Microclimatic characteristics of blue grouse Dendragapus obscurusroost-sites: influence on energy expenditure

Energetic models which incorporate environmental measures have demonstrated that significant thermoregulatory savings are accrued from nocturnal winter roost-sites, usually from reduced wind speed and radiated heat loss. Because blue grouse Dendragapus obscurusoccupy high elevation, snowbound coniferous stands in the Rocky Mountains during winter, selection of a favourable microhabitat is likely their primary thermoregulatory behaviour. Therefore, we measured the microclimatic conditions at diurnal and nocturnal roost-sites of blue grouse to determine whether their choice of roost-sites reflects thermoregulatory behaviour. Temperature, wind speed, and solar radiation were measured at 17 diurnal Douglas-fir Pseudotsuga menziesiiand 17 nocturnal subalpine fir Abies lasiocarparoost-sites and compared to those of an open control site in Logan, Utah, 1985–1986. Temperature varied <2°C between the roost-sites and the control site. Wind speed was significantly lower in 15 of 17 diurnal ( = 0.71 m/sec) and all nocturnal roost-sites ( = 0.24 m/sec) than in the control site ( = 1.75 m/sec). Wind speed was reduced >75% at all but one nocturnal roost-site. Solar radiation at the diurnal roost-sites ( = 51 W/m-2) was significantly lower than at the control site (201 W/m-2); however, five roost-sites had maximum values >90% of the control maximum. Douglas-fir roost-sites had significantly greater solar radiation, diurnal, and nocturnal wind speed than subalpine fir roost-sites. Reduction of convective heat loss was the major thermoregulatory contribution of both diurnal and nocturnal roostsites. Diurnal roost-sites also afforded measurable radiant energy and, presumably, grouse could track the sun in roost trees to maximize such heat input. Daily energy costs predicted from metabolic equations incorporating temperature and wind speed were below the metabolizable energy intake of captive blue grouse. Application of the average microclimatic conditions from both roost trees to an energetic model revealed that a blue grouse would realize a 50% greater reduction in convective heat loss, and a 10% greater net energy savings, by roosting overnight in a subalpine fir rather than a Douglas-fir. This difference may explain why blue grouse show affinity to subalpine firs for nocturnal roosting, and points to the energetic importance of specific coniferous habitats to wintering blue grouse.