Where Do Northern (baltimore") Orioles Spend the Winter?"

he forested dune ridge that separates Lake Manitoba from Delta Marsh, Manitoba, has been the focus since 1974 of an ongoing investigation into the ecology of a community of densely nesting species of insectivorous birds and one dove species (study area and bird community described by Sealy 1980a, b; MacKenzie 1982; MacKenzie et al. 1982).Since this work began, 2,191 Northern Orioles of the "Baltimore" subspecies (Icterus galbula galbula] have been banded, and in most cases uniquely color marked.Two of these orioles were encountered off the forested dune ridge.One recovery offers a clue to where individuals of this population spend the winter.The other recovery, supported by observations of color-marked individuals and mist-net captures during the late nestling and post-breeding periods on the study area, suggests that SY males leave the study area in mid-summer, presumably destined for their wintering grounds, up to one month before the ASY males and AHY females migrate southward.The recoveries mentioned above are detailed in the present paper.In addition, the records of the encounters of banded "Baltimore" Orioles (Table 1), furnished by the Migratory Birds Branch, Canadian Wildlife Service, which date back to 1924, provide additional information on where "Baltimore" Orioles spend the winter.I have broken down these encounters as follows: (1) orioles banded during spring migration or the breeding season and recovered within the subspecies' known wintering range (see Fig. 1) during the following or a subsequent winter, (2) birds banded in the wintering range and recovered during the following or a subsequent breeding season, and (3) birds banded on the wintering grounds which repeated at the same site or returned there in a subsequent winter.Recoveries of orioles from the eastern United States where their relatively recent presence in winter apparently has been influenced by the provisioning of food at feeding stations (see Erickson 1969) have been ignored in this analysis.Results and Discussion An s¾ male 1861-035291, banded and color marked on the dune-ridge study area on 20 June 1977, was found dead by N. Hanson on 10 July 1977, near Goodridge, Minnesota, about 245 km southeast of the study area.The implication of this recovery is that SY males leave the breeding grounds not long after their prebasic molt has begun (see Sealy 1979), presumably destined for their wintering areas.The lack of mist-net captures, repeats, and sightings of SY males on the study area after mid-July (Table 2) also suggests strongly that these males leave the study area, whereas it is known that the ASY males and AHY females complete their prebasic molts before migrating south in late August (Sealy 1979).It is not known whether the prebasic molt of SY males is interrupted while they migrate south, and completed after they arrive on the wintering grounds.Why do SY male "Baltimore" Orioles leave the breeding grounds about one month or more before the adults?As such, the early departure of these males from the dune-ridge forest, and probably also other areas, occurs before the collapse of the food supply (see Sealy and Biermann 1983).Thus, it is unlikely that such early fall migration southward is directly related to food availability on the breeding ground (see also Rappole et al. 1979).Sealy (1979) argued that because at least some SY males nest successfully on the study area, with their numbers varying from year to year (Sealy, unpub.data), it is possible that those that move southward early and while molting did not breed or were failed breeders.The continued absence of SY males after mid-July (Table 2) suggests that indeed all of these males leave early.Such early departure from the breeding ground should mean that these males arrive first on the wintering ground, but confirmation of this is needed.Sealy (1979) believed that the less experienced males may be better able to compete for food and space on the wintering grounds if they arrived before the ASY males, as Morton (1976) argued for the early-migrating Yellow Warbler (Dendroica petechia].Age-related differences in the ability to hold territories on the wintering grounds have been found in Yellow Warblers (Morton 1976) and Dark-eyed Juncos (Junco byemalls] (Ketterson and Nolan 1979) and are probably common in many passerine species (see also Ewald and Rohwer 1980).Where do Northern ("Baltimore"] Orioles spend the winter?

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