Energetic Benefits of Communal Roosting by Acorn Woodpeckers during the Nonbreeding Season

We examined the thermal consequences, energy benefits and patterns of nighttime communal roosting in Acorn Woodpeckers (Melanerpes formicivorus) during the non-breeding season, the period when they are most reliant on nutrient-poor acorn stores. Because the oak limbs (Quercus) in which Acorn Woodpeckers excavate roost cavities cool more slowly than the surrounding air, the nighttime temperature inside unoccupied woodpecker roost cavities averaged 4.3°C higher than the outside ambient temperature, when the latter was approximately 0°C. The temperature of occupied roosts was further augmented in proportion to the number of birds sharing the roost. A single woodpecker increased the cavity temperature an additional 1.2°C, whereas four birds increased it an additional 6.0°C. Acorn Woodpeckers did not huddle together at low air temperatures when placed in a simulated roost-cavity metabolism chamber. Consequently their nighttime, fasted oxygen consumption was independent of group size (one, two, or four birds) and was described by the relation: ml O2/(g hr) = 3.33-0.055 Ta. From this relation and our cavity temperature measurements, we estimate that at an outside temperature of 0°C a single cavity-roosting woodpecker would reduce its heat loss by at least 9%, whereas four birds would reduce their heat loss by at least 17%, and even more in the presence of wind. This energy savings may contribute to the higher winter survival noted for male Acorn Woodpeckers that live in larger groups.