Incubation scheduling by African Black Oystercatchers: effects of weather, tide phase, and time of day

Constant nest attentiveness, which benefits avian embryos, and body maintenance activities (e.g., foraging trips) by incubating parents are mutually exclusive activities. To balance these, parents must schedule the time spent on and off their nests according to a wide range of environmental conditions that change the costs and benefits of breaking or continuing an incubation bout. Here, we used data from 38 camera-monitored nests over two breeding seasons on Robben Island, Southern Africa to examine how African Black Oystercatchers Haematopus moquini, a biparental incubator, varied incubation bout length in relation to environmental variables that would have affected energy/water budgets: tide phase, air temperature, windspeed and time of day (day versus night). We predicted that incubation bout duration would decrease at high temperatures and wind speeds when energetic and water needs increase and increase at high tides when foraging opportunities decrease. Oystercatchers’ overall nest attentiveness was high at 95%. Incubation bout length was longer at high tide and at night than at low tide and during the day. Incubation bout length decreased with increases in temperature but had a non-linear relationship with windspeed that varied by the time of day: it increased sharply with windspeed and plateaued at high windspeed during the day, but largely declined with moderate to high windspeeds at night. These results suggest that oystercatchers’ energetic, thermoregulatory, and hydration needs may have increased with high heat loads during the day but decreased at night and with increased windspeed during the day, and oystercatchers responded by varying incubation bout lengths accordingly. The proximity of their nests to the intertidal zone, which means short travel time and quick access to food, water, and thermoregulatory opportunities, seems to be of vital importance to their capacity to maintain high nest attentiveness and deal with increasing heat loads.