Nocturnal Energy Expenditure of Dark-Eyed Juncos Roosting in Indiana during Winter

To determine the thermoregulatory significance of roosts used by Dark-eyed Juncos (Junco hyemalis) wintering in Indiana, we measured microclimatological variables inside and outside the roost and combined these measurements with earlier laboratory studies carried out in a wind tunnel. Air temperatures and vapor densities measured inside the roost did not differ from those outside the roost; however, the roost did afford significant shielding from winds present outside the roost, reducing wind speeds by about a factor of five. Shielding from wind was about four times as important to the birds' heat budget as was shielding from long-wave radiative exchanges. Despite the great reduction in wind speed, energetic cost of occupying the roost was 2.1 × SMR as opposed to 2.3 × SMR outside the roost. The selection of open roosts by small birds has limited advantages for energy conservation at low air temperatures. Fasting time, rather than direct thermoregulatory stress, is probably the critical factor for small birds roosting in winter.