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Below the lower critical temperature (LCT), the metabolic costs of thermoregulation linearly increase at decreasing air temperatures. UCT: Upper critical temperature; TNZ: thermoneutral zone; BMR: basal metabolic rate. Represented values of LCT and UCT are approximates for the studied species of forest passerines (Kendeigh et al. 1977). 

Below the lower critical temperature (LCT), the metabolic costs of thermoregulation linearly increase at decreasing air temperatures. UCT: Upper critical temperature; TNZ: thermoneutral zone; BMR: basal metabolic rate. Represented values of LCT and UCT are approximates for the studied species of forest passerines (Kendeigh et al. 1977). 

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Aim: To disentangle the subtleties of the relationship between the thermal environment and the distribution of wintering birds, at multiple spatial scales. Specifically, to analyze the relative influence of temperature on species distributions, its interaction with various thermal and non-thermal factors, and the context-dependence of these relatio...

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... analyzing species abundance through quantile regression analysis (Cade & Noon 2003) the emphasis is put on the areas where temperature is actually limiting abundance over other factors, even taking into account uncertainties in the response variable associated with data quality in extensive sampling. Thus, the limiting influence of temperature is measured at the maximum response of organisms (i.e., maximum ecological abundances; Figure 1a). By means of the use of linear and quadratic terms of temperature, its curvilinear influence can be estimated, allowing us to obtain two important parameters describing the response to thermal gradients: the 'environmental preferred temperature' that maximizes the species' abundance within the thermal span of the study region; and the breadth of the thermal response considering the area under the curve parameterized by the regression coefficients, which represents the plasticity of species in utilizing the thermal space (Figure 1b). These analyses can be carried out considering the partial effect of temperature, either in climate- only models that consider the linear and quadratic terms of temperature and precipitation, or in more complex models where other environmental predictors are ...
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... analyzing species abundance through quantile regression analysis (Cade & Noon 2003) the emphasis is put on the areas where temperature is actually limiting abundance over other factors, even taking into account uncertainties in the response variable associated with data quality in extensive sampling. Thus, the limiting influence of temperature is measured at the maximum response of organisms (i.e., maximum ecological abundances; Figure 1a). By means of the use of linear and quadratic terms of temperature, its curvilinear influence can be estimated, allowing us to obtain two important parameters describing the response to thermal gradients: the 'environmental preferred temperature' that maximizes the species' abundance within the thermal span of the study region; and the breadth of the thermal response considering the area under the curve parameterized by the regression coefficients, which represents the plasticity of species in utilizing the thermal space (Figure 1b). These analyses can be carried out considering the partial effect of temperature, either in climate- only models that consider the linear and quadratic terms of temperature and precipitation, or in more complex models where other environmental predictors are ...
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... richness was significantly lower in winter 2009-2010 than in 2008-2009 for both average number of species per census ( Figure 1B) and the accumulated number of ground- foraging species (P<0.005 in the two paired ...
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... has shown an increasing trend in Spain in the last 25 years (de Castro et al. 2005), consistent with the global increase in mean surface air temperatures detected in the mid-and high-latitudinal continental regions of the northern hemisphere since 1976 (IPCC 2001). This increase has led to important changes in the onset of breeding, clutch size and hatching and fledging success in Mediterranean montane populations of several passerine species (Sanz et al. 2003, Potti 2009). Nevertheless, little is known about the influence of temperature on winter bird biology in this region (Senar and Borras 2004). If survival of small birds is particularly sensitive to the combination of low temperatures and food shortage in winter (Newton 1998), then an increase in temperature could play a prominent role in those populations inhabiting cold areas-habitats, such as forests in montane areas or northern latitudes (e.g. Butler et al. 2007, Gregory et al. 2007). Our results are consistent with these expectations, as the short term variation of bird abundance is tightly correlated with nocturnal winter temperature. Moreover, there have been marked general increases of these species in the last decade in the same region of cold Mediterranean climate (northern Madrid province; Palomino et al. 2006; significant yearly population growth rates for Nuthatch 11.6%, Great Tit 4.8% and Blue Tit 8.6%). This effect may be acting despite the fact that fewer young usually fledge with warmer temperatures (see Potti 2009 for the study region). Recently, Figure 1. Partial residual plots illustrating the influence of food treatment (a) and average night temperature (b) on the relative abundance of tree-gleaning birds using artificial feeders in oakwoods of Central Spain during winter [2009][2010]. n = 40 oakwood plots (20 oakwoods with the experimental addition of artificial feeders, and 20 not supplemented). Partial residuals are obtained after controlling for the remaining variables considered in the analysis of Table 2. Vertical bars show mean+standard error. Cresswell et al. (2009) have shown that the increase of 6.5ºC from 1995 to 2005 in mean daily winter temperature decreased the starvation risk of Great Tits in England (birds responded to this scenario by decreasing their body mass). Therefore, global warming may exert a positive influence in woodland temperate avifaunas, favouring winter residency and enhancing winter survival, thus translating into positive population trends on a regional ...
