Description and ecology
The Wood Thrush (Hylocichla mustelina) is a large migratory songbird (20 cm long). It is a member of the True Thrush Family, Turdidae. The Wood Thrush is the only species of Hylocichla in the world, this genus is thus termed a monospecific genus by evolutionary ecologists. In Ontario, Wood Thrushes are most often observed during the height of spring, in the month of May during migration or just after they arrive in their breeding habitat – dense forests. The male birdsong, among the most beautiful in the world, is heard from May through July – recognized by a flute-like “ee-oh-lay” mnemonic, the middle of their three-part song. Wood Thrushes have a large breeding range throughout Eastern North America, from Lake Superior to northern Florida, Missouri to the Atlantic Coast. Between October and April, Wood Thrushes winter in the cordilleras and lowland rainforests of Mesoamerica from southern Mexico (Yucatan, Quintana Roo) through Panama.
Both sexes of Wood Thrush are cinnamon-brown above, white with brown spots on their underparts, giving them a dotted appearance on the belly. Juveniles show a somewhat muted version of the same pattern. All Wood Thrushes have a bold, white eyering. Males are slightly more vibrant than females.
Wood Thrushes arrive in the Lake Erie Region in the first week of May. Males arrive a few days before the females. Almost immediately, male Wood Thrushes will begin singing and established a breeding territory, filling the forest atmosphere their with gorgeous birdsong. They sing persistently, often all day, into the evening hours. Male Wood Thrushes will chase females through the forest canopy as they try to attract them. Once a breeding pair has been established, the nest, a circular cup in the forest understory, is built by the female Wood Thrush. She lays up to 3-4 eggs, each turquoise with no markings. Incubation is completed primarily by female Wood Thrushes over a period of 13 days, while the nestling raising, achieved by both parents, takes a further 14 days. Wood Thrushes raise up to 3 broods per season (especially in the southern areas of their distribution close to the Gulf of Mexico), in Ontario, 1-2 broods is typical.
Male Wood Thrushes will continue to sing throughout breeding season. This lasts from May through July – during late spring and early summer, or a period called “high summer”, where day length, plant growth, insect activity, and bird song are all at their maximum levels. As July fades into August, summer begins to wane. One of the first ecological signs of summer’s aging is the diminishing of Wood Thrush birdsong. As nestlings are raised, male Wood Thrushes will sing less frequently, with emphasis shifted over to parental duties. During the last weeks of July, Wood Thrushes will begin their post-breeding moult (the larger of the two annual moults), where all their feathers are replaced over a period of 2-3 weeks. Wood Thrushes move to foraging areas by August, with strong focus on building up their fat reserves for their upcoming autumn migration in September. During this time, Wood Thrushes do not sing – silent even during dawn. August is the time when nights, for months only a few hours long, begin to grow in length, days begin to diminish, and birdsong ceases, signalling summer’s decline – although the days may still be warm and humid. Around a month later, when the autumn crickets become loud, late goldenrods and aster’s begin to flower – summer is all but over, except visually (much of the forest canopy is still deep, but more darker green). The now September canopy, now almost devoid of birdsong, will remain green for around another month, until the first frosts transform it into a plethora of red, orange, and yellow characteristic of Canadian autumns. But by then, Wood Thrushes are long gone. They left by late September, and by mid October, are firmly in the tropics.
Wood Thrushes are insectivores in the spring and early summer. They especially prefer beetles (Coleoptera), flies (Diptera), caterpillars (Lepidoptera), and ants (Hymenoptera). In late summer, to accumulate fat reserves for autumn migration, Wood Thrushes will begin to consume fruits, moving dense forests to canopy gaps or more open forests – where fleshy fruits are more abundant. Spicebush (Lindera benzoin), grapevines (Vitis sp.), blueberries (Vaccinium sp.), cherries (Prunus sp.), elderberries (Sambucus, especially the Red Elderberry, Sambucus racemosa), Pokeweed (Phytolacca americana), holly (Ilex sp.) dogwoods (Cornus sp.) and Black Tupelo (Nyssa sylvatica).
Ontario, with it’s dense deciduous forests around Lake Huron, is an important breeding ground for Wood Thrushes. Unfortunately, many of southwestern Ontario’s original forests were cleared, resulting in a large population reduction for this gorgeous songbirds in intensively agricultural areas like Perth County. However, Wood Thrushes have thrived in the larger surviving woodlots in southwestern Ontario. The Long Point Basin is home to some of the largest Wood Thrush populations in southwestern Ontario, in no small part due to the proportionately larger forest cover (>35% in many areas) preferred by these birds. As land conservation efforts continue to restore ecosystems, including forests, to Norfolk and Elgin counties, the Wood Thrush population may benefit.
Why are the birds are so quiet in late summer? Wildlife phenology and seasonal lag
Ecologically, wildlife behaviour is closely in sync with Earth’s photoperiod, or daylength, while temperature and weather events tend to follow patterns of heat distribution, making them somewhat delayed (i.e., the warmest day of the year, usually in late July or early August, is around a month later than the day with highest insolation, June 21, the summer solstice). This delay is a phenomenon called seasonal lag. So although it may still be warm in August, even at night, wildlife quickly shifts focus to autumn activities – they are more in-sync with the sun, and mid-August is already autumn from a solar point of view. In contrast, wildlife activity (birds, insects, plant growth, etc.) is at maximal activity in early summer, near the solstice in late June.
Written by Matthew Palarchio, HBSc in Environmental Sciences, Western University
All photos are Stock photos retrieved from Canva
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