The influence of nest size on heat loss of Penduline Tit eggs

The size and quality of nest influence mate choice and parental behaviour in several birds. Previous works in Penduline Tits showed that females prefer to mate with males that build large nests, and females desert small nests whereas they incubate eggs in large nests. We tested whether female preference may be related to the benefits of having large nests by investigating the influence of nest size (height, volume and thickness) on cooling rates of eggs and terminal egg temperatures in the laboratory using two ambient temperatures (10 degrees C and 25 degrees C), and clutch sizes (3 and 9 dummy eggs). Cooling rate of eggs was slower at 25\degrees C than at 10 degrees C, and 9 eggs cooled slower than 3 eggs. In addition, cooling rates decreased with nest thickness, although they were unrelated to nest height and volume. The influence of nest thickness on egg cooling rates was stronger at 10 degrees C than at 25 degrees C indicating that nest thickness is particularly important to insulate the eggs in relatively cold environment. Our results are thus consistent with a previous field study that showed a stronger female preference for large nests early in the breeding season, when ambient temperatures are low, than late in the season.