Do energetic demands constrain incubation scheduling in a biparental species?

The high energetic demands of incubation in birds may be an important ecological factor limiting the evolution of life-history traits, such as clutch size. In biparental species, however, the demands of incubation may not be a major constraint because there may always be sufficient feeding time available for the off-duty bird to regain energy used during an incubation bout. We investigated whether the energetic demands of incubation constrain optimum incubation bout length in a biparental incubator by decreasing the energetic demands of incubation. We put an insulated cup around the lining of semipalmated sandpiper nests so that the rate of cooling of eggs was reduced by 21%. Semipalmated sandpipers responded by increasing their mean incubation bout length of around 11.1 h by about 10%. Bout lengths in unmanipulated natural nests became longer as hatch approached (incubation stage), and this was independent of weather. Bout lengths may have decreased with increasing rainfall and were independent of time of day. The results suggest that bout length in semipalmated sandpipers is constrained by their cumulative energetic expenditure during an incubation bout, and this is determined partly by the high costs of steady-state incubation. The results also suggest that the incubating bird determines the bout length rather than the returning bird. Semipalmated sandpipers may have maximized incubation bout length to minimize changeovers during incubation because these probably increase predation risk. Selection to minimize the frequency of changeover may their be a factor contributing to the evolution of biparental care and life-history traits in semipalmated sandpipers.