BEHAVIORAL THERMOREGULATION BY TREECREEPERS: TRADE-OFF BETWEEN SAVING ENERGY AND REDUCING CRYPSIS

Abstract

We studied the effect of solar radiation on the winter biology of Short-toed Treecreepers Certhia brachydactyla inhabiting a montane forest in Spain. We hypothesized that, in temperate latitudes of cold winter climate, with low cloudiness and under windless conditions, birds should select sunlit sites (i.e., forest sectors or trunk patches with high levels of exposure to sunlight) to reduce the metabolic cost of thermoregulation. At a within-habitat scale, a hypothesis of “only metabolic benefits” predicts that birds should select sunlit patches at shade temperatures (Tshade) below the birds' lower critical temperature (Tlc). They should shift to a random use of sunlit and shaded patches at temperatures above Tlc. Alternatively, there could be added costs (e.g., travel costs, predation risk) in using sunlit patches. If higher visibility leads to diminished crypsis at sunlit patches (“trade-off with predation risk” hypothesis), birds should select only shaded patches at Tshade values above Tlc (to enhance crypsis). They should increasingly select sunlit patches as Tshade decreases below Tlc. Treecreepers were selective in their use of sun–shade patches, across different spatial scales. At the among-plots scale, treecreeper abundance was positively related to the availability of sunlit trunks, after we controlled for the effects of tree density, prey availability, and altitude. At the within-plot scale (i.e., selection of foraging patches on trunks and thick branches), Ivlev's electivity for sunlit patches decreased linearly as Tshade increased. Birds preferred to forage on sun-exposed surfaces (electivity >0) when Tshade was lower than ∼4°C, but they tended to forage on shaded surfaces (electivity < 0) when Tshade was higher than ∼9°C. The selection of sunlit trunk patches at low temperatures was not a by-product of higher food availability, because numbers of prey were much less predictable than temperature as a function of trunk exposure (sun vs. shade), and the negative relationship between use of sunlit patches and temperature remained significant when we controlled for the effects of prey availability. Thus, the selection of sunlit patches at low temperatures can be interpreted as a behavioral thermoregulation strategy allowing birds to save energy. However, the Ivlev's electivity for sunlit patches became negative at temperatures well below Tlc, and no sunlit patches were used when Tshade ≈ Tlc, which led us to reject the “only metabolic benefits” hypothesis. Photometric measurements of treecreeper taxidermic mounts realistically positioned on trunk surfaces, and detection times by simulated (human) predators, suggest that treecreepers were more detectable under direct solar radiation than in deep shade. Crypsis diminished in sunlit patches because of a higher image contrast and an increased difference in perceivable coloration between bird and background, relative to shaded patches. Average detection times were significantly lower for sunlit mounts. Moreover, focal birds scanned more frequently in sunlit than in shaded patches. Thus, the observed temperature-dependent variation in the selection of sunlit substrata is consistent with the “trade-off with predation risk” hypothesis predicting that prey should avoid patches where they are more detectable to potential predators. We interpret the distribution and behavior of treecreepers as indicative of a trade-off between the energy savings (through higher operative temperature and reduced metabolic costs) and the increased risk of predation (through higher visibility and diminished crypsis) afforded by sunlit foraging patches.