After recording, the video frames were analysed with the
software INVAS (Buschmann and Dambach, 1997) for periods
of transitions showing a changeover of breeding between the
parental animals of the nest. The duration of this relieve deter-mined the period of light exposure to the eggs, where the start
was defined as the first frame with a completely exposed egg and
the end time as the subsequent coverage of at least 50% of the
egg by one of the breeding birds. The advent of eggs and hatching
could be seen during the first cover up after the egg was laid
or the offspring hatched. Finally, the times during disturbances
due to water/food change or cleaning were excluded from the
measured time periods and the number of relieves.