Rhythms in the Breeding Behaviour of the European Wren

Abstract

Two nesting cycles of the European Wren Troglodytes t. troglodytes were studied by means of recording apparatus at the nest augmented by direct observation. Data were obtained concerning nest-building and -lining, egg-laying, incubation, feeding nestlings and roosting. The first incubation probably began in the afternoon. In both nestings under-brooding was compensated for by an unusually long session after an unusually long recess. The relationship between the hourly amount of brooding and temperature is represented by a curvilinear regression from a maximum of about 75 % around 40° F. With rising temperature recesses tend to increase in length and sessions to decrease. The average session-plus-recess, defined as the incubation-cycle span, was found to remain constant, independently of environmental conditions, within certain restricted limits of diurnal variation. This span differs for different individuals. During the first brood Wrens frequently settle on the eggs well before sunset but after the hatch the length of the working day increases slightly in the morning and considerably in the evening. During the incubation period in 1952 the male apparently visited the nest on several days between 08.00 and 12.00. Hatching of all the eggs in one nest took about 21 hours. In 1951 the male went to the nest several times with food on the day of the hatch but did not give it to the nestlings. He fed the first-brood chicks only during the second week of the nestling period. In 1952 the male never fed the nestlings of the first brood. Regular interruptions in the day to day increase in the frequency of visits during the nestling period may have been due to physiological strain. Daily trips with food for the nestlings showed approximately 5-hourly peaks of activity in 1951 and 4-hourly peaks in 1952. Probably these represent an endogenous rhythm. A diurnal decline in tempo with a ratio of about 3 : 2 was noted in the number of sorties by one incubating female and in the foraging sorties of both females during the latter part of the nestling period. The evidence suggests another endogenous rhythm. During the early phase of the nestling period, activities showed a major morning and minor evening peak, but later this effect almost disappeared. Brooding of the young decreased by 6%-8 % daily. At one of the nests the male helped increasingly to feed the chicks as the female's efforts decreased but reduced his efforts as hers increased. This inverse relationship suggests that the importunity of the nestlings is of importance in regulating the amount of food brought to the nest. In polygynous birds, such as the European Wren, the male may act as a reserve available to forage for the young in adverse circumstances. Polygyny in the Wren is integrated with other adaptations including multiple nest-building and persistent song. It implies family-size adequate to maintain the race but not too large to be beyond the capacity of the female to feed. One of the nests was visited by the female on a number of occasions after the young fledged. The fledglings roosted in the disused nests of other species.