Physiological ecology of Mediterranean Blue Tits (Parus caeruleus) I. A test for inter-population differences in resting metabolic rate and thermal conductance as a response to hot climates

Abstract

Blue Tits (Parus caeruleus) are widely distributed throughout Europe, reaching the southern limit of their range on islands in the Mediterranean and in northern Africa. On Corsica, one population located at Pirio in the Fango Valley breeds roughly one month later than populations in adjacent valleys or on the continent, thus exposing nestlings to high ambient temperatures (T(a)). We tested the hypothesis that nestlings and possibly adult Blue Tits at Pirio would exhibit a reduction in resting metabolic rate (MR) and an increase in thermal conductance as a physiological response to high T(a). We compared the thermoregulatory response and evaporative water loss for nestlings from Pirio in Corsica and one continental site (Vic-le-Fesq) and for adults from two Corsican (Pirio and Muro) and one continental site (La Rouvière). At 12-14 days of age, nestlings from Pirio showed two distinct thermoregulatory patterns. Nestlings under 8.0 g behaved as heterotherms, whereby MR was correlated only with body temperature. At body masses above 8.0 g nestlings progressively acquired the ability to regulate T(b) and at masses >9.0 g they behaved as homeotherms. When considering homeothermic nestlings and adults, population of origin did not affect either thermal conductance or resting MR. For homeothermic nestlings, mass-specific resting MR (mW x g-(1)) was 15.5 +/- 2.6 and 17.5 +/- 2.5 for nestlings from Vic-le-Fesq and Pirio, respectively. For adults, mass-specific resting MR (mW x g-(1)) was 17.5 +/- 2.0, 17.8 +/- 1.6, and 17.9 +/- 1.0 for birds from Pirio, Muro, and La Rouvière, respectively. Although there was a weak but positive effect of T(a) on evaporative water loss for homeothermic nestlings, no such trend was evident for adults over the range of T(a) tested in this study. We thus find no evidence to indicate that either nestlings or adults exhibit the exponential increase in evaporative water loss associated with the non-convective regulation of T(b) within the range of T(a) tested (roughly <or=35 degrees C). We conclude that there is no evidence for a specific physiological adaptation in the Pirio population. Measures of nestbox temperatures indicate that nestlings rarely experience temperatures in excess of 33 degrees C. We conclude that, although some years may be hot enough to impose a thermal stress, temperatures at Pirio are not high enough to consistently impose a selective pressure for physiological adaptations to heat.