Alcid Feathers Wet on one Side Impede Air Outflow Without Compromising Resistance to Water Penetration

. While diving, small alcids appear wet, suggesting that their feathers repel water poorly. Nevertheless, once wet, their plumage resists water penetration effectively. This study's objective was to evaluate the effect of feather wetting on the critical penetration pressure of air (maximum pressure for resistance to air penetration, Pa) and water (Pw). We measured these values in breast feathers of Xantus's Murrelet (Synthliboramphus hypoleucus) and Cassin's Auklet (Ptychoramphus aleuticus) under two conditions : dry (feathers with separated barbs) and wet on one side (feathers with aggregated barbs). Dry feathers of both alcids did not resist air penetration at any pressure, but feathers wet on one side resisted 1.25 kPa. For Xantus's Murrelet Pw was 1.44 kPa, for Cassin's Auklet 1.36 kPa. These results support the hypothesis that plumages that appear wet could minimize heat loss by reducing the exchange of air within the plumage while the bird is diving.