Seasonal plumage condition variation and the thermal value of the feather coats of house sparrows (Passer domesticus)

Feathers are critical to how birds thermoregulate, and thus their total energy balance. The feather coat insulates birds by trapping air next to the skin and acting as a physical barrier to heat loss. Despite previous work studying thermal balance in birds, relatively few studies have focused on the thermal contribution of the feather coat alone; most studies have focused on physiological and behavioral responses. Moreover, to our knowledge, no studies have directly measured the effect of feather wear through the annual cycle on the thermal performance of the feather coat. To address this, we used a thermal camera to measure the temperature at the surface of the feather coat of live house sparrows (Passer domesticus) in winter (post-molt, unworn feathers) and summer (pre-molt, worn feathers), as well as flat-skins of the same specimens in order to isolate the thermal effect of the feather coat alone. We predicted that worn feather coats would lose more heat than unworn feather coats in both live birds and flat skins. Surprisingly, we found that feather wear had no effect on the thermal performance of the feather coat across seasons. The thermal balance of birds will be better understood when the thermal contribution of the feather coat is directly measured across more species and conditions.