Diversity of incubation rhythms in a facultatively uniparental shorebird - the Northern Lapwing.

In birds, incubation by both parents is a common form of care for eggs. Although the involvement of the two parents may vary dramatically between and within pairs, as well as over the course of the day and breeding season, detailed descriptions of this variation are rare, especially in species with variable male contributions to care. Here, we continuously video-monitored 113 nests of Northern Lapwings Vanellus vanellus to reveal the diversity of incubation rhythms and parental involvement, as well as their daily and seasonal variation. We found great between-nest variation in the overall nest attendance (68-94%; median = 87%) and in how much males attended their nests (0-37%; median = 13%). Notably, the less the males attended their nests, the lower was the overall nest attendance, even though females partially compensated for the males' decrease. Also, despite seasonal environmental trends (e.g. increasing temperature), incubation rhythms changed little over the season and 27-day incubation period. However, as nights shortened with the progressing breeding season, the longest night incubation bout of females shortened too. Importantly, within the 24h-day, nest attendance was highest, incubation bouts longest, exchange gaps shortest and male involvement lowest during the night. Moreover, just after sunrise and before sunset males attended the nest the most. To conclude, we confirm substantial between nest differences in Lapwing male nest attendance, reveal how such differences relates to variation in incubation rhythms, and describe strong circadian incubation rhythms modulated by sunrise and sunset.