Ecological Energetics and Foraging Behavior of Overwintering Bald Eagles

Abstract

The ecological energetics and foraging behavior of overwintering Bald Eagles (Haliaeetus leucocephalus) were studied in 1978—1979 and 1979—1980 on the Nooksack River in northwestern Washington and in the laboratory. We investigated eagle energy requirements and energy relationships in a winter environment, and adaptations to winter energy and food (chum salmon [Oncorhynchus keta]) stress. We developed a model to predict the energy metabolism of free—living eagles. Information of food consumption and energy requirements of captive Bald Eagles was supplemented with laboratory and field data to determine energy costs of free existence. Basal metabolic rate, as measured by oxygen consumption on four eagles, was 11.595 kJ°g—1°h—1; a lower critical temperature of 10.6°C was recorded. Standard metabolic rate increased linearly with decreasing temperature. Thermal conductance was 0.347 kJ°g—1°h—1°°C—1. The diel range in body temperature was from 38.9° to 41.2°. Energy metabolism was measured in response to artificially induced rainfall. During 40 trials of 4 h of continuous rain, metabolism increased up to 9 and 21% at 6.1 and 22.2 cm/h rain levels, respectively. Effects of rain on heat loss by wild eagles on the Nooksack River were negligible even though precipitation was high. Data on metabolic responses were incorporated in a black—body heat—budget model which predicted energy costs of free existence. Ambient temperature, wind velocity, long—wave radiation, and rainfall data collected at three microhabitats used by eagles also were incorporated into the heat—budget model to determine heat production by wild birds. Activity budgets were assessed by tracking four radio—tagged eagles for 38 d. Flight activity occurred only during 1% of the 24—h day, and energy costs of flight (110 kJ/d) were included in the model. The daily energy budget (total energy metabolized), daily energy consumption (total food energy required), and daily food (salmon) consumption (total mass of food required) were 1703 kJ, 2068 kJ, and 489 g, respectively, for a 4.5—kg Nooksack eagle. Energy and food needs of free—living eagles were °10% more than for captive eagles. Most eagles roosted in coniferous rather than deciduous forest even though they expended more energy to travel there. By roosting in conifers they accrued a net daily energy savings of 61 kJ, or °5% of the daily energy budget after accounting for energy costs of flight to and from the roost. Energy savings in this protected microclimate were afforded by higher ambient temperature and long—wave radiation levels, and lower wind velocity; reduced rainfall had little effect. Social interactions by feeding eagles were quantified during 46 d of observation. Adult eagles were dominant over younger birds and were more successful at kleptoparasitizing (stealing) food. Kleptoparasitism was the primary means by which food was acquired. Interaction frequency averaged 0.27 interactions/min during feeding and was positively correlated with the size of the feeding group. Juveniles and subadults were less effective at feeding than adults. They consumed 410 and 459 g°bird—1°d—1 of salmon, respectively, while the adults ingested 552 g°bird—1°d—1. Young eagles failed to acquire the needed 489 g°bird—1°d—1 of salmon. The effects of this socially mediated food deprivation resulted in a less than optimal use of time and energy by young birds. When food is limited, young eagles probably incur the highest mortality rates. Our data indicate that overwintering Bald Eagles are effective at exploiting and conserving energy. They maximize energy gain by foraging in groups, gorging, and assimilating more energy during cold stress. They minimize energy loss by becoming sedentary, seeking protective microclimates, reducing nocturnal body temperatures, and reducing foraging costs by living in groups. Because Bald Eagles are suspected to be food—limited, protective management policies that reduce energy stress could reduce overwinter morality.