Any parent will tell you how useful it can be to have family living nearby, giving a helping hand when raising your children. In humans, relatives and even non-relatives act as childminders. Such behaviour is widespread in other animals too, particularly birds.
Authors
- Jennifer Morinay
Postdoctoral researcher, University of Sheffield
- Ben Hatchwell
Professor of Evolutionary Ecology, University of Sheffield
In our recent study we explored why long-tailed tits, one of the UK's tiniest bird species, often act as foster parents instead of raising their own brood.
Since Darwin's musings on apparent altruism in honeybees , scientists have discovered that individuals of many species help each other, including bacteria, insects, shrimps, mammals and especially birds. We now know that almost 10% of bird species cooperate , helping to raise nestlings that are not their own. This is called cooperative breeding.
The question that has puzzled scientists since Darwin is why do other animals (and why do we) cooperate? The answer usually lies in a shift in focus from the individual to the genes that make them. The revolutionary work of evolutionary biologist Bill Hamilton in the 1960s (popularised in Richard Dawkins' The Selfish Gene in 1976), showed that helping family members can improve the transmission of shared genes to the next generation.
But when is it more advantageous to help family members than to raise your own kids? That is the question we asked of long-tailed tits, the most cooperative of the UK's birds. In our recent study published last month, we summarised 30 years of research on the cooperative breeding system of long-tailed tits.
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These tiny and highly social birds with their characteristic chrrr-rr-rr calls are common in UK woods and gardens. In winter, they hang out in groups of 10-20 birds . These flocks often contain close family members , such as parents with offspring and siblings. They forage together during the day and, at night, they sleep in tight huddles on branches , to keep themselves warm.
In February or March, when they sense that spring is just around the corner, the groups disband, and each bird finds a partner. Then follows what must seem like a marathon effort. Their domed nests, usually built in spiky bushes or high in tree forks, are elaborate constructions that may take weeks to complete. Females lay 9-11 eggs that they incubate until hatching two weeks later. Nestlings are fed by both parents for 16-18 days, when they fledge.
However, small carnivores and other bird species are waiting for this bonanza of eggs or chicks, and about 70% of long-tailed tit nests are destroyed by such predators every year. Pairs must then start all over again, building a new nest and laying eggs. No wonder that they can only raise one brood per year.
By early May, the season is too advanced to re-nest, and if a pair has lost their brood they give up for the year. Some lucky ones manage to reproduce, of course. Is it luck? Maybe, in part - but the secret for half of the successful nests is that they got help from family members.
Birds that fail to breed often become helpers, moving to another nest and assisting that pair in raising their offspring. Around half of all broods have helpers, typically just one or two, but up to eight at a single nest. The extra food that helpers provide increases the survival of offspring . Helpers normally choose the nest of relatives, increasing the number of birds carrying their genes in the next generation. For birds that may live for just a couple of years, this is the next best option after failing to breed successfully themselves.
Small birds with persistent family bonds
In our study, we aimed to understand how these family bonds persist and are important for the long-tailed tits. First, they tend to stay and reproduce near where they were born, especially males. As a consequence, we observed that long-tailed tits live in "kin neighbourhoods". Second, even when they move and decide to settle further afield, long-tailed tits do so with sisters and brothers, maintaining their family ties as they move.
These family ties can persist over thousands of kilometres. Long-tailed tits in the UK stay in the same area all year. However, populations in the Baltics migrate and spend the winter in central Europe. By catching groups on their southward and northward journeys , we discovered that long-tailed tits travel over large distances in family groups and end up nesting next to each other. These strong family bonds enable them to keep their support network in place.
Males help more than females, and only birds in good condition help. But, most importantly, it is the strength of the family bond - how closely related and familiar they are - that affects this decision. We also identified external factors that promote cooperation. When predation is intense there are more failed breeders seeking helping opportunities. And when the weather limits time for breeding, long-tailed tits are more likely to help others. Out of adversity comes opportunity.
We have unpicked the web of causes and effects that explain why long-tailed tits have complex social lives and a cooperative breeding system. Now, our aim is to understand how they recognise family members, whether it is with their calls, their smells, or simply because they built strong friendships over time with them.
Ben Hatchwell receives funding from a Horizon Europe Guarantee grant from UKRI, via EPSRC, for a project selected by the ERC.
Jennifer Morinay does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
