Delayed incubation leads to hatching failure of a Burrowing Owl (Athene cunicularia) nest

Birds face strong selective pressures to complete individual nesting attempts as quickly as possible to minimize exposure of nests to predators and weather, maximize renesting potential, and maximize hatching success. As a result, the duration of developmental periods and overall nest periods are often relatively constant within species. However, birds may sometimes be subject to acute energetic constraints that may preclude them from initiating incubation at the optimal time. We report an extraordinary case of delayed incubation by a female Burrowing Owl (Athene cunicularia) in Oregon, USA, which contrasted sharply with a large sample of nests monitored by motion-activated video cameras in western North America from 2015 to 2018. The focal female delayed incubation by 11 days following clutch completion and subsequently experienced near-total hatching failure of her clutch. This observation corroborates previous experimental studies regarding the limits to egg viability and trade-offs between the timing of incubation onset and hatching success. These findings illustrate the acute trade-offs faced by nesting birds, and the cost of delayed incubation that we observed in this owl could help explain the cause of asynchronous hatching.

Methods
We placed motion-activated infrared video cameras into Burrowing Owl nest boxes (underground "artificial burrows") at five study sites in western North America including at the Umatilla Army Depot in Oregon, USA. Each camera was connected to a mobile DVR programmed to record a 2.5-minute clip each time there was motion inside the burrow. We reviewed all videos in their entirety from the laying and hatching stages, to identify the time each egg was laid and hatched. From the laying of the final egg through the hatching of the first egg, we reviewed at least four 2-hour periods per day, including at least two nocturnal hours. We recorded the start time and stop time of each incubation bout and incubation recess to the nearest second. We ignored incubation bouts and recesses lasting <60 seconds in duration (which often involved an incubating bird repositioning herself or turning the eggs) because we assumed that changes in egg temperature during bouts <60 would be negligible (Naylor et al., 1988, Cooper & Voss, 2013). 

To calculate daily attentiveness, we defined the laying stage as the period from the day of nest initiation through the day of clutch completion, and we defined the incubation stage as the period from the first full day after the last egg was laid through the last full day before the first egg hatched. We did not include the last day of laying or the first day of hatching in our calculation of daily nest attentiveness during the incubation stage because nest attentiveness is typically lower when the clutch is not complete. We defined Incubation Period Length as the days elapsed between the laying of the final egg and the hatching of the final viable egg. We calculated hatching success (Proportion Hatched) as the number of hatched eggs divided by the number of eggs in the full clutch.