|M.Sc Student||Mordechay Tamar|
|Subject||Physiological and Behavioral Significance of the Spread-wing|
Posture in Cormorants
|Department||Department of Biology||Supervisors||Professor Emeritus Zeev Arad|
|Professor David R. Jones|
|Full Thesis text|
Cormorants (Phalacrocoracidae) are water fowl that feed exclusively on fish through pursuit-diving in both saline and fresh waters. They are known to be both good divers and good fliers. Birds are primarily adapted to a reduction in body density in order to minimize flying costs. However, a low body density means high buoyancy while diving. Cormorants cope with buoyancy by a reduction of the air volume trapped in the plumage and by temporary reduction of the plumage air volume. In theory, these mechanisms would reduce insulation and thus increase thermoregulatory costs since substantial heat would be lost during diving and drying. Nevertheless, cormorants do not consume more food compared to other water fowl. Cormorants are distinguished by their wing-spreading behaviour after emerging from a dive. Three major theories have been suggested for this behaviour: 1) as an intra-specific communication signal which indicates social rank in the colony, fishing success or sexual display. 2) aiding in drying the feathers thus reducing flying costs or to retrieve insulation, which is also a part of 3) thermoregulation, including warming via solar radiation or via heat generated by the breast muscles while wing-flapping, or heat dissipation while warm.
The present study examined the behavioural and physiological aspects of wing-spreading behaviour of cormorants, focusing on its thermoregulatory role.
No correlation was found between wing-spreading and social rank among individuals in the experimental group of Double-crested cormorants (n=5). However, a significant positive correlation was found between the amount of cold fish (4°C) consumed and flapping duration. There was no such correlation when they were fed with the same amount of warm (23°C) fish. A counter-current heat exchange system was found along the humerus, presumably allowing control of heat loss or heat gain, depending on the cormorant's thermal state. I conclude that cormorants are able to utilize heat generated by the breast muscles to warm the cold ingesta. Thermal images of stuffed and live birds revealed that the feathers can warm up substantially when exposed to solar radiation and thus reduce heat loss and even warm up. I found that factors that enhance heat loss while diving (number of dives, body mass), reduce heat loss while drying (direct solar radiation, higher ambient temperature) or inhibit drying (higher relative humidity), all promote wing-spreading. I conclude that wing-spreading behaviour aids in thermoregulation, primarily via feather drying but also through regulation of heat exchange through the wing.