Temperate Zone Birds Research Articles

INTERNAL AND ENVIRONMENTAL CONTROL OF BREEDING

Summary. 1 There exists in seasonal birds a partly endogenous, partly exogenous rhythm of reproduction involving successive phases of post-nuptial regeneration, acceleration and culmination which can be considered as a cogwheel which is engaged at various times by changing environmental “teeth” that are helpful, or otherwise, to reproductive success and therefore to the existence of the species. The possible neuro-endrocrine basis of these phenomena is discussed. 2 After the spontaneous progression from the regeneration phase (which seems to be constant in its duration) to the acceleration phase (when sex hormones are again liberated and spasmodic display occurs) the neuro-endocrine machinery of each individual comes under the week-to-week influence of two antagonistic sets of external factors, i.e. accelerators and inhibitors Dependent upon its length, and upon the environment in which the species exists, the regeneration (=“refractory”, “negative” or “preparatory”) phase may be an important internal “timing” device in many species. 3 Light is only one of the important post-regeneration accelerators of the sexual cycle of temperate-zone birds under natural conditions. Others are warmth (and probably sunshine per se), territory, adequate food, nest site, nesting materials and behavioural inter-actions. Inhibitors are cold, inclement weather, hunger, fear and lack of appropriate nesting material and the traditional nest-site. This is particularly true of the female. 4 External culminators (which are often also accelerators) bring the sexual cycle to a successful conclusion. No single over-all factor (such as day-length) is responsible for the timing of avian breeding seasons. The breeding season is kept in step with the sun essentially by the external factors that influence the female exteroceptor organs and ultimately permit nidification and ovulation on the habitual breeding ground. Of these, the most important may be appropriate nesting materials, a safe nesting site, the nest, mild weather and an abundance of the food on which the young are traditionally fed. Until the environment presents the pair with its species requirement the mutual behaviour essential to reproduction will not occur. 5 The sexual cycle and timing of reproduction in Arctic and xerophilous species, equatorial and transequatorial migrants, species such as the Sooty Tern of Ascension Island, as well as temperate zone birds, are discussed in the light of the above. 6 Various seasonal phenomena, the nature and significance of which are in dispute, are also discussed.