Barn swallows depend on abundant flying insects for food and show population declines across Europe linked to collapse of aerial invertebrate communities.
Swallow chicks in nests with insufficient insect provision face slow starvation as adults cannot provision them adequately in poor insect years. Weather events causing prolonged cool wet periods during breeding season cause mass chick mortality from starvation and hypothermia. Loss of traditional farm buildings with open access doors removes nest sites that populations have used for generations. Adults in poor nutritional condition from inadequate insect supply enter autumn migration in compromised condition, reducing survival to the following breeding season.