Philadelphia vireos are small, stocky vireos that appear very similar to red-eyed vireos. Philadelphia vireos measure 13.3 cm in length, have a wingspan of 20.3 cm, and weigh in at only 12 g. Like all vireos, they feature a distinctive slender, hooked beak which in Philadelphia vireos is shorter than most other vireo species. They have olive-gray backs and wings, with a gray-blue cap. They feature thick white supercilia, dark gray eye-stripes and dark lores. Undersides are a mix of bright and dull yellow, with the brightest yellow at the throat. Their tails are olive-gray and relatively short. Legs and beaks are slate gray to black in color. Males and females of this species look alike and juvenile plumage is not distinctive.
Philadelphia vireos are often confused with red-eyed vireos and warbling vireos. Red-eyed vireos are larger, with longer beaks and lack the bright yellow throats and undersides of Philadelphia vireos. Warbling vireos have paler facial features and paler underparts. The brightest yellow is on their flanks, and their throats are white as opposed to yellow. (Ehrlich, et al., 1988; Moskoff and Robinson, 1996; Sibley, 2000)
Philadelphia vireos inhabit North and Central America. These birds breed across southern Canada and select portions of the northern United States. They migrate through the entire state of Michigan, and may breed in the upper peninsula and northern regions of the lower peninsula. Philadelphia vireos overwinter in Central America from southern Mexico through Panama. Their migration range covers the Gulf Coast and the eastern half of the United States, excluding the southeast. (Ehrlich, et al., 1988)
Philadelphia vireos primarily breed in the boreal forests of southern Canada. They may also breed in second growth or early- to mid- successional forests composed of aspen, birch, alder and ash trees. They seem to prefer dense habitats with tree stems and branches at many different heights. They often forage in the tops of trees, and thus they prefer habitats with a thick canopy layer. During migration these birds may be found in similar habitats, but more often in dense, shrubby thickets. While wintering in Central America, this species is often found along scrubby edge habitats and occasionally in plantations or gardens. They have been recorded at elevations of up to 1,600 m in Central America. (Bennett, et al., 2000; Moskoff and Robinson, 1996)
Philadelphia vireos are monogamous, meaning that one male and one female form a pair to breed and raise their young each breeding season. It is unknown whether the same pairs form from year-to-year. Pairs form soon after arriving at the breeding grounds or possibly during migration. Courtship often consists of males snapping their bills, tail-fanning and raising their crest feathers. Females respond to or initiate this ritual by wing-quivering. Males will follow their mates and actively defend them from other males. (Ehrlich, et al., 1988; Moskoff and Robinson, 1996)
For Philadelphia viroes, the breeding season occurs annually between May and August. These vireos depart from the wintering grounds relatively late and typically do not arrive on the breeding grounds until late May. Males begin singing in early June and pair-formation occurs two weeks after arrival. After pair-formation, females begin searching for suitable nesting sites with their respective mates following close behind to defend them from intruding males. The female selects a forked branch high in the tree canopy, typically 9 to 24 m above the ground. Females alone construct the hanging, cup-shaped nests using birch bark, grass, feathers, vegetation and spider webs as materials. Typical clutch size is 4 but may range from 3 to 5. Eggs measure 19 mm in length and are white and speckled with brown or black.
Both males and females perform incubation and thus males feature a small brood patch during this time. Incubation lasts 11 to 13 days, after which the newly hatched young are fed and brooded by both parents. At 12 to 14 days after hatching, the young fledge (leave the nest) but remain with the parents for an additional 10 days. The age at which Philadelphia vireos can reproduce is unknown. (Ehrlich, et al., 1988; Moskoff and Robinson, 1996)
Parental investment begins with time, energy, and risk involved with selecting and defending nesting territory or mates. After mating occurs, females select a suitable microhabitat in which to construct a protected nest. Both males and females incubate the clutch and males develop a small brood patch. After the clutch hatches, both parents gather food to provide the altricial young. Parents often forage outside their typical area which puts them at risk for interspecies aggression or predation. Philadelphia vireos often nest alongside more aggressive red-eyed vireos which will attack any intruder to their territory. Philadelphia vireos often have to defend their nests and young from aggressive encounters with these close relatives. Once the young fledge, parents continue to provide care for at least an additional 10 days. If red-eyed vireos are nesting nearby, the parents must actively defend their defenseless fledgers against aggressive red-eyed vireos that perceive the fledgers as intruders. Observations have been made of young Philadelphia vireos being ushered by parents from open areas to nearby shrubs for shelter. (Moskoff and Robinson, 1996)
The longest-lived Philadelphia vireo was 8 years and 10 months old. Little is known regarding causes of mortality for this species. They are occasional hosts to parasitic brown-headed cowbirds, but this does not cause significant damage to population numbers. Some fatalities occur during migration from impact with television towers, though this is also not thought to be a significant cause of mortality. (Moskoff and Robinson, 1996)
Philadelphia vireos are neo-tropical migratory birds that perform two migrations annually: one in spring and another in fall. During the breeding and non-breeding seasons this species is diurnal but like most songbird migrants, they migrate at night. During migration and non-breeding seasons, these vireos will form groups with other migratory species including warblers and other vireos. They are an arboreal species that forages and nests within tall canopy trees.
