Yellow-throated Vireo

A regular breeding resident and migrant primarily in the deciduous forest belt of Minnesota. The Yellow-throated Vireo was a common species during the Minnesota Breeding Bird Alas (MNBBA).

An inhabitant of the eastern deciduous forests, the Yellow-throated Vireo is broadly distributed across the eastern United States from the eastern Great Plains, where its distribution often follows major river valleys, south to central Texas and east to the Atlantic coast. To the north, it can be found in scattered localities in southern Manitoba, Ontario, and Quebec. Sparsely distributed through its summer range; east-central Minnesota is one of the few areas where breeding densities are a little higher (Figure 1).

Assigned a Continental Concern Score of 9/20 by Partners in Flight.

A long-distance migrant that winters in Central America and northern South America.

Primarily an insectivore that gleans the foliage and bark of tree canopies.

A tightly woven open-cup nest bound together with caterpillar silk and spider webs and placed in the canopy of deciduous trees.

In Minnesota, Roberts (1932) considered the vireo a common summer resident south and west of the northern boreal forest region, “its range extending northward through the Red River Valley in decreasing numbers as far as southern Manitoba.” A nesting record near Onamia, in Mille Lacs County (an adult feeding young), suggested to him that the species’ range might eventually extend farther into northeastern Minnesota. As younger, deciduous forests replaced the old-growth conifers that were harvested in the early lumber days, they would create more opportunities for this deciduous-loving species. At the time of his writing, Roberts compiled confirmed nesting records (nests with eggs or young, or adults incubating) from Hennepin, Ramsey, and Wright Counties in east-central Minnesota, and from Mahnomen County in the northwest. Inferred breeding records (nests, or adults feeding young) were available from Isanti, Sherburne, and Wasceca Counties and from the Mille Lacs region.

When Green and Janssen (1975) published an updated account of the species, confirmed nesting records were available from 7 additional counties: Pennington, Stearns, Benton, Sherburne, Anoka, Winona, and Houston. Although the species still remained absent from a large area of northeastern and far north-central Minnesota, inferred breeding evidence was now reported as far north as St. Croix State Park in Pine County and Bay Lake in Crow Wing County. There also were summer observations in Duluth.

A few years later, Janssen (1987) provided an updated distribution map for the species that excluded northeastern Minnesota and the adjacent counties of Koochiching and Itasca, and nearly all of southwestern Minnesota and southern portions of the Red River valley. Even though Itasca County was excluded from his map, Janssen noted that the species was continuing to slowly expand northward, as summer observations were reported from Cass and southern Itasca Counties in north-central Minnesota as well as from Kittson, Roseau, and Lake of the Woods Counties in the northwest, and Carlton and southern St. Louis Counties in the east. He delineated 12 counties where nesting had been confirmed since 1970: Anoka, Brown, Carver, Clay, Crow Wing, Goodhue, Morrison, Ramsey, Sherburne, Stearns, Pennington, and Wright. Hertzel and Janssen (1998) later added Grant and Winona Counties.

As they conducted intensive surveys across the state, field staff with the Minnesota Biological Survey (MBS) documented 924 breeding season locations for the Yellow-throated Vireo. In addition to numerous observations in the Eastern Broadleaf Forest and Tallgrass Aspen Parklands Provinces, the species was common throughout counties in the western half of the Laurentian Mixed Forest Province, including from Clearwater, southern Beltrami, and Itasca Counties south to central Minnesota. Two observations were also reported from central St. Louis and central Lake Counties in the Arrowhead region. The species was common along the entire length of the Minnesota River valley, and there were scattered reports in the southwestern corner of the state (Minnesota Department of Natural Resources 2016).

During the MNBBA, observers reported a total of 1,457 Yellow-throated Vireo records from 21.4% (1,014/4,742) of the surveyed atlas blocks and from 29.2% (683/2,337) of the priority blocks. Breeding evidence was gathered from 30 blocks. The birds were observed in all but 7 of Minnesota’s 87 counties: Cook and Lake Counties in the northeast; Wilkin County in the west; and Cottonwood, Jackson, Murray, and Pipestone Counties in the southwest. Confirmed breeding was documented in 19 counties. 3 blocks with confirmed nesting straddled both Scott and Dakota Counties. Fourteen of the counties were additions to the map published by Hertzel and Janssen (1998) and included several northern counties: St. Louis, Carlton, Pine, and Aitkin. Overall, the species’ distribution was very similar to that documented by the MBS, with the exception of even more observations in northeastern Minnesota and observations in regions of north-central Minnesota that had not yet been surveyed by MBS staff.

