When their body temperature increases, blood flow to the areas of bare skin increases as vessels dilate.The continual renewal of the skin acts to repel parasitic microorganisms.
Handbook Of The Birds Of The World Volume 2 Skin Increases AsThe skin of birds also produces and supports feathers. With feathers, the skin also plays an important role in thermoregulation. Although largely covered by feathers, the integument is unfeathered on the beak, feet, and, in some species, other areas. In contrast to mammals, avian skin does not have sweat glands and sebaceous glands. The outer layer, the epidermis, is generally very thin and pliable. Handbook Of The Birds Of The World Volume 2 Free Nerve EndingsThe dermis is thicker than the epidermis and contains blood vessels, fat deposits, nerves and free nerve endings, several types of neuroreceptors, and smooth muscles that move the feathers (Lucas and Stettenheim1972). The epidermis is thinnest in areas covered by feathers (both feather tracts and apteria; Figure 1) and thickest in exposed, featherless areas, including the covering of the beak (rhamphotheca) and feet (podotheca). The epidermis has two main layers a superficial stratum corneum and a deeper strateus germinativium (Figure 2). The stratum corneum consists of flattened, keratinized cells. These cells are called keratinized because they contain a protein called keratin (and, specifically, beta keratin) that, along with extracellular lipids (fats) produced by epidermal cells, provide a tough, permeability barrier that prevents excessive evaporative water loss. The stratum corneum can be viewed as having a brick-and-mortar organization, with the keratin-enriched cells forming the bricks and the extracellular lipids the mortar (Elias and Menon 1991). However, compared to reptiles and even mammals, cells in the stratum corneum have less keratin and, as a result, this barrier is less stringent and can facilitate evaporative cooling while retaining the capacity for facultative waterproofing (Menon et al. The high body temperatures of birds, increased heat production during flight, insulation by plumage and the lack of sweat glands, require a higher rate of evaporative cooling through a relatively leaky epidermal permeability barrier. ![]() For example, Menon et al. Zebra Finches can reduce water loss via the epidermis by 50 by the rapid secretion of epidermal lipids. A similar ability to influence water loss by regulating secretion of epidermal lipids has been reported in larks (Haugen et al. House Sparrows ( Passer domesticus; Muoz-Garcia and Williams 2008), and the tropical Dusky Antbird ( Cercomacra tyrannina; Muoz-Garcia and Williams 2007). In addition, epidermal lipids are used for cosmetic coloration in the Japanese Crested Ibis ( Nipponia nippon; Wingfield et al. Before breeding, the skin of the neck and head starts secreting a black substance that the ibises apply to their white plumage (Figure 3). The extent of the secretory skin area and how much of the plumage is covered by the cosmetic varies among individuals and this variation plays a role in mate choice. For example, the skin on the head is unfeathered to varying degrees and distinctively colored in guineafowl, vultures, colies ( Colius ), and many storks, ibises, spoonbills, and cranes. The skin around the eyes is unfeathered and distinctively colored in other birds, such as cariamas, falcons, sheathbills ( Chionis ), parrots, cuckoos, broadbills, bare-eyes ( Phlegopsis ), lyrebirds ( Menura ), and helmet-shrikes ( Prionops ) (Stettenheim 2000). More generally, patches of bare skin, other than the bill and legs, can be found in birds belonging to at least 19 different orders and 62 families (Negro et al. ![]() A,FJ, reproduced with permission from VIREO; B,C,L, reproduced with permission from Kenneth Fink; H, reproduced with permission from Nate Rice; D, reproduced with permission from Steve Zack; K, reproduced with permission from Tom Schulenberg (From: Prum and Torres 2003). For example, skin in the unfeathered areas of a Crested Caracaras ( Polyborus plancus ) head has a much denser supply of blood vessels than skin in feathered areas (Figures 5 and 6). Although they sometimes feed on carrion, the bare skin of caracaras is most important for thermoregulation. Caracaras are relatively large birds with generally dark plumage that are typically found in relatively hot areas.
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