Philadelphia vireos form pairs during the breeding season and defend feeding and nesting territory by singing advertisements and occasionally physical aggression. Males sing on their territories and warn intruding birds using body postures. These warning postures include raising the crest feathers, perching with head thrust forward and beak agape, and fanning the tail feathers. Both sexes produce a harsh 'ehh' sound when intruders are near or as a contact call between mates. Males will chase intruders away or even attack and grapple if the intruder is persistent.
Interspecific competition with red-eyed vireos often occurs as they prefer to breed in similar habitats. When both species are present, aggressive encounters are frequent and there is significant competition over food, nesting sites and other resources. When this occurs, the two species seem to share the habitat by foraging and nesting in different areas of the forest canopy. Philadelphia vireos move up into the highest layer of the canopy while red-eyed vireos move down to the middle canopy layer. This is called "resource allocation". Philadelphia vireos also prefer white ash and yellow birch as canopy species, whereas red-eyed vireos use any tree species. (Moskoff and Robinson, 1996; Robinson, 1981)
Territory size for Philadelphia vireos varies in response to population density and resource availability. Size has been recorded ranging from 0.3 to 0.8 hectares. (Moskoff and Robinson, 1996)
Like all birds, Philadelphia vireos perceive the environment through the senses of sight, smell, taste, touch and sound. They communicate with others of their own species or other species by using calls, body postures and physical encounters. The typical Philadelphia vireo call, given only by males, is mnemonically described as "cherrie-o-witt, cheree, sissy-a-witt, tee-o". This is very similar to that of red-eyed vireos but it is frequently higher-pitched with longer pauses between phrases. Both males and females use a harsh "ehh" call in response to intruders or between mates.
Typical warning body postures include erecting crest feathers, fanning tail feathers, gaping the beak or holding the head low with body horizontal. These are typically used to warn off intruders and a physical attack may follow if the intruder does not retreat. Philadelphia vireos are typically not aggressive, but will aerially chase, peck, or grapple with threatening individuals.
Currently, known courtship rituals solely consist of body postures. Males will raise crest feathers, fan their tails, and rarely sway back and forth. Females will respond or initiate these rituals with crouched wing-quivering. ("Whatbird.com: Field Guide to Birds of North America", 2007; Moskoff and Robinson, 1996)
Philadelphia vireos are primarily insectivores but will seasonally eat berries. Berries constitute 7% of the diet year-round, but up to 20% of the diet during winter. They use a foraging technique known as "gleaning" meaning they prefer to capture stationary insects from leaves while flying by or hovering. Philadelphia vireos have been shown to prefer foraging on white ash and yellow birch, specifically. The most frequent dietary item is caterpillars and Philadelphia vireo populations may increase in response to a high abundance of caterpillars. Double broods have been reported in some years of caterpillar outbreak. Other dietary items include: butterflies and moths, beetles, ladybugs, leaf-eating beetles, weevils, wood-boring beetles (Buprestidae and Cerambycidae), click beetles and flies. (Ehrlich, et al., 1988; Moskoff and Robinson, 1996; Robinson, 1981)
No observations of predation on Philadelphia vireos have been made. Philadelphia vireos do exhibit mobbing behavior in response to potential predators and have been observed mobbing blue jays, squirrels, and common grackles. The olive-gray coloration of these vireos likely serves as camouflage in the dense canopies they inhabit. (Moskoff and Robinson, 1996)
As primarily insectivores, Philadelphia vireos likely impact the local insect communities, especially during times of caterpillar outbreaks. They also consume wood-boring beetles which reduces damage to local trees. Philadelphia vireo nests are occasionally used by parasitic brown-headed cowbirds. Parasites that utilize Philadelphia vireos are currently unknown. (Moskoff and Robinson, 1996)
There are no known adverse effects of Philadelphia vireos on humans.
Philadelphia vireos' consumption of wood-boring beetles and caterpillars during outbreaks indirectly affects humans by protecting trees which are used for harvest, oxygen production, or prevention of soil erosion. Philadelphia vireos also produce a beautiful song and are attractive birds that many bird watchers enjoy. (Moskoff and Robinson, 1996)
Currently, Philadelphia vireos are of least concern to the IUCN Red List due to an increasing population size and abundance over a large geographic range. They thrive in successional forests that are currently created nearly exclusively by human disturbance, specifically select harvesting or clear-cutting. Due to their preference for white ash and yellow birch, harvesting of these species may negatively affect their populations. As migratory birds, Philadelphia vireos are protected under the United States Migratory Bird Act. Philadelphia vireo populations are stable in Michigan. Forest harvesting continues to be prevalent across the state of Michigan which may create more habitat for Philadelphia vireos. ("Birdlife International", 2009; Moskoff and Robinson, 1996)
Rachelle Sterling (author), Special Projects, Tanya Dewey (editor), University of Michigan-Ann Arbor.
2009. "Birdlife International" (On-line). Vireo philadelphicus. IUCN Red List of Threatened Species. Accessed March 28, 2011 at http://www.iucnredlist.org/apps/redlist/details/146470/0.
2007. "Whatbird.com: Field Guide to Birds of North America" (On-line). Philadelphia vireo. Accessed March 28, 2011 at http://identify.whatbird.com/obj/693/_/Philadelphia_Vireo.aspx.
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Robinson, S. 1981. Ecological relations and social interactions of Philadelphia and red-eyed vireos. Condor, 63: 16-26.
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