The MNBBA predicted breeding distribution map depicts low breeding densities throughout western and central Minnesota north as far as southern St. Louis County, Itasca County, and northern Koochiching County (Figure 4). The species’ association with floodplain forests is clear in portions of southeastern and south-central Minnesota and throughout northwestern Minnesota. Moderate breeding densities occur throughout the Mille Lacs Uplands, the Pine Moraines and Outwash Plains, and the Hardwood Hills Subsections. The landscape in these regions is characterized by numerous lakes and wetlands embedded in a matrix of farmland, deciduous forests, and grasslands, a perfect landscape for this semiopen deciduous forest species. Very small pockets of higher breeding densities are found scattered along the Mississippi River south of the Twin Cities and in northwestern Minnesota. Although the model predicts a sparse distribution in southeastern Minnesota, the number of MNBBA records suggest that species is encountered a little more frequently than predicted.

As Roberts (1932) speculated would happen, nearly 100 years ago, the Yellow-throated Vireo has continued to move farther into the Laurentian Mixed Forest Province, spurred by the loss of upland conifer cover and increased landscape fragmentation by residential and recreational development and small farmlands. Today, the species is common along the southern and western border of the province but remains quite rare east and north of southern Itasca County (Figure 2). A similar change has been observed in Michigan, where the species has become more abundant in the northern regions of the Lower Peninsula and in the Upper Peninsula (UP). In the 20 years between the state’s first atlas (1983–1988) and its second atlas (2002–2008), there was a 57% increase in the number of blocks where the species was reported in the UP (Chartier et al. 2013). In Wisconsin, the species was documented breeding across the state during their first atlas (1995-2000). Prior to 1963, however, the Yellow-throated Vireo was largely absent from the states’s northern counties. It is unclear if the atlas documented a northern range extension or the findings simply reflected the lack of survey work conducted in the region prior to 1963 . Elsewhere within its breeding range, there is little evidence of any large-scale change in the species’ breeding distribution apart from small-scale changes due to habitat changes (Rodewald and James 2011).

*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.

Although occasionally found in mixed deciduous-coniferous forests, the Yellow-throated Vireo’s primary habitat is mature, deciduous woodlands including both uplands and floodplains (Figure 5). Because it prefers more open stands, it also resides in parks, orchards, and small towns where groves of mature shade trees are available (Rodewald and James 2011). The species never nests in dense forests, unless forest openings are available, and rarely in conifers (Danz et al. 2007; Rodewald and James 2011).

The National Forest Bird (NFB) Monitoring Program, conducted on four national forests in northern Minnesota and Wisconsin, demonstrated a very strong preference of the Yellow-throated Vireo for oak forests. Aspen, lowland hardwoods, mixed upland hardwoods, and other forest types were used infrequently (Niemi et al. 2016). Habitat data collected from MNBBA point counts depicted the species’ strong association with northern hardwoods and pine forests (Figure 6). Two factors may explain the association with pine forests.  First, in the central part of the state where Yellow-throated Vireos are most abundant, pine forests often include an oak component.  Second, planted or natural pine forests in the central part of the state are embedded in a landscape dominated by oak forests which may be the primary feature the vireos are likely responding to.  The association with conifers, however, also has been noted in Wisconsin where Cutright and his colleagues Cutright et al. 2006) wrote that in the northern regions of the state, Yellow-throated Vireos are found in oak forests that are mixed with “maple, beech, yellow birch, and conifers.”

Despite its preference for forest edges, numerous studies have pointed to the species’ area sensitivity. A study in Wisconsin (Bond 1957) found the vireo was 50% more common in forest stands larger than 32 ha. On the East Coast, one of the factors correlated with the species’ relative abundance was the percentage of forest cover within 2 km (Robbins et al. 1989); another study in the northeastern United States demonstrated that numbers decreased in forests that were smaller than 100 ha (Robbins 1979). Yellow-throated Vireos may initially select large forest tracts but then selectively use the outer edges of those stands. Unfortunately, these edges are also sites where rates of cowbird parasitism and predation are often higher (Rodewald and James 2011).

Population estimates for the Yellow-throated Vireo have been generated using long-term monitoring data from the federal Breeding Bird Survey (BBS). The estimated number of individuals in North America is 4.4 million birds, only a small percentage of which reside in Canada (Rosenberg et al. 2016). In 2013, biologists estimated that Minnesota supported 3.9% of the continental population (Partners in Flight Science Committee 2013). When that percentage is applied to the most recent population estimate, the statewide population is projected at 172,000 breeding adults. This compares to a statewide estimate generated using MNBBA point count data of 430,000 breeding adults (95% confidence interval of 317,000 – 811,000), more than twice the BBS-derived estimate.

