Ornis Hungarica. vol.15-16. (2008) p.7-17.
Effects of experimental nest cooling on the behaviour and body mass of female Great Tits (Parus major) during the brooding period
We cooled the nests of brooding Great Tits for 6 mornings, and examined which method the females choose to compensate and whether the greater energy need is reflected in the body mass changes over the week following hatching. In cooled nests the females brooded in shorter bouts, however their nest attentiveness did not decrease. Visit rates were 1.6 higher compared to the control group, so females with cooled nests had shorter foraging time. According to the adaptive mass loss hypothesis, females loose mass after hatching to reduce the energy costs of flight and to decrease the risk of being predated while feeding the nestlings. The stress hypothesis emphasizes that brooding is energy-demanding and that's why females loose mass. Females with cooled nests lost 0.75 gram more mass than the controls over the first week. An incubating female with cooled nest of eggs plus a single nestling also lost mass in this period, however in a smaller extent. Based on these results we think that females always loose mass after hatching, but its timing and degree is influenced by the environmental conditions over the breeding period and the energy need of brooding.