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... relationship between population density and temperature would be dependent on resources, because individuals adjust their foraging activity to meet the energetic demands of the environments they occupy and bird densities often change in relation to varying food supplies (Newton 1998). Therefore, at low resource levels (control plots) we could expect a tighter relationship between abundance and temperature than at high resource levels (supplemented plots). However, we found that temperature affected abundance independently of resource levels. According to species-energy theory this suggests that our study area in winter is, despite food supplementation, a low- energy scenario where the relationship between biodiversity and energy is predicted positive linear ( Phillips et al. 2010). Indeed, average minimum night temperatures were low and within the range of 0.05 to 2.54 ºC (see Fig. 1b), so birds spend a very long proportion of the winter day without foraging and at temperatures approximately 20 ºC below the thermoneutral zone (average night duration: 14.4 h or 60% of a winter day). Moreover, there must be a limit to acclimatization capacity, even in overabundance of food, mainly because there is a limit to the amount of reserve a bird can carry to survive overnight (McNamara andHouston 1990, Witter andCuthill 1993). In such conditions, temperature would be limiting even if food resources are abundant. Therefore, even under abundant food, birds may have remained limited by ambient temperature. Their limit of cold tolerance could have improved due to food availability, but not to a point where they can physiologically escape from the effects of ...
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... the seven a priori models exploring the among woodland plot variation in bird abundance (Table 2), the model including night temperature and food supplementation was the one with the highest strength of evidence (model weight, W i = 0.772; R 2 = 0.335). Its weight of evidence was considerable higher than that of the model including only habitat characteristics (W i = 0.006). The food- supplemented factor and average night temperature were the predictor variables with the highest magnitude effects (weighted averages of β = 0.42 for both variables). Density of large trees was also an important predictor variable, although its magnitude effect was considerably lower (weighted average of β = 0.294). The rest of predictor variables played a minor role determining bird numbers of small passerines exploiting artificial feeders (absolute values for weighted averages of β < 0.17; Table 2). Thus, birds were more abundant in mature oakwoods supplemented with food and with higher average nocturnal temperatures (see Fig. 1 for the partial residual plots of food supplementation and night temperature with the relative abundance of birds after controlling for all other ...
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... one half of the observed spatial variation in wintering bird species richness (49.4%) was explained by an average model including oakwood maturity (first vegetation PCA axis in Table 1) and minimum night temperature as the most influential variables (ΣW i >0.8 and highest magnitude effects; two models with ΔAICc<2; Table 2). Winter species richness increased in parallel to minimum night temperature and oakwood maturity (Fig. ...
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... air temperature was highly correlated with minimum night temperature across oakwood plots in the two study winters 2008-2009 and 20092010 Figure ...
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... applicability of the habitat use pattern described in this paper cannot be generalized to every environmental condition or geographical location. Geographic and orographic variation in winter sun incidence defines the background where the phenomenon may be ecologically relevant for these species. For example, cloudiness and day length vary from less than 50 % of the 9-10 h of winter daylight in southern Europe to more than 70 % of less than 5 h of daylight at latitudes above 55º (Henderson-Sellers 1986). Thus, possibilities for minimizing costs of thermoregulation by selecting sunlit patches are scarce at higher latitudes with few clear days available in winter. Moreover, this behavior can only operate in mountainous areas where the hill-shade effect introduces a large variation in sun radiation at small spatial scales. For example, in the study area, the potential sun radiation received at different nest box locations varied enormously both in length (from 15 min to 7 h) and in magnitude (from 15 to 3,604 kJ/m 2 in the winter solstice day; Fig. 1). Therefore, selection of roost sites according to differential solar radiation received by a forest patch could play an important role at mountainous areas of cold climate with frequent anticyclonic ...
