Bimodal Occurrence of Breeding in an Equatorial Sparrow.

In an earlier paper' I have reported findings concerning the male reproductive cycle in a population of Andean sparrows, Zonotrichia capensis, at equatorial latitudes. It was shown that each male has a double cycle in the course of a year so that there are two periods of breeding capability, the length of each averaging about four months. Thus any one bird is at reproductive capacity eight months out of the year and, because the cycles of individuals are not necessarily synchronized in the population, males in breeding state are available at all times. In the analysis of cyclic breeding activity in birds it is most important to draw a sharp distinction between the culminative phase2 of breeding, namely the actual involvement in nesting and production of fertile eggs, and the phase of male reproductive potentiality, which potentiality may or may not be exercised. The environmental stimuli involved in timing the attainment of a functional testis and of the culminative phase can be distinctly different. Moreover the culminative phase is controlled in large degree by the female,3 which does not have the rather prolonged recrudescent period of the gonad which is requisite in the male. In the Andean sparrow, to judge from histologic evidence in the ovary, in which there are ova 1 to 11/2 mm in diameter at all times, the female would appear always to be able to enter the culminative phase and thus to nest and lay eggs within a week or ten days. Individual adult females do undergo long periods when they do not nest, even though they seem capable of doing so and are associated in pair bond with a male that is at breeding level and which is singing and courting. It is not known whether or not these non-nesting periods of the females result from some aspect of the innate cycle of the adenohypophysis-a situation strongly suspected as paralleling that in the male-or from the lack of appropriate environmental stimuli for the induction of nesting and ovulation. It is most probable that there is an interplay of the two. The data here reported on the seasonal occurrence of nesting in equatorial Andean sparrows are, then, to be viewed as an expression primarily of the culminative phase of the female cycle. The environmental correlates that are drawn to notice may provide stimuli that in part control this seasonality. Methods.-The population of Andean sparrows investigated consisted of freeliving, individually marked birds that were repeatedly observed and retrapped over a period of one year, as already described.' Because the timing of events in a nesting history had been well worked out, from the date of laying through the development of the young and the loss of their juvenal plumage in the postjuvenal molt,4 observations of a variety of kinds could be used to permit extrapolation of the time of deposition of the normal two-egg clutch. The chronology of events thus derived permitted utilization of data gained from observations of nestings, handling of trapped females that had brood patches, observation and measurement of dependent and independent young, and collection and preservation of sample