Risking life for wife
The male willow tit incurs great danger to take care of his mate.
Willow tits pair for life. The male protects the female from other aggressive and dominant males and alerts her as soon as he detects deadly enemies, such as sparrowhawks and small owls.
But the males don’t do this just to be nice.
A male willow tit has a high metabolism, and it is energetically costly for the male to sacrifice time to keep the female alive through the winter. But he doesn’t do this out of altruism – helping out has clear advantages for him.
Since winter mortality is consistently highest in females, a widowed male could have trouble finding a new mate quickly enough for spring breeding. Chances are also high that he would be replaced by a young pair if he loses his mate. Therefore it is important to take good care of the female he’s with.
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The tits’ positions in trees are far from random when they congregate in the winter. The birds compete to forage for food high up. Adult birds look for food in the upper half of the trees and juveniles in the lower half. The adult female keeps to the top, well protected by her mate, who forces the young males to stay further down. Success for predatory flying owls or hawks is greatest when they attack low, since they are less easily detected there than if they fly high.
Hierarchy in the winter flock
The male defends his territory with a little help from the female. The willow tit is one of the few northern perching birds that defend their territory throughout the year. During the winter, the pair flocks together with unrelated juveniles who have settled down after their brief fall wandering.
A strict hierarchy exists within the flock: a territory is dominated by the alpha male, the undisputed number one. The juvenile males are number two and dominate the adult female. The young females are at the bottom of the ladder.
During periods of cold weather and high energy needs, the flock stays together to reduce the risk of being preyed upon. More pairs of eyes can detect an enemy faster than fewer. Birds’ placement in trees is often a result of social rank. (See figure above.)
When the adult female visits a feeding place, she is constantly on guard and checking for safety. If everything seems safe, she can forage 60 per cent of time. If her mate is near, she can be less vigilant and can increase her foraging time. As soon as a juvenile male approaches, she becomes more cautious and loses 20 percent of foraging time compared to when she is alone. (translation of x axis: Alene= Alone; Med Make= With Mate; Med Ung Hann= With Young Male. Translation of y axis: Prosent0 Per cent)
The adult male chases away juvenile males that are aggressive towards his mate.
The benefits
By protecting the female through the winter, the male keeps his mate in good condition for the breeding season. By spending more time eating in winter, she can get an early start with egg laying – a necessary condition for raising robust young. Young birds that leave the family flock before other flocks have the best chances of securing a territory and carrying on their parents’ genes.
Leaving the family flock only a few days later than other young birds stacks the odds against the laggards. They often end up as floaters who fly around with minimal opportunities to breed.
That means that the male’s altruism toward the female also results in a better chance for grandchildren, since the early-out juveniles have the greatest chance of obtaining their own territory.