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... test the hypotheses in Figure 1, local abundance of birds before the cold wave (B-CW), during the cold wave (CW) and after it (A-CW) were compared using t-tests. The comparison was carried out by Monte Carlo analyses ( Davison and Hinkley 2007). First, we obtained the t-test statistics for each species comparing the averages of the 15 woodland plots in B- CW vs. CW, and in CW vs. A-CW. Second, a randomization process was carried out maintaining the data within each plot (i.e., the values of the average number of birds recorded in [B-CW / CW] and [CW / A-CW]). The aim of this randomization procedure was to preserve the spatial structure of the data, accounting for the spatial autocorrelation of the data and for the possible pseudoreplication derived from the fact that the same individual bird may be present in more than one woodland plot on different days. Third, the t-tests were carried out considering the randomized data for each species in the B-CW vs. CW, and CW vs. A-CW comparisons, thus obtaining a null t statistic; this process was repeated 9,999 times. And fourth, the actual figures of the t statistic testing B-CW vs. CW, and CW vs. A-CW were compared with 9,999 null t values obtained. Significance of t-tests was estimated considering the position of these true t statistics within the null distribution of t figures, by means of the percentiles using a two-tailed ...
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... study was conducted on 52 days around winter solstice (from 23 th November 2010 to 13 th January 2011), i.e., the most stringent period of the winter considering the relative duration of day and night (9:14 h -14:46 h, respectively, at winter solstice). The study area has 159 wooden nest boxes installed in 2008 in a grid at a regular distance of c. 50 m between adjacent boxes (c. a density of 2.6 nest boxes per hectare; Fig. 1). These nest boxes have been used as regular breeding sites since 2008 by the pied flycatcher (Ficedula hypoleuca; transaharian migrant) and the following three resident species: blue tit (Cyanistes caeruleus), great tit (Parus major) and nuthatch (Sitta europaea). In a pilot survey outside the study period of 52 days, we observed that nest boxes were only used as winter roost sites by blue tits, great tits, and nuthatches and that each nest box was only used each night by one individual (i.e., communal roosting was never observed). We used wooden nest boxes (base 11 × 13 cm, height 20 cm, 15 mm thickness of walls, centered entrance of 30 mm diameter), with a plastic pipe of 9 cm long installed in the entrance to avoid predation by Mustelidae, the garden dormouse (Eliomys quercinus) and the great spotted woodpecker (Dendrocopos major). Nest boxes were hanged from branches with a metal hook at an average height of 3.76 m (0.49 SD). The occupation of nest boxes may be favored by the scarcity of natural cavities in this managed forest ( Camprodon et al. 2008). Occupation of nest boxes by blue tit, great tit, and nuthatch was registered during the preceding breeding season (spring 2010; data obtained by OG in the context of other research ...
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... General Linear Mixed Model (all effects: F 151,228 = 1.41, p = 0.010, 48.2% of the variance accounted for; Table 1) shows that distance to cover (partial regression coefficient, β = -0.352) and temperature (β = 0.266) had significant effects on feeding intensity in the ten foraging patches (Figure 1). Birds spent more time feeding at 'near' (mean + se, 47.8 + 5.8 s / 10 h, n=38 birds) than at 'far' feeders (12.0 + 2.6 s / 10 h), and at five warmer (48.6 + 6.6 s / 10 h) than at five colder feeders (18.4 + 3.8 s / 10 h). Neither the species nor the individual birds and the interaction terms birds x predictors reached significance (temperature, light intensity and distance to cover), which means that the described pattern of feeder use is generalizable among species and individuals within species. Moreover, the interaction terms between distance to cover and temperature (F 1,74 = 0.302, p = 0.584) or distance x light intensity (F 1,74 = 0.318, p = 0.574) were also non- significant, showing that the positive influence of temperature, or the lack of effect of illumination, did not change between near and far from vegetation cover. Distance to cover, which may provide refuge against predators, was the predictor variable with the highest magnitude effect (partial  2 = 0.66), followed by temperature at feeders (0.25). Therefore, the influence of distance to refuge was 2.6 times higher than that of temperature in determining the foraging intensity at feeding patches, while light intensity had a negligible and no significant ...
