Physiological ecology of Mediterranean Blue Tits (Parus caeruleus) 1. A test for inter-population differences in resting metabolic rate and thermal conductance as a response to hot climates

Blue Tits (Parus caeruleus) are widely distributed throughout Europe, reach ing the southern limit of their range on islands in the Mediterranean and i n 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 i ncrease-in thermal conductance as a physiological response to high T-a. We compared the thermoregulatory response and evaporative water loss for nestl ings from Pirio in Corsica and one continental site (Vic-le-Fesq) and for a dults from two Corsican (Pirio and Mure) and one continental site (La Rouvi ere). At 12-14 days of age, nestlings from Pirio showed two distinct thermo regulatory patterns. Nestlings under 8.0 g behaved as heterotherms, whereby MR was correlated only with body temperature. At body masses above 8.0 g n estlings progressively acquired the ability to regulate Tb and at masses >9 .0 g they behaved as homeotherms. When considering homeothermic nestlings a nd adults, population of origin did not affect either thermal conductance o r resting MR. For homeothermic nestlings, mass-specific resting MR (mW . g( -1)) was 15.5 +/- 2.6 and 17.5 +/- 2.5 for nestlings from Vic-le-Fesq and P irio, respectively. For adults, mass-specific resting MR (mW . g(-1)) was 1 7.5 +/- 2.0, 17.8 +/- 1.6, and 17.9 +/- 1.0 for birds from Pirio, Mure, and La Rouviere, respectively. Although there was a weak but positive effect o f T-b on evaporative water loss for homeothermic nestlings, no such trend w as evident for adults over the range of T-a tested in this study. We thus f ind no evidence to indicate that either nestlings or adults exhibit the exp onential increase in evaporative water loss associated with the non-convect ive regulation of T-b within the range of T-a tested (roughly less than or equal to 35 degreesC). We conclude that there is no evidence for a specific physiological adaptation in the Pirio population. Measures of nestbox temp eratures indicate that nestlings rarely experience temperatures in excess o f 33 degreesC. We conclude that, although some years may be hot enough to i mpose a thermal stress, temperatures at Pirio are not high enough to consis tently impose a selective pressure for physiological adaptations to heat.