Philadelphia Vireo

A regular breeding resident and migrant; the Philadelphia Vireo was rare during the Minnesota Breeding Bird Atlas (MNBBA).

The Philadelphia Vireo is primarily found distributed across a narrow band in Canada from Labrador to northwestern Alberta. Its breeding range also extends southward into northern portions of Minnesota, Wisconsin, and Michigan (Figure 1). Populations are generally low, but the highest densities have been observed in parts of Ontario and Quebec.

Assigned a Continental Concern Score of 10/20 by Partners in Flight and designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.

Long-distance migrant that over-winters in southern Central America.

Small arthropods gleaned from foliage.

Pensile cup nest suspended in a fork usually high in a tree.

Simply described by Roberts (1932) as “no positive breeding records for Minnesota, but it seems probable that it will be found nesting in the northern part of the state.” Even breeding observations appeared to be limited because he only lists Hibbing in St. Louis County and the city of Pembina in North Dakota as locations where individuals were collected. The latter was a citation in 1873 where he quotes Coues as saying that the species “undoubtedly breeds about Pembina, in the heavy timber of the river bottom.”

More than forty years later, Green and Janssen (1975) emphasized its primary distribution in northeastern Minnesota as far south as Duluth. They found two breeding records with young being fed in eastern Becker County (1961) and a bird banded on June 16, 1966, in Little Falls, Morrison County (1966). Besides “inferred” nesting in Becker County, they also list confirmed nesting in Lake and St. Louis Counties. A few years later, Janssen (1987) provided a more restrictive distribution primarily in the northern tier of counties from Lake of the Woods County and northern Beltrami County east to northern Cook County. Janssen (1987) and Hertzel and Janssen (1998) only included confirmed nesting in Cook County since 1970. Both Green and Janssen (1975) and Janssen (1987) emphasized that the species was scarce throughout its breeding range, but identification is complicated because of its similarity to the substantially more common Red-eyed Vireo.

The Minnesota Biological Survey (MBS) included only 19 breeding season locations; all were found in Cook, Lake, and St. Louis Counties, except for one location in central Becker County (Minnesota Department of Natural Resources 2016).

The MNBBA included a total of 32 records in 22 blocks. Similar to the observations by the MBS, most detections were in northeastern Minnesota, including Cook, Lake, and St. Louis Counties, plus one in northeastern Becker County (Figure 2). There were three confirmed nesting observations: two in St. Louis County and one in Cook County. Probable nesting was also limited to one in St. Louis County; possible nesting observations were scattered throughout Cook County, southern and central Lake County, and in northern St. Louis County (Figure 3; Table 1). Note that all MNBBA records of the Philadelphia Vireo were carefully reviewed before acceptance because of the notorious difficulty in identifying this species.

The difficulty in the identification of the Philadelphia Vireo is well known. For instance, Moskoff and Robinson (2011) emphasized that its “obscurity” is related to the overwhelming abundance of the Red-eyed Vireo. The Philadelphia Vireo is very easy to overlook even by experienced observers. The first nesting in Wisconsin was not detected until 1997 in Bayfield County during Wisconsin’s first breeding bird atlas (Cutright et al. 2006). In addition, another confirmed nesting was identified in Pierce County in 1999 near the St. Croix River, adjacent to Washington County in Minnesota. Because of these complications in detection, no information on any historical changes in its breeding distribution is available.

The Philadelphia Vireo is well represented by nesting activity in the southeastern region of Manitoba (Bird Studies Canada 2017) and by high relative abundance in the western regions of Ontario, areas that are adjacent and north of Cook and Lake Counties (Cadman et al. 2007). Observations of vireos in June and early July should be scrutinized throughout the forested areas of Minnesota and pictures of any observations are important sources of confirmation.

*Note that the definition of confirmed nesting of a species is different for Breeding Bird Atlas projects, including the definition used by the Minnesota Breeding Bird Atlas, compared with a more restrictive definition used by the Minnesota Ornithologists’ Union. For details see the Data Methods Section.

Primarily deciduous forests comprised of aspen, birch, and ash in early to mid-successional stages (Erskine 1977; Rice 1978; Godfrey 1986). Holmes and Sherry (1986) in the northeastern United States found the species in mixed conifer and older forests as long as deciduous trees were present. The Philadelphia Vireo was also described as preferring woodland edges, aspen parklands, second growth on burned and logged areas, and shrubby riparian areas (Moskoff and Robinson 2011).

Green and Niemi (1978) identified open woodlands, burned-over areas, and streamside willows and alders in the Superior National Forest as areas of habitat. Brewer et al. (1991) emphasized mature deciduous forest primarily of aspen but often with maple, birch, and oak. The National Forest Bird Monitoring (NFB) program (Niemi et al. 2016) has only recorded 14 breeding detections from 1995 to 2010. Among the reasons for the paucity of detections is the requirement of the NFB program to observe the individual and not rely on song identification alone. Time is also a serious limiting factor because visual observations can be time-consuming.

Partners in Flight (Rosenberg et al. 2016) estimated a breeding population of 3.7 million breeding adults, with an estimated 99% of the population in Canada. Environment Canada (2014) also estimated a wide adult population that ranged from 500,000 to 5,000,000 adults. No population estimate was attempted from the MNBBA because of a paucity of observations.

The federal Breeding Bird Survey (BBS) trends for Minnesota were unreliable. Because of the species rarity, estimates were also unreliable for Michigan, Wisconsin, and the provinces of Manitoba and Ontario. The most reliable estimate was for the survey-wide BBS routes (n = 478). These trends indicated a significantly increasing population of 2.31% per year from 1966 to 2015 (Figure 4). An equivalent magnitude in population increase was also found in Canadian survey routes. Partners in Flight (Rosenberg et al. 2016) estimated an overall increase in the population of 87% from 1970 to 2014.

No reliable population density estimates are available for Minnesota due to its rarity in quantitative surveys. Average numbers counted on BBS routes were less than 1 per route in Minnesota and in Ontario, but mean counts in Quebec were 4.73 detections per route per year (Sauer et al. 2017).

The Philadelphia Vireo has a moderate score of 10 of 20 by Partners in Flight, although it appears to have an increasing population (Rosenberg et al. 2016). Where its primary breeding population exists in Canada, it is considered “unlikely to face significant habitat shortages in the near future” (Environment Canada 2014). However, the species has been identified as a priority for conservation in some regional strategies. The species is listed as a Species in Greatest Conservation Need in Minnesota primarily because it is rare, has potential declining or vulnerable habitat, and has unknown causes for a decline (Minnesota Department of Natural Resources 2015).

The Philadelphia Vireo was identified as susceptible to communication tower kills, with an estimated annual mortality of 38,431, or approximately 1% of its total population (Longcore et al. 2013). Moskoff and Robinson (2011) suggested some sensitivity to Brown-headed Cowbird nest parasitism and predation due to fragmentation of forests. However, they also advised that the species might be dependent on forest disturbance because of its potential disappearance from late-successional forests. Cutright et al. (2006) reported it was found nesting in Bayfield County in aspen forests where the canopy was “opened by logging activity or natural disturbance.”

Terborgh (1989) pointed out that the Philadelphia Vireo had a restricted winter distribution in southern Central America, but it seemed to accept disturbed habitats in its wintering grounds. In its review of bird species’ susceptibility to climate change, Langham et al. (2015) and the National Audubon Society (2015) labeled it as “climate threatened.” The rationale for this decision was based on the possibility that “less than 10 percent of climate space suitable at the present time is forecast to be in that condition by century’s end.” The society emphasized the significance of the species’ ability to adapt to new climatic conditions in the northern portions of its breeding range.