; it enables tits to co-operate with each other when there is mutual advantage in doing so.
Amy Leedale et al. Cost, risk, and avoidance of inbreeding in a co-operatively breeding bird. PNAS. 2020