Other male birds are also two-timing their ladies, but it was always assumed that the first female partners weren’t negatively affected by this, since the males would mainly work to support their primary brood.
PhD researcher Koosje Lamers and professor Christiaan Both at the RUG discovered that this is not, in fact, the case. Females who are forced to share their mate with another female have lower survival chances and tend to die more often in spring.
For twelve years, the ecologists studied a large population of European pied flycatchers brooding in nest boxes in Drenthe. The majority of the population is monogamous, but some males were entertaining multiple females.
These males leave the nest as the female is brooding, flying over to another nest box to visit another female. Lamers and Both concluded that especially females who lay their eggs early in the season have to contend with their mate having a second female.
In one experiment, the researchers moved Dutch pied flycatcher females to Sweden, where they brood earlier than in the Netherlands. The males these females had as mates also hit the road more often to get themselves a second mate.
Early brooding can result in a female having to share her mate, with considerably negative effects. ‘Not for her young so much as for herself’, says Lamers.
In their research, which was recently published in the Journal of Avian Biology, the researchers write that the number of young who survive their first year is not dependent on whether their father is monogamous. The females, however, are affected. Those who had to share their mate had a much harder time surviving until the next year.
Lamers and Both suspect that females get less support from their mates if the latter is two-timing her. As she most likely takes on a larger part of the care of her young, this is at the expense of her own health and future.