Yellow-throated Vireos are rather sparsely distributed throughout their eastern breeding range (Figure 1). The average number of birds seen on BBS routes survey-wide is just under 1 bird. Even states with relatively high breeding densities report an average of only 2 to 3 birds per route each year (West Virginia, Virginia, Kentucky, and Connecticut; Sauer et al. 2017). Most studies that report local breeding densities have been conducted in the southeastern United States. There, numbers range from as low as 3 territorial males per 40 ha in floodplain forests in Maryland, to as high as 19 in virgin deciduous forest stands in Maryland and mixed hardwood forest stands in West Virginia. In northern Minnesota, the NFB Monitoring Program found an average of 0.4 pair of Yellow-throated Vireos per 40 ha in the Chippewa National Forest, and an average of only 0.01 pair per 40 ha on the Superior National Forest. The data strongly demonstrate the species’ preference for the more open, drier upland deciduous forests found on the Chippewa than the more densely forested, mixed coniferous-deciduous landscape of the Superior. By comparison, its close relative the Red-Eyed Vireo averaged 20.1 pairs per 40 ha in the Chippewa National Forest, and 13.4 in the Superior (Niemi et al. 2016).

Because of their low abundance, population trend data collected by the BBS lack statistical precision in most states, including Minnesota, as well as throughout the BBS monitoring region. Nevertheless, there is only one state, Delaware, where Yellow-throated Vireos demonstrated a significant declining population. Every other ecological region, province, and state demonstrated either a stable or significantly increasing population. Because the heart of the vireo’s breeding range in Minnesota is in the Eastern Broadleaf Forest Province, the regional BBS data for the Prairie Hardwood Transition region, which encompasses Minnesota’s Eastern Broadleaf Forest Province, are a good surrogate for assessing the species’ population trend in the state. The data for the region are statistically robust and demonstrated a significant population increase, averaging 2.67% per year from 1966-2015 (Figure 7). BBS data that are available for Minnesota also suggest a strong increasing population trend (Sauer et al. 2017). Range-wide, biologists estimate the vireo’s population has increased 62% from 1970 to 2014 (Rosenberg et al. 2016). In Minnesota, the statewide increase may be attributed in part to expanding numbers of birds now occupying large portions of northern Minnesota where development and timber activities have converted older mixed and coniferous stands to younger stands of aspen-birch.

On the Chippewa National Forest, the Yellow-throated Vireo demonstrated a nonsignificant downward population trend. Its abundance on the forest is so low, however, that any trend, positive or negative, needs to be interpreted with caution. Farther east, on the Chequamegon and Nicolet National Forests in Wisconsin, Yellow-throated Vireo populations were relatively stable (Niemi et al. 2016).

The vireo’s range-wide increase is attributed in part to increased forest cover throughout the eastern United States, especially in the Northeast. Following two centuries of deforestation and intensive farming, a significant decline in agricultural land use has created a landscape that is increasingly forested and maturing over time (Foster et al. 2008). Sites that originally were cleared for small farms have since been abandoned, allowing them to succeed once again to young forests.

Given the Yellow-throated Vireo’s broad distribution and increasing population, it is not considered in immediate need of conservation action and has been assigned a moderate Continental Concern Score of 9/20 (Rosenberg et al. 2016). Nevertheless, since a high percentage of its global population occurs within one biome, the eastern deciduous forest, in 2004 Partners in Flight designated it a Stewardship Species for that region (Rich et al. 2004).

The “State of the Birds 2010 Report on Climate Change” ranked the vireo has having medium sensitivity to climate change (North American Bird Conservation Initiative 2010). A recent analysis of the impacts of warming temperatures on North American birds by the National Audubon Society classified the species as “climate threatened.” Its models predicted that the species could lose 73% of its current summer range by the year 2080, forcing it to move northward into Canada should suitable habitat be available (Langham et al. 2015; National Audubon Society 2016).

Despite its dependence on forest communities, few studies have examined the relationship between forest management practices and the Yellow-throated Vireo’s population abundance and reproductive success. In the absence of recommendations, retention of large forest stands is likely the most important conservation measure that can be implemented.

Finally, the species’ susceptibility to the use of pesticides may be a concern, particularly in residential areas. Although populations have been increasing in recent years, the species suffered some major declines in the early 20th century in the northeastern United States, disappearing from many small cities and towns. Because the vireo is often found in shade trees, small woodland groves, and orchards, Bent (1940) and others speculated that its disappearance was due to extensive spraying programs to control Dutch elm disease and gypsy moths (Rodewald and James 2011).