THE INCUBATION PATCH OF BIRDS

Summary 1. The incubation patch of birds forms in the areas of the ventrum devoid of contour feathers (apteria) by processes involving defeathering of down, dermal and subdermal hypervascularization, oedema, and hyperplasia of the epidermis and dermal connective tissue. Its formation facilitates the transfer of heat to the eggs and hatched young. 2. In general, the sex that incubates develops a patch; this may be the female, both sexes, or the male, and the species within any one order tend to exhibit a common pattern. In some orders (e.g. Pelecaniformes) no patch develops. 3. The incubation patch begins to form before egg laying in passerine birds and during egg laying in Galliformes. Most of the patch responses in passerines are completed earlier in the reproductive cycle (late egg laying to middle incubation) than in Galliformes (middle incubation to early brooding). 4. In the passerines studied, in which usually only the female develops a patch, oestrogen and prolactin synergize to cause patch development; oestrogen given alone is effective because it synergizes with endogenous prolactin in intact birds. The role of progesterone is unclear, but it seems to mimic the effects of prolactin and also plays a role in the increase in skin sensitivity characteristic of patch development in the canary. 5. In the one galliform studied (California quail), in which both sexes develop a patch, either oestrogen or androgen synergizes with prolactin; oestrogen alone is ineffective in non‐breeding quail because of insufficient endogenous prolactin levels. Prolactin alone causes epidermal hyperplasia and vascularization, and progesterone seems mainly to be involved in defeathering. In the starling also both sexes develop a patch, but testosterone plus prolactin is not effective as it is in the quail. 6. In the phalaropes, birds in which only the male develops a patch, androgen and prolactin are the effective synergists in patch development. Thus, there is a correlation between the sex which develops a patch in the wild and the steroid (oestrogen or androgen) which synergizes with prolactin. 7. In the brown‐headed cowbird, a parasitic species which neither develops a patch nor incubates, there are no responses to exogenous hormones. The absence of patch development in one sex or in both can be due either to an absence of appropriate hormone levels or to a lack of sensitivity of the skin to hormones. 8. More study is needed of the transfer of heat from the patch area to the eggs or young, including the relationships between patch structure, surface area, surface temperature, type of nest, and the number and size of eggs. 9. Patch formation is affected by and influences behaviour. 10. More research is needed in regard to (I) the natural development and endocrine control of the incubation patch in various orders of birds: (2) the effects of patch formation on behaviour, and vice versa; (3) the mode of action of the hormones in patch formation, that is, whether it is direct or indirect, e.g. through the release of another hormone; (4) the specificity of the ventral skin to hormone administration; (5) the effects of stimulation of the patch by eggs in the nest on prolactin and gonado‐trophin secretion; (6) the possible role of other, as yet untested, hormones on patch development; (7) the levels of endogenous hormones in relation to natural patch formation; and (8) possible correlations between the structure of the patch, the chronology of its formation, the clutch size and the condition of the young when hatched in various species.