Female birds are moving to busier areas, it seems, leaving behind lonely males; and that could worsen the declines of smaller populations.
A long-term study of thousands of Willow Warblers has shown that females tend to favour busier breeding areas, with abundant resources, and male to female ratios there are more equal.
But in the smaller territories formed due to habitat fragmentation, males are outnumbering females, leading to a lack of offspring, worsening further the declines in those areas.
The study showed that in 1994, male to female ratios of Willow Warblers were approximately 50:50, but by 2012 males formed nearly 60% of the population, with the greatest imbalances occurring in the areas with lower numbers of birds.
The effect is made worse by the fact that female birds also tend to have lower survival rates than male birds.