Authors: POLO, V. and CARRASCAL, L. M.
Published: Volume 44(2), December 1997. Pages 215-224.
Language: Spanish
Original Title: El ciclo diario de ganancia de peso durante el invierno en una población silvestre de Parus cristatus en el centro de España
Keywords: body weight, Central Spain, daily cycle, temperature, wild population and winter.
Summary:
The winter diurnal cycle of body weight variation was studied in a wild population of Crested Tit Parus cristatus living in a mixed coniferous forest in Central Spain (Pinus sylvestris, Castanea sativa, Acer spp.; continental cold Mediterranean climate; 1500 m.a.s.l., 40º45'N 04º01'W, Sierra de Guadarrama). Supplemented food (husked peanuts) was available throughout the study period. Mean daily temperature during the study period (January 2 - February 17, 1997) was 4.2 ºC. Crested tits were trapped using especially designed funnel-traps. Body mass was measured to the nearest 0.01 g with an electronic balance. Body weight was negatively related to average temperature (p=0.029; an average decrease of 0.027 g per ºC, Table 1). Average diurnal increase in body weight was 0.08 g/h and 6.9% of body weight at dawn, being these figures lower than in other wild populations of Northern Europe. This increase in body weight was markedly and significantly exponential (Figures 1 and 2; determined by means of nonlinear regression using the model W=a+bT+c[H-7]e, where W: weight, T: temperature, H: hour GMT). The exponent e (Table 1) was significantly larger (p=0.001) than the expected value for a linear model of diurnal weight increase (e=1). The percentage of variance accounted for by the nonlinear model (Table 1) was significantly larger (p=0.028) than that the percent variance explained by a linear model (W=a+bT+c[H-7]). Body weight gain (Figure 3) was lower one hour after dawn (0.04 g/h) than at midday (0.08 g/h), and was also lower at midday than one hour before dusk (0.12 g/h). The discrepancy between our findings for the studied Mediterranean population with previous reports from laboratory studies and field studies carried out at higher latitudes (model e<1 in Figure 1) is explained on the basis of the higher winter temperatures (-5 to 10 ºC) and the shorter nights (13-14 h) which occur in Central Spain as compared to winter conditions farther north, as well as taking into account the strong starvation treatments performed in most experimental studies with captive birds. The benefits of carrying lower levels of body reserves may be related to a reduction of metabolic costs during activity and to an improved manoeuvrability, both of adaptive value against predation risk.
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