Why Don't Boblinks Breed in Brazil?

The Bobolink, Dolichonyx oryzivorus, experiences two sets of "long" and "short" days each year during the transequatorial migration between North and South America. Increasing daylength normally stimulates gametogenesis in birds, and thus it is perplexing that Bobolinks do not also respond to the southern summer and attempt to breed in Brazil. Previous hypotheses regarding transequatorial migration are discussed; four experiments relating to these hypotheses are presented and evaluated. The testis cycle, moult pattern, body weight fluctuations, and beak color of male birds were followed on simulated natural daylength cycles for 15 months at 20-day intervals. Body weight and testis cycles appeared normal on this simulated natural cycle; beak color patterns and moult desynchronized. Absolute insensitivity to light terminates by late October; thereafter the birds are sensitive to "long days" for 6 months, yet they do not respond to an increasing daylength cycle comparable to 200S latitude. Birds were also exposed to 17 months of continuous bright light: body weight cycles were regular for 6 months, then desynchronized; the testes regressed but spontaneous recrudescence was observed in 3 of 13 birds; recrudescence was not synchronous. Bobolinks did not respond to the 6L-30D lighting cycle that indicates a circadian control mechanism in other birds. A summarization of the photoperiodic responses of the Bobolink is included. The Bobolink, Dolichonyx oryzivorus, undergoes an unusual photoperiodic experience each year during its extensive migrations. The flights from the Canadian Wheat Belt to and from Brazil correspond closely to the spring and fall equinoxes, and the birds experience two sets of short "winter" days as they cross the equator and two sets of long "summer" days in both North and South America. Increasing daylength normally stimulates gametogenesis in birds, and thus it is perplexing that Bobolinks do not also respond to the southern summer and attempt to breed in Brazil. Three prior hypotheses have attempted to resolve this apparent dilemma. According to the first, "prolonged photorefractory periods" (Bissonnette 1937, Farner 1954, Wolfson and Westerhoff 1960; and see Engels 1962, for review) transequatorial migrant birds do not breed twice a year because they are physiologically refractory to photostimulation when daylengths are increasing in the opposite hemisphere, but such a simple explanation would require these migrants to have a photorefractory period considerably longer thar that of most other birds, and the available data dc not indicate this to be the case. The only transequatorial migrant yet shown to have a photorefractory period is the Bobolink (Engels 1961 1962; Wolfson and Westerhoff 1960), and Engeh has demonstrated that the normal photorefractory period, when tested by 14 hour "long days," is terminated by November 1, giving an estimate for the photorefractory period of about 2 months which is about normal for most birds but insufficient for a transequatorial species. This estimate 'Received September 24, 1969; accepted April 9, 1970. may be confounded additionally by the possibility that the photorefractory period may actually consist of two phases, a situation seen in House Finches (Hamner 1968). Thus, if Bobolinks are photoperiodically similar to House Finches, the previous tests for photorefractory termination in the Bobolink may be invalid (see Hamner 1968, for further discussion of this point), and it would be necessary to reexamine the photorefractory period of the Bobolink. The second hypothesis regarding transequatorial migration appears to have been applied only to Bobolinks, this being that retardation of breeding in South America ". . . could be due . .. to a very slow rate of response following the termination of refractoriness" (Engels 1%1). To our knowledge this pattern of gonadal development would be different from that seen in all other birds, and thus seems unlikely. Nonetheless this possibility could be tested by obtaining data on the normal testicular cycle throughout the year. Since Bobolinks have not been studied in South America it would be interesting to simulate a natural lighting regimen in the laboratory and record the testicular cycle. Gifford and Odum (1965) did expose Bobolinks to simulated cycles in the laboratory for energetic purposes and recorded food consumption, body weight, water uptake, Zugunruhe, and moult, but unfortunately they did not record changes in beak color in the males, and so we do not have even indirect information about the normal testis cycle. One of the purposes of the present investigation was to obtain laboratory data on the testis cycle of Bobolinks on simulated natural photoperiods for at least 1 year. This content downloaded from 157.55.39.111 on Sat, 17 Sep 2016 05:42:14 UTC All use subject to http://about.jstor.org/terms 744 WILLIAM M. HAMNER AND JOHN STOCKING Ecology, Vol. 51, No. 4 The third hypothesis concerning transequatorial migration postulates that an endogenous annual rhythm is the major component of the system (Marshall 1959), with the endogenous cycle perhaps synchronized by a variety of environmental stimuli. Although the only appropriately controlled laboratory data that substantiate this view are, in our opinion, equivocal (Benoit, Assenmacher, and Brard 1956; see also Farner and Follett 1966), the field data of Chapin (1954), Chapin and Wing (1959), Dorward (1962), Ashmole (1962), Snow (1965), and M. Harris (personal communication) on equatorial sea birds leave no doubt that natural cyclical breeding cycles are real. Spontaneous changes in body weight and Zugunruhe under constant 10-hour daylengths (Gifford and Odum 1965) suggest that other physiological systems in the Bobolink might also have an autonomous component, and hence it was additionally important to place Bobolinks under constant and continuous illumination for at least a year to see if their testes would also exhibit spontaneous cycles of growth and regression. The three hypotheses listed above may not be mutually exclusive explanations. Indeed it is possible that each of these suggestions will prove meritorious when more data are obtained, but it is difficult to formulate rigorous hypotheses regarding the photoperiodic mechanism of Bobolinks in what we believe to be the absence of sufficient basic information. We have been impressed with the exceptional complexity of the photoperiodic systems in plants and with the wealth of new data being accumulated on Starlings, previously considered to have rather simple photoperiodic responses. Unless the seasonal timing mechanisms in birds prove to be much simpler than those already described for plants (a situation that seems unlikely) then the extant photoperiodic hypotheses for birds are probably somewhat simplistic. It was our intent, therefore, simply to collect basic information that would relate to the hypotheses listed above for Bobolinks but which would also relate to the more general body of information regarding seasonal breeding cycles of all birds. We believe that formulation of other hypotheses at this stage of the investigation would have been