Blue tits divorce their partners if they turn up late to annual breeding season.
About 85 per cent of birds are socially monogamous, meaning they form couples and share the workload of raising their young. Staying together long-term is thought to be beneficial because they can focus on breeding and parenting rather than having to look for new mates.
However, break-ups have been observed in 92 per cent of these socially-monogamous species, including blue tits.
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To work out why some blue tits divorce, at the Max Planck Institute for Ornithology in Germany and her colleagues studied over 100 breeding pairs in a southern German forest for 8 years. The birds were microchipped to identify when they visited 277 breeding boxes in the area.
The study revealed advantages to being faithful: females that mated with the same male during consecutive breeding seasons laid eggs 1 day earlier, had 0.5 more eggs, and produced 0.7 more chicks on average. The couple’s familiarity meant they could get down to business quicker and capitalise on optimal food and laying conditions.
Tits call it quits
In spite of this, only 20 to 50 per cent of blue tit couples stayed together from one year to the next.
Unlike in other bird species, this didn’t seem to have anything to do with the couple’s previous level of breeding success or competition from other potential suitors.
Instead, most divorces appeared to be caused by one partner showing up at the annual spring breeding session later than the other. If one partner had to wait more than 3 days for the other to arrive, they were 5 times less likely to stay faithful.
“For a species with high mortality like the blue tit, it makes sense to pair with a new mate rather than waiting around, because they don’t know if their former mate is alive or dead,” says at Monash University in Melbourne, Australia. “They need to hurry up while the conditions are getting just right to start breeding,” she says.
According to Gilsenan, the reason some blue tit pairs may get out of sync is because they end up in separate winter flocks that return to breeding sites at slightly different times in spring.
This seems like a sensible theory, says Peters. However, it doesn’t explain divorce in all bird species, because many factors like habitat, competition and life expectancy are at play, she says. For example, her research has found that bust-ups in Australian purple-crowned fairy-wren couples are usually driven by with better nesting conditions.
