Reproductive Effort of Widowed Birds

(1) Parent male and female great tits and blue tits (Parus major and P. caeruleus) were removed when their nestlings were at various ages and the breeding success, development of the nestlings and the change of foraging behaviour were analysed. (2) There was no significant difference in the survival of blue tit nestlings more than 6 days old and great tit nestlings older than 8 days, whether they were fed by widows or two parents (control nests). This crucial point coincides with the time when nestlings reach approximately half of their maximum weight. All nestlings younger than 3-4 days old being fed by widowed males died, and there was a significant difference in nestling mortality between control nests and those of single parents widowed before 6 and 8 days old for the blue tits and great tits respectively. (3) In first broods the mortality of the nestlings of widowed blue tits was higher than that of great tits. In second broods this relationship was reversed; changes in the environment as the season progresses may affect the breeding success of the blue tit less than the great tit. (4) Both sexes of the two species increased their feeding frequency and the number of changes of departure direction after they became widows. The higher feeding frequency was apparently stimulated by more frequent begging of the hungry nestlings. The diversity of the food delivered by widowed parents increased during the 3-4 h after their mates were removed. The feeding frequency and the change of foraging patches by the widowed parents were determined jointly by the food source and the immediate demands of the nestlings. (5) The body weight of the widowed parents was significantly lower than that of the parents feeding with their mates.