MIGRATORY RESTLESSNESS IN CAGED BOBOLINKS (DOLICHONYX ORYZIVORUS, A TRANSEQUATORIAL MIGRANT)

Abstract

1. The bobolink (Dolichonyx oryzivorus) is a transequatorial migrant which breeds (June-July) above Lat. 40° N. in North America and "winters" (November through March) below about Lat. 10° S. in South America. The northward migration occurs during April and May.2. Observations on nocturnal restlessness, molt, body weight and the testicular cycle were made on some captive bobolinks at Lat. 36° N., caged out-of-doors and exposed to natural as well as to lengthened photoperiods, and to normal outdoor air temperatures. "Restlessness" was recorded every quarter-hour, by an electronic counting device, as the number of hops made by each bird on the perches of its cage.3. Restlessness was almost completely suppressed by the low air temperatures of winter. Nevertheless, intense nocturnal unrest set in rather abruptly in late March when air temperatures at night were still regularly below the freezing point (natural photoperiods).4. When 14-hour photoperiods were superimposed on natural day-lengths, beginning November 28, restlessness set in about three weeks earlier than in the controls which experienced only natural day-lengths. This was interpreted as evidence at least of a photoperiodic influence on, if not photoperiodic induction of, nocturnal restlessness.5. The onset of pronounced unrest was always associated with a marked rise in body weight, due to the deposition of subcutaneous and intraperitoneal fat, usually following a molt. In most cases this onset of restlessness preceded, by an appreciable interval, the appearance of black beak pigmentation, which indicates testicular recrudescence. These temporal relationships correspond to a sequence of events in nature and are tentatively interpreted to mean that the restlessness reflects the induction of a migratory state.6. The long delay (about three months) in the response to the presumed stimulation by long photoperiods is similar to the delay found in the response of the testicular cycle to photostimulation. An interpretation in terms of an internal rhythm as a primary seasonal initiator, with other factors (such as photoperiod) acting as accelerators-inhibitors, is not precluded.