Factors Affecting Feeding and Brooding of Gray Catbird Nestlings

Abstract

-Gray Catbirds (Dumetella carolinensis) were studied to document patterns of feeding and brooding nestlings and to relate these to nestling age, brood size, time of day, time of season, and weather. Major factors affecting feeding were nestling age and brood size. The average number of food deliveries* brood-l h-1 increased linearly with the log of nestling age for both male and female parents. Also, volume of food delivered/feeding trip increased linearly with nestling age. As brood size increased, feeding frequencies/brood increased but food deliveries/nestling decreased. Brooding and shading were affected mainly by nestling age and ambient temperature. Time spent brooding decreased substantially as nestlings grew older. Shading was important when the sun shone directly on the nest. Females spent significantly more time brooding nestlings when rain increased. As ambient temperature increased, time spent shading increased and time spent brooding declined. Received 7 May 1980, accepted 27 April 1981. ONE of the greatest stresses on birds during the breeding season is caring for their nestlings. Frequent feeding trips are required to satisfy the food demands of the rapidly developing young, and nestling body temperature must be maintained at a relatively constant level. Rates of feeding and brooding by parent birds can be related either to requirements of the nestlings or to environmental factors affecting the adults. Knowledge of these relationships is important to understand better the nestling period of altricial birds, but few workers have addressed these topics. Although the Gray Catbird (Dumetella carolinensis) is a common species throughout much of the United States and its nesting biology has been extensively documented (for a review see Johnson and Best 1980), factors affecting feeding and brooding frequencies have not been reported. Slack (1973) documented effects of temperature, time of day, and day of incubation on female attentiveness during incubation but reported very little concerning brooding of nestlings. Zimmerman (1963) collected limited information on attentive periods of female catbirds relative to nestling age. The objectives of this study were (1) to document patterns of feeding and brooding nestling Gray Catbirds and (2) to relate these to nestling age, brood size, time of day, time of season, and weather. STUDY AREA AND METHODS This study was conducted in a shrubby and partly wooded pasture near Ames, Iowa during the springs and summers of 1977 and 1978. The area covered 16 ha of which approximately 50% contained shrubs, primarily gooseberry (Ribes spp.) and multiflora rose (Rosa multifiora), suitable as nesting sites for catbirds. A stream meandered through the study site. During 1976-1977, a drought occurred in central Iowa. Rainfall in Ames was 52 cm less than the normal of 116 cm during the 15-month period, May 1976-July 1977 (Natl. Ocean. Atmos. Admin. 1976, 1977, 1978). The drought ended in August 1977, and during April-July 1978, precipitation was 9 cm above the normal of 43 cm. To locate all active nests, intensive nest searches were conducted weekly over the entire study area. The status of active nests was checked at least once daily. Nearly all adult catbirds on the study area were captured by using mist nets and marked with colored leg bands. Additionally, a spot of paint was placed on the head of one member of each pair. Nestlings were marked with a spot of paint on the top of the