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... to our prediction, wintering birds did not show any preference for roosting at lower altitudes (Tables 2 and 3). Due to orographic effects in our study area, the patches located at higher altitudes turned out to be those with a higher sun incidence (Table 3, Figure 1). This counterintuitive preference for high elevations due to increased sunshine was also shown in black-and-white snub-nosed monkeys wintering in the Trans- Himalayans ( Quan et al. ...
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... responses to climate extremes may range from temporal migration of individuals, to population crashes, to long-term changes in community structure or ecosystem function (see reviews of Jiguet et al. 2011 and Smith 2011b, and references therein). In a global warming scenario, the emphasis is rarely put on extreme cold events (e.g., Jentsch andBeierkuhnlein 2008, Cattiaux et al. 2013). Nevertheless, severe weather might imperil the energy balance of wintering small passerines beyond normal thresholds. At temperate latitudes, birds have to cope with winter temperatures well below their thermoneutral zone King 1974, Kendeigh et al. 1977), and their metabolic costs of thermoregulation increase with decreasing temperatures and increasing wind speeds (eg. Wolf and Walsberg 1996). Accordingly, these species select the warmest areas at a landscape scale to overwinter, where the Figure 1. Three possible mechanistic scenarios of response of bird populations to the studied cold wave (grey area). All are postulated to work over the background steady increase in population abundance usually registered in these forests from the winter solstice to the end of the winter season (thin dashed line). Postulated tendencies over the study period (thick solid line) and tendencies until the end of February (thick dashed line) are highlighted. The cold wave could either: not influence species abundance at all, promote a temporal migration of individuals, or provoke a widespread mortality of the population. thermoregulation costs are minimized (e.g., Canterbury 2002, Meehan et al. 2004, Evans et al. 2006, La Sorte et al. 2009, Carrascal et al., 2001, ...
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... birds were able to identify small variations in temperature across foraging patches, devoting more time to relatively warm patches and likely reducing the thermoregulation expenditure while searching for food ( Figure 1A). Birds also perceived the potential risk of being predated, spending more time foraging at safer patches with a close refuge available to escape from predators ( Figure 1B). Noticeably, minimizing predation risk was much more important than reducing thermoregulation metabolic costs in these Mediterranean forests of relatively mild winter ...
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... birds were able to identify small variations in temperature across foraging patches, devoting more time to relatively warm patches and likely reducing the thermoregulation expenditure while searching for food ( Figure 1A). Birds also perceived the potential risk of being predated, spending more time foraging at safer patches with a close refuge available to escape from predators ( Figure 1B). Noticeably, minimizing predation risk was much more important than reducing thermoregulation metabolic costs in these Mediterranean forests of relatively mild winter ...
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... we analyze the resistance to change of the abundance and local distribution of three resident woodland passerines facing a cold wave that hit Europe in February 2012. The studied species belong to a tree-gleaning guild wintering in montane areas of the Iberian Peninsula. We postulate three possible mechanistic scenarios of response to this extreme cold wave (Figure 1), that would work over the steadily increase in population abundance usually registered in these forests from the winter solstice to the beginning of the following breeding season (O Gordo, JJ Sanz and J Potti, com pers). First, the cold wave may have not negatively influenced species local abundance, because they have local adaptations to cope with severe weather (e.g., changes in foraging behavior and habitat use). In this case, the natural steady increase in abundance should be maintained or only slowed down during the cold wave. Second, the cold wave should impose a temporal migration of part of or the whole wintering populations, which would return after the end of the cold wave to their winter territories and restore normal abundance levels. And third, the cold wave could provoke widespread population mortality, so local abundance would decrease during the cold wave but would not recover after reaching previous levels. To test these hypotheses, we monitored local abundance of birds before, during and after the cold wave, measuring the local environmental conditions in terms of temperature, wind speed and wind chill. We also analyzed the environmental determinants of the spatial variation in abundance within the study area, to detect if individuals staying during the cold wave would move to the warmer and less windy forest patches to mitigate the deleterious effects of wind ...
