- Vander Haegen, William Matthew
The Graduate School, University of Maine
Bioenergetics of female American woodcock (Scolopax minor) was studied from 1987-1989 at Moosehorn National Wildlife Refuge, Maine. A model of daily energy expenditure was developed from laboratory-derived data on metabolic rates; from data on activity and microclimates collected in the field; and from body component analysis of collected birds.
Energy demands incurred by female woodcock on the breeding grounds were highest during the pre-nesting (60.3 kcal/day) and laying (89.1 kcal/day) periods. Availability of food (earthworms [Lumbricidae]
0) is normally sufficient during theses periods, but shortages such as the one caused by persistent soil frost in the spring of 1989 can delay nesting and affect productivity. Female woodcock feed throughout the diet period prior to incubation, obtaining nutrients for reproductive tissues and to store fat for use during incubation. Incubating females spend only 8% of the day active and used endogenous reserves to supplement energy derived by feeding, losing about 75% of their body fat over incubation. In March and April 1989, energy intake was too low to initiate egg production as nesting did not occur until the frost melted and earthworm availability returned to normal, 3-4 weeks later than the typical nesting date in Maine.
Woodcock chicks are not homeothermic until 15-20 days old, and there is an inverse relationship between air temperature and brooding requirements. At air temperatures typical of the brood period, a drop in mean air temperature of 5ºC can result in a 40% decrease in time spent active, with a concomitant loss of foraging time. Rainfall also increases the brooding requirements of chicks, reducing by 30% the time spent active by chicks , 10 days old. Reduced foraging time lowers both energy intake by the female and her ability to feed the chicks.
Lack of snow cover, and freezing temperatures, influence the depth of soil frost and can reduce both food availability in spring and woodcock productivity. In addition, rather during the brood period and condition of the female at the end of incubation play important roles in determining the number of offspring produced. Thus, habitat management should strive to provide high earthworm biomass in a variety of suitable feeding sites to ameliorate the effects of weather.