Does the thermal environment influence vigilance behavior in dark-eyed juncos (Junco hyemalis)? An approach using standard operative temperature
Introduction
One of the more studied aspects of over-wintering passerines is foraging behavior coupled with anti-predator vigilance. Most studies have focused on vigilance responses to group size, group composition, and environmental features such as distance to protective cover (Elgar, 1989; Roberts, 1996). However, little attention has been given to the influence of the thermal environment on vigilance. There are nevertheless many reasons to expect that the thermal environment will have a major impact on thermoregulatory metabolism, and thus on the feeding rate and vigilance of small birds.
One would intuitively expect that birds would feed faster and be less vigilant as thermal stress increases. This expectation follows from the fact that avian metabolic rates increase as the thermal environment becomes colder (Scholander et al., 1950; Gessaman, 1972; Robinson et al., 1976; Mayer et al., 1979; Bakken et al., 1991). Thus, in colder thermal environments, more food must be ingested to both offset higher energy costs and maintain fat reserves for overnight roosting, assuming that digestive efficiency has not been altered. To increase food intake within a fixed amount of time available for feeding, a bird must feed faster and reduce its time spent vigilant for predators.
This intuitive expectation has been observed in some bird species. Yellow-eyed juncos (Junco phaeonotus; Caraco, 1979), willow tits (Parus montanus; Hogstad, 1988), tufted titmice (Baeolophus bicolor; Pravosudov and Grubb, 1995), and redshanks (Tringa totanus; Cresswell, 1994) become less vigilant as air temperature decreases. However, vigilance in male chaffinches (Fringilla coelebs; Beveridge and Deag, 1987) increased with decreasing temperature; a similar but non-significant trend was noted by Lima (1988) for dark-eyed juncos (Junco hyemalis). Apparently, foraging and vigilance responses to temperature are not necessarily intuitive.
Our goal here is to test the hypothesis that, as cold stress increases, dark-eyed juncos increase feeding rates at the expense of anti-predator vigilance. The interpretation of any correlation of vigilance with cold stress is complicated by the fact that some aspects of the micrometeorological environment have both thermal and non-thermal effects. For example, increasing wind increases convective heat loss, and so is a component of cold stress. However, wind also produces a mechanical force which results in motion of vegetation and noise. These may, in turn, impede predator detection and ultimately influence vigilance (Hilton et al., 1999).
The thermal and non-thermal effects of the microclimate may be separated by using standard operative temperature, Tes, to describe overall thermal stress (Santee and Bakken, 1987). Therefore, we measured both standard micrometeorological parameters and Tes. However, for ground-foraging birds, computing Tes from the micrometeorological data as did Santee and Bakken (1987) requires an uncertain extrapolation from instrument level to ground level. Therefore, we improved on their procedures by measuring Tes in the same space occupied by the foraging juncos and independently of the general micrometeorological data by using newly developed standard operative temperature sensors (Bakken et al., 2001). These sensors respond to all components of the thermal environment (air temperature, wind, etc.) experienced by juncos. Dark-eyed juncos are excellent study animals for this work, not only because their vigilance behavior is relatively well characterized (see Lima et al., 1999), but more critically because past work on their physiological responses to thermal stress allowed for the calibration of our thermal sensors (Bakken et al., 2001).
Section snippets
Study site and species
We conducted this study during February through mid-March 1995 at a site approximately 9 km southwest of Terre Haute, Indiana. The study site consisted of a 4×5 m ground-level concrete pad surrounded by mowed grass, with mature deciduous forest 5 m to the east, and tall herbaceous (old field) vegetation about 10 m to the south and west. An artificial brush pile held within an open wooden frame was placed adjacent to the eastern edge of the concrete pad; this provided protective cover for the
Proportion of time spent scanning
Vigilance, as measured by proportion time scanning (PTS), clearly decreased as flock size increased (Fig. 1A). Vigilance group-size effects of this basic form have been demonstrated in numerous studies (Elgar, 1989; Roberts, 1996). As outlined in Section 1, one might expect that a more stressful thermal environment would lead to a general decrease in vigilance. However, a visual comparison of PTS data from warm and cold periods (Fig. 1A) suggests that the thermal environment had little effect
Acknowledgements
C. J. Amlaner made several helpful comments on earlier versions of this ms, and kindly allowed the use of his laboratory during the lengthy videotape analyses. This work was supported in part by NSF grant IBN-9221925 to SLL.
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