The use of incubation behavior to adjust avian reproductive costs after egg laying

Reproduction in birds requires the input of time and energy during discrete breeding phases leading to investment trade-offs between laying date, clut ch size, body mass, and incubation constancy. We investigated costs during incubation by experimentally enlarging 25 clutches of white-tailed ptarmiga n Lagopus leucurus. The experiment was conducted in 2 years, one with harsh weather that forced a natural delay in reproduction. When forced to delay egg-laying, females began incubation with poorer bed; condition and foraged more during incubation. Rates of mass loss during incubation were not affe cted by clutch enlargement, and did not differ between harsh and benign yea rs; however, females that were heavier at the start of incubation lost more mass than lighter females. Clutch-enlarged females had reduced nest attend ance compared to control birds in both years and incubation periods increas ed by up to 2 days relative to controls. In the harsh year, there was a tre nd for clutch-enlarged females to have lower nest success, but there was no effect on overwinter survival. Different behavioral responses by females i n the 2 years showed that incubation costs may depend on other factors such as female quality, food supply, or weather conditions. Incubation is a dyn amic period during which birds may adjust energy balances by varying body c ondition and food intake.