A novel non-invasive method for measuring the body mass of hole nesting birds during breeding

Capturing birds during egg-laying or incubation to determine their condition often results in brood abandonment – up to 40% of broods for the great tit Parus major. At the same time, the weight of birds during the feeding of young is most often completely different (lower) than during incubation. Hole nesting birds are frequently the object of research, as they easily accept artificial nesting sites, i.e., nest boxes. We tested the possibility of determining the weight of an incubating female great tit without its capture by modifying the nest box to make it easily removable, allowing the female to be weighed along with the box. By taking measurements in such a way, we were able to determine the weight of the incubating female without breeding losses and brood abandonment. We also present changes in the weight of the same individuals (N = 15) during successive broods in the same season. These females were weighed twice during the incubation and feeding of nestlings during two broods (6 measurements in total per season). The presumed pattern was found, i.e., reduction in weight during the feeding of young. Females at the beginning of incubation of the first and second broods had similar weights, indicating that they rebuild their condition very quickly after the first brood's young have fledged. The proposed method is particularly recommended for determining the weight of birds during the initial stages of reproduction (nest building, egg laying and incubation) during which birds are most vulnerable to disturbances after having been captured.