The image on the left of a Chinese Pond Heron (Ardeola bacchus) by Chris Lee a.k.a. chrisle023 shows the bird with its feathers ruffled and raised.
Such a situation usually occurs when the heron adopts a threat display. According to Martinez-Vilalta & Motis (1992), bitterns of the genus Botaurus, “crouches with its neck arched backwards and its bill pointing towards the intruder; the wings are spread and held slightly forwards, while the feathers of the crest are erected and those of the neck and the scapulars bristled up in order to make the bird appear larger.” This posture was taken in the presence of intruders, including photographers. In this Chinese Heron, although the bird was standing, it was most probably adopting a threat display.
Birds also fluff their feathers to create an insulating layer of air that keeps the heat trapped against the body surface (Evans & Heiser, 2004).
1. Evans, H. E. & J. B. Heiser, 2004. What’s inside: Anatomy and physiology. In: Podulka, S., R. W. Rohrbaugh Jr & R. Bonney (eds.), Handbook of bird biology. The Cornell Lab of Ornithology, Ithaca, NY. Pp. 4.1-4.162.
2. Martinez-Vilalta, A. & A. Motis, 1992. Family Ardeidae (Herons)]. In del Hoyo, J., A. Elliott & J. Sargatal (eds.), Handbook of the birds of the world. Vol. 1. Ostrich to ducks. Lynx Editions, Barcelona. Pp. 376-429.
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