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... show triangular-shaped (see example in Figure 1 and data in Appendix S2). However, the importance of temperature in determining winter bird distribution depends on the species, as there is a broad range of versatility of how species utilize the available winter thermal space (i.e., the thermal breadth of the studied species varies from 0.225 to 0.975; Appendix S2; Moussus et al., 2011). Our results suggest that winters will be less restrictive for birds wintering in the Iberian peninsula under future climate warming scenarios (IPCC, 2007;Brunet et al., 2009), though the impact of changes will depend on species-specific thermal plasticity, and will be more intense in those species with a narrow thermal breadth that prefer higher winter temperatures and that mainly rely on arthropods and fruits as winter food (e.g., Alcedo atthis, Upupa epops, Ptynoprogne rupestris, Luscinia svecica, Cisticola juncidis, Phylloscopus collybita, Sylvia melanocephala, Carduelis chloris, Serinus serinus). Future studies should explore the biological traits associated with interspecific differences in winter thermal preferences, considering the phylogenetic relatedness and differences among species in body mass, habitat selection, diet, metabolic characteristics and biogeographic ...
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... distributions in temperate ecosystems are strongly influenced by environmental temperature at various scales (e.g., Shields and Grubb 1974, Root 1988, Wachob 1996, Meehan et al. 2004, Evans et al. 2006, Butler et al. 2007, Davies et al. 2007, Kalmar and Currie 2007, Whittaker et al. 2007, MorenoRueda and Pizzarro 2008, Quian et al. 2010, Velky et al. 2010, Zuckerberg et al. 2011). This may be especially true in the energy-limiting period of winter, in which food resources are scarce, the time available for foraging is shortened, adverse weather conditions are unpredictable, and the low temperatures continuously challenge homeothermic animals ( Gibb 1954, Fretwell 1972 Figure 1; Figure 2). Survival in this scenario depends on maintaining a positive energy balance, obtaining enough food for self- maintenance and reducing the metabolic costs of thermoregulation (e.g., Newton 1998, Rogers and Reed 2003, Cuthill and Houston 2008. Thus, species are expected to overwinter at warmer areas, where thermoregulation costs are reduced and the foraging environment is indirectly improved (i.e., higher fruit production, higher arthropod activation and reduced snow cover on foraging substrates; Avery and Krebs 1984, Honek et al. 1997, Carrascal et al. 2001, Robinson et al. ...
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... was a very broad variation in environmental variables among nest box locations (range: min-max); potential cumulative sun radiation: 15-3,604 kJ/m 2 (on winter solstice), altitude: 1,281-1,599 m, average minimum night temperature (from 1 st December to 13 th January): -0.60 to -0.01 ºC, average diurnal temperature (from 1 st December to 13 th January): +3.80 to +4.90 ºC, oak height: 4-15 m, oak density: 6-127 oaks per 314 m 2 . Only 38 of a total of 159 available nest boxes were occupied for winter roosting (24%). Most nest boxes chosen by birds for winter roosting were not occupied the previous breeding season (81%), and only 12% of nest boxes used for breeding were later used for winter roosting (Fig. 1). Table 2. Table 2. Mean and standard deviation (SD) of seven variables describing the characteristics of occupied (OCCUP, n=38) and unoccupied (UNOCCUP, n=121) nest boxes for winter roosting. Statistical significance of the differences between means were obtained using t-tests and Monte Carlo analysis building null distributions of t statistics (9,999 randomization trials; see Data analyses for more details). Significance tests are one-tailed according to predictions in Table 1. Potential sun radiation is the cumulative radiation obtained in an anticyclonic sunny day in winter solstice. According to predictions derived from the hypotheses explaining winter preferences for roost sites (Table 1), characteristics of woodland locations with occupied nest boxes significantly differed from those with unoccupied nest boxes in only the amount of potential sun radiation and average oak height (Table 2). Birds preferred woodland locations with higher potential sun radiation and taller oaks around nest boxes. Nest box occupancy in the preceding breeding season did not show the predicted effect: wintering birds preferentially roosted in nest boxes not used during the previous breeding season. Altitude and temperatures did not show the predicted effects: occupied roost sites were 52 m higher, had nearly identical minimum night temperatures, and were 0.13 ºC colder during the day than unoccupied nest boxes (one-tailed p's > 0.85). Oak density around the nest boxes did not influence their occupancy. Therefore, selection of woodland locations to spend the night under identical controlled situations (i.e., roosting within the same wooden nest boxes), was only dependent on subtle variations in sun incidence related to topography and maturity of the ...
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... Birds are small endotherms prone to rapid heat loss, whose metabolic thermoregulation costs are known to directly increase with decreasing temperatures (Figure 1), increasing wind speed and diminishing sun radiation (see references in the ...

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