Seasonal variation in nest predation rates in boreal forests

Nest predation is the most important factor causing reproductive failure in open-cup nesting birds. Therefore, it represents a strong selection agent shaping reproductive strategies. To understand how predation affects avian populations, it is crucial to understand how predation effects on nesting success vary in space and time, and which ecological factors determine variation in predation rates. Artificial nests are commonly used to evaluate nest predation rates, but their use is criticised for not providing predation rates comparable with natural nests, either because timing of predation or predator species targeting the nests differ. However, use of artificial nests allows us to standardize nest characteristics between sites and across time, even outside the breeding season of most birds. Here, we present data on 576 artificial nests gathered over several years that explore how nest predation rates vary over the year. We found that predation rates on artificial nests show seasonal variation where rates are high during the bird breeding season and low outside the breeding season. Avian and mammalian predators tend to display different nest predation behaviours. The frequency of predation events by avian predators follows a similar seasonal pattern as predation rates, whereas mammalian predators show elevated, but stable, frequency of predation events across the breeding season. Our study supports the commonly reported temporal variation in nest predation rates, indicating that avian predators actively search for bird nests as food during the bird breeding season, while mammalian predators seem to prey on bird nests incidentally.