Philadelphia Vireo Vireo philadelphicus

The Philadelphia Vireo breeds across much of southern Canada from British Columbia to Newfoundland, but its distribution in the u.s. is restricted. The species is known to breed in Maine and in the White and Adirondack mountains, but was only known to nest in Vermont from old reports (Daven­ port 1907). Atlas Project confirmation was first achieved in 1979. Observations made during that and subsequent years revealed that the Philadelphia Vireo could be locally common in preferred habitat in northern Vermont. For example, 14 birds were seen in the Cold Hollow Mountains, south of (Barlow and Power 1970; Rice 1978) helps Montgomery Center, along less than a mile to maintain territorial boundaries, but how of logging road in an area that had been females distinguish correct mates-a selec­ selectively cut 5 to 10 years before. Subse­ tive process often assumed to be a function quent observations revealed that, though of song-is not clear; mismatches are un­ rare in the state, the Philadelphia was far known. Philadelphia Vireos nest higher in more widely distributed and frequently en­ the canopy than Red-eyeds, but Rice (1978) countered than observers had previously determined that no significant differences in suspected. foraging heights of the two species existed. The Philadelphia Vireo is particularly in­ The Philadelphia Vireo arrives in Vermont teresting to ecologists because of its simi­ in mid May. Like most other members of the larity to the Red-eyed Vireo. The two spe­ genus, the Philadelphia Vireo builds its nest cies overlap at the southern and northern in the fork of a horizontal branch; the nest limits of their respective ranges. In the area is 3-12 m (10-40 ft) above the ground. of sympatry they compete interspecifically June 16 is the only date for nest construc­ for similar habitats (Rice 1978). In Ver­ tion in Vermont. From 3 to 5 (usually 4) mont, Philadelphia and Red-eyed vireos brown-spotted, white eggs constitute a com­ have been found to respond strongly to plete clutch. In marked contrast to the Red­ tapes of each others' songs. Ethologists in­ eyed Vireo, the male Philadelphia assists in terpret various differences in their behavior incubating the clutch. He may even sing and reproductive strategies as reproductive­ while on the nest (Lewis 1921). Incubation isolating mechanisms (Barlow and Power lasts 13 to 14 days, and the nestling period 1970). is approximately equivalent. Young are fed a Although in Canada the Philadelphia variety of insects captured by gleaning, hov­ Vireo inhabits aspen-birch forests, most Ver­ ering, and occasional hawking. Age to com­ mont records are from cutover hardwood plete independence is unknown, but is prob­ forest; occasionally they are from open ably similar to that of Red-eyed Vireos. Both mixed woodlands. Rice (1978) found that Vermont fledgling dates are from late June. Philadelphia Vireos in Canada occupied ter­ Rice (1978) found that Philadelphia Vireos ritories of 0.3 ha (0.8 a), nearly identical in experienced much less nest parasitism by size to territories used in the same habitat Brown-headed Cowbirds than did Red­ by Red-eyed Vireos. However, territories eyeds, but failed to identify behavioral traits were for the most part mutually exclusive. that might account for the observed differ­ Interspecific aggressive response to song ences. In Vermont, Philadelphia Vireos oc-

270 Species Confirmed as Breeders in Vermont 7)' n'

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7)' ,,' cur in areas where cowbirds are relatively delphias. Most of the state's forests have uncommon (D. P. Kibbe, pers. observ.); para­ been selectively cut in the past, but increas­ sitism rates within the state are unknown. ing tracts, particularly in the Northeast Philadelphia Vireos are common but un­ Highlands, are now being clear-cut. While obtrusive fall migrants. Most pass through clear-cutting is not as beneficial to Phila­ Vermont in late August and early September, delphia Vireos as selective logging, the edge but stragglers may be found through Sep­ habitats created are probably better habitat tember 28. The species winters from central for the species than unbroken forest. Addi­ Guatemala to Panama and Colombia. tional studies of the dynamics of coloniza­ The historic status of the Philadelphia tion by Philadelphia Vireo populations in Vireo in Vermont and current trends in the cutover areas, and analyses of distribution population are open to speculation. Perkins records in subsequent years, are needed to and Howe (r90r) and Spear (1976) consid­ enlighten our view of the status of these un­ ered the Philadelphia Vireo an uncommon obtrusive residents. summer resident. Selective logging activity DOUGLAS P. KIBBE appears to create habitat suitable for Phila-

Philadelphia Vireo 271