Initiation and Termination of Daily Feeding in Dark-Eyed Juncos: Influences of Predation Risk and Energy Reserves

Abstract

After an overnight fast, small wintering birds such as dark-eyed juncos (Junco hyemalis) may wish to initiate feeding as soon as light permits. Initiating feeding in dim light, however, entails a considerable risk of predation, not only because predators are difficult to detect but also because nocturnal and diurnal predators may be active simultaneously. Experimental manipulations showed that juncos initiated feeding in very dim light only when food was available in the relative safety of protective cover. Furthermore, juncos with reduced energy reserves initiated feeding before those with normal reserves, strongly suggesting that feeding initiation reflects a trade-off between starvation and predation. Additional work showed that juncos terminated daily feeding at much higher light intensities than those at which they initiated feeding; they relinquished up to 1 h of potential feeding in the evening. Juncos apparently apportioned their daily foraging so as to avoid the vening increase in risk as dimness approached. It is argued that the risk of feeding on dim light should be accepted only when energy reserves are low, such as at dawn.