Effects of Extended Tropical Photoperiod and Temperature on the Dickcissel

Abstract

The interplay between exogenous and endogenous factors in the regulation of the annual cycle of birds has received considerable discussion and experimental attention since the work of Rowan (see Marshall, 1961; Wolfson, 1960a). It has been argued that the annual cycle of tropical residents must be under predominantly endogenous control because of the constancy of their environment; but there are few instances reported for which such influence has been suggested to be primary (Chapin, 1954; Marshall and Roberts, 1959; Miller, 1959). With most tropical residents, environmental factors still operate to adjust the annual cycle so that the species breeds at the time most propitious for the survival of the young (Keast, 1959; Marshall and Disney, 1957; Skutch, 1950; Voous, 1950). It has also been suggested that temperate-zone breeders wintering in the tropics likewise must be under a high level of internal control, and that exogenous factors must operate primarily during the period of the year at higher latitudes, annually setting the cycles of internal physiological events (Curry-Lindahl, 1958; Marshall and Williams, 1959). Merkel (1963) compared the long-term effects of constant photoperiods on two species of passerines having different migratory ranges. He concluded that the gonadal and weight cycles are more internally established in the Whitethroat, Sylvia communis, which is an equatorial migrant, than in the Robin, Erithacus rubecula, which remains within the North Temperate Zone during its migration. These results again suggest that tropical-wintering species depend to a great extent on endogenous control of physiological events. The success of the Dickcissel, Spiza americana, which breeds in temperate North America and winters in the neotropics, is related to the photoperiod to which it is exposed during the breeding season. The longer photoperiods of the temperate summer result in the attainment of a positive energy balance of sufficient magnitude to facilitate successful breeding, even though the mean temperatures at these latitudes are a few degrees lower than those of its winter range during the same months (Zimmerman, 1965b). While still on the contranuptial area, Dickcissels initiate gonad recrudescence, accumulate depot fat, and captive birds show Zugunruhe. Although the photoperiodic control of these events has not been experimentally verified, it is probable that the developments of the reproductive and migratory states are partially dependent on light stimulation, as they are in temperate-zone (Dorst, 1955; Farner, 1964; Wolfson, 1963) and transequatorial migrants (Engels, 1959; Wolfson, 1963). To suggest that in the tropics these birds are unresponsive to the relatively constant daylengths is to disregard the conclusions of Wolfson (1952, 1960b) and Farner et al. (1953) on the nature of the effects of light. Through natural selection a species could adapt to the light periods that are experienced in its annual distributional pattern either by changes in the rate of response to stimulation (Wolfson, 1959a) or the inclusion of a refractory period of appropriate length (Marshall, 1960), or both. In order to ascertain the dependency of Dickcissels upon their natural photoperiod regime, birds were held for as long as 21 months under conditions of temperature and photoperiod that approximated those of their wintering grounds. This