Experimentally advancing morning emergence time does not increase extra-pair siring success in blue tit males

Extra-pair paternity occurs frequently in socially monogamous birds, but there is substantial variation in extra-pair siring success among males. Several studies have shown that siring success relates to the timing of morning activity, with the earliest active males being more successful, suggesting that early activity is important for acquiring extra-pair copulations. However, these studies are correlational, and it, therefore, remains unclear whether the relationship between timing and extra-pair siring success is causal. An alternative explanation is that successful extra-pair sires tend to be active earlier (e.g., because they are of high quality or in good condition), but that early activity in itself does not increase siring success. We experimentally advanced the emergence time of male blue tits by exposing them to light about half an hour before their natural emergence time. Although males that were exposed to the light treatment emerged from their roost substantially earlier than males that were exposed to a control treatment, light-treated males were not more likely to sire extra-pair offspring. Furthermore, whereas control males showed the expected relation between emergence time and siring success (although not statistically significant), there was no relation between emergence time and extra-pair siring success among light-treated males. Our results suggest that the timing of emergence from the roost is not an important factor underlying extra-pair siring success. Offspring sired outside the pair-bond occur regularly in socially monogamous birds. Males that are active earlier in the day tend to be more successful in siring extra-pair young, but it is unclear whether this relationship is causal. We experimentally advanced the emergence time of male blue tits, but this did not increase their extra-pair siring success. Our results challenge the idea that early activity per se increases extra-pair siring success

Automated summary

Extra-pair paternity is common in socially monogamous birds, but there is variation in siring success among males. Previous studies suggest that early activity is important for acquiring extra-pair copulations, but it is unclear whether this relationship is causal. We experimentally advanced the emergence time of male blue tits, but found no increase in their extra-pair siring success.