Elsevier

Journal of Thermal Biology

Volume 26, Issue 6, November 2001, Pages 605-612
Journal of Thermal Biology

Does the thermal environment influence vigilance behavior in dark-eyed juncos (Junco hyemalis)? An approach using standard operative temperature

https://doi.org/10.1016/S0306-4565(01)00007-9Get rights and content

Abstract

One might expect that increased thermal stress would cause wintering birds to forage faster in order to meet the increased metabolic demand. Faster foraging should, in turn, lead to a reduction in vigilance, since feeding and vigilance are mutually antagonistic activities. We examined these intuitive behavioral expectations using newly developed standard operative temperature sensors designed specifically to characterize the thermal effect of the microclimate environment (rain excluded) on wintering dark-eyed juncos (Junco hyemalis). These sensors allowed us to distinguish the behavioral effects of thermal stress from non-thermal effects associated with micrometeorological conditions (e.g. wind noise). Our analysis indicated that neither thermal nor non-thermal aspects of the physical environment influenced the proportion of time spent vigilant by juncos. However, the rate of food ingestion (measured as pecking rate) exhibited a negative correlation with thermal stress per se. This unexpected result may reflect the effect of thermal stress on feeding posture, peripheral muscle cooling, or both. The effect of thermal stress on pecking rate was nevertheless minor in comparison to the effect of flock size, which exerted by far the largest effect on both vigilance and pecking rate. Our overall results suggest that birds experiencing thermal stress will not necessarily lower their vigilance, but rather increase feeding bout length to compensate for the greater metabolic demand. This interpretation is consistent with theoretical models of vigilance in a non-time-limited environment, and may help explain the contradictory results to date on the effect of thermal stress on vigilance.

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.

References (30)

  • F.M Beveridge et al.

    The effects of sex, temperature, and companions on looking-up and feeding in single and mixed species flocks of house sparrows (Passer domesticus), chaffinches (Fringilla coelebs), and starlings (Sturnus vulgaris)

    Behaviour

    (1987)
  • Blomstrand, E., 1985. Muscle metabolism during intensive exercise—influence of subnormal muscle temperature. Acta...
  • Boysen, A.F., 1998. Over-wintering dark-eyed junco (Junco hyemalis) foraging and vigilance behavioral responses to the...
  • G.S Campbell et al.

    An Introduction to Environmental Biophysics

    (1998)
  • T Caraco

    Time budgeting and group sizea test of theory

    Ecology

    (1979)
  • Cited by (0)

    1

    Present address. 175 Chittenden #2B, Columbus OH 43201, USA

    View full text