The Wilson’s Warbler is a familiar and characteristic species of wet habitats in the boreal and Pacific coastal zones. It is the most widespread of its genus. It breeds throughout Alaska, most of Canada, and south through the western US to southern California and New Mexico and is often the most abundant warbler in these regions.
Adult Male (spring/summer)
Head has black cap; face is bright yellow with dull yellow auricular; large black eye; chin, breast, belly and undertail coverts bright yellow. Nape, back, rump and uppertail coverts olive-yellow. Tail and wings olive with yellow feather edging.
Adult Female (spring/summer)
Similar to male but with smaller or absent black cap.
Juvenile
Similar to adult but dull with smaller or absent black cap.
General:
Very small, long-tailed warbler with diagnostic black cap. Length: 10-12cm. Wing: 14-17cm. Weight: 5-10grams.
Behaviour:
Picks insects from foliage, twigs, and grasses low to the ground. It often catches flying insects on the wing like a flycatcher.
Habitat:
Breeds in shrub thickets of riparian habitats, edges of ponds, lakes, bogs, overgrown clearcuts of montane and boreal areas.
Information:
Being a widespread species, Wilson’s Warblers will nest anywhere from sea level to alpine; they build a cup-shaped nest using ground vegetation such as moss, grass and leaves. Usually the nest is placed on the ground, hidden by a clump of grass, but Pacific coastal lowland birds tend to place their nests low in shrubs. The female builds the nest herself and she will incubate the eggs for an average of 12 days; the clutch typically consists of 3-6 cream-white eggs with reddish-brown spotting. There are three subspecies of this bird that are currently recognized.
Similar species:
Orange-crowned Warbler, Yellow Warbler.
Conservation Status:
Listed as Least Concern but a a downward trend in numbers in western populations due to large-scale habitat loss.
Capture Rates:
Although caught in every month from April through October, capture rates of Wilson’s Warbler go through a large peak in May as they pass through Colony Farm to their nearby breeding areas. As long distance migrants, these warblers move south for the winter and are the only migrant warbler found in the tropical high plains (paramo).
Molt Summary:
PF: HY partial; PB: AHY complete; PA absent-limited
Preformative molt usually includes all med & gr covs, but not the greater alula, terts or rects
PA if present, is limited to feathers of the head.
Juveniles have the entire plumage washed brownish or buff with yellow lores and two buffy-yellow wing bars. Juv M=F.
In addition to molt, a criteria which is helpful in ageing Wilson’s Warblers is the variation in the extent of the crown patch being glossy black on adults and extending further forward and backward (see AHY/ASY below) than on hatch year birds like the two birds immediately below which is less extensive and shows greenish yellow mottling.
Ageing wood warblers in late summer/early fall requires close scrutiny as molt limits can be difficult to see when hatch year (HY) birds still have very fresh retained juvenal feathers.
The wings of the HY birds in August / September below are good examples of this. Under magnification the contrast between replaced lesser, median, greater coverts and carpal covert and retained primary coverts and main alula feathers (A2/A3) can often be seen but these molt limits can be quite subtle and for this reason, especially after adults of these same species begin to show signs of approaching the end of their complete prebasic molt age should always be confirmed by additional criteria such as skull ossification.
This HY in September is showing molt limits indicated with red arrows between the outer greater covert and inner primary covert, carpal covert and adjacent primary cover and greater alula covert (A1) and main lower alula feather (A2).
Notice also the outer primary coverts are narrow and tapered with very feint green edging contrasting markedly with the slightly fresher and olive-green edged greater coverts.
This HY in August is showing a similar pattern of replaced of replaced lesser, median and greater coverts (red arrows).
Notice again the narrow and tapered outer primary coverts with virtually no greenish edging contrasting with the slightly fresher and olive-green edged greater coverts. Notice also the thin and tapered main alula feathers (A2 & A3) with no green edging and compare these to the adult birds below.
Another HY in August showing a similar pattern of replaced lesser, median and greater coverts (red arrows).
Notice again the narrow and tapered outer primary coverts with virtually no greenish edging contrasting with the slightly fresher and olive-green edged greater coverts and the thin and tapered main alula feathers (A2 & A3) with no green edging.
Tail shape is another helpful clue to ageing birds in the hand. It is important not to use tail shape alone however as it is generally not very reliable because of a) individual variation and b) the possibility of accidental loss and replacement (called adventitious molt).
The tail of this HY in August is showing thin, tapered outer rectrices with sharp points to the tips. Notice also the fault bar in a straight line across the rectrices.
‘Fault bars’ are the result of environmental or nutritional stress that the bird encountered while it was growing in the feathers. Groups of feathers on hatch year birds are grown concurrently so a stress that results in a fault bar on the feathers is distributed in an even line. Adult feathers are sequentially grown so a fault line on the feathers is normally distributed in an uneven line.
Fault bars are caused when actively growing feathers cease to grow resulting in a dark line across the feathers. Young birds begin to grow their rectrices as nestlings and growth continues after fledgling. At this time the birds are dependent on their parents for their nutrition (the food required for the energy and proteins necessary for feather synthesis) but these adults are typically not only feeding themselves but also several hungry fledglings. This is a tremendously energetically taxing time for adults often resulting in undernourished young and the presence of fault bars.
Fault bars can be useful as an ageing criterion but banders should be aware that adults do sometimes lose all of their tail feathers at once accidentally. When such accidentally lost feathers are regrown adventitiously (outside the normal molt cycle), it is not unusual for such adult replacement rectrices to have prominent fault bars like those more often seen in juveniles.
Correctly sexing a bird depends on correctly ageing it first always remembering that young (HY/SY) males can look a lot like adult (AHY/ASY) females.
This adult (ASY) female Wilson’s Warbler in May is showing an extensive black cap which could easily be confused with a second year (SY) male.
The wing however shows definitive adult plumage, with no discernible molt limits among the coverts or alula, nicely edged primary coverts and truncate brightly edged green remiges (primaries and secondaries).
Adult (AHY/ASY) males have glossy black crowns extending far to the back of the crown like this ASY male in April.
This AHY male wing in August is a good example of what definitive adult plumage with no discernible molt limits looks like in the fall. The outer primary coverts are truncate with distinct olive-green edging not contrasting in colour or wear with the great coverts.
Notice also the shape of the main alula feathers with distinct olive-green edging and compare them to the alula of the HY/SY birds above.
This AHY male in August is also showing truncate outer primary coverts with distinct olive-green edging not contrasting in colour or wear with the great coverts. Notice again the shape of the main alula feathers with distinct olive-green edging and with no contrast between the greater alula covert (A1) and lower main alula feather (A2), both with distinct olive-green edging.
Wilson’s Warblers are long distance migrants and the only migrant warbler found in the tropical high plains (Páramo). By spring the retained juvenal feathers of SY birds are worn having been used for one long return Neotropical migration.
Adult birds like this ASY male in May although showing a degree of wear are still relatively fresh. Notice the truncate outer primary coverts and lower main alula feathers with distinct olive-green edging not contrasting markedly in colour or wear with the greater coverts or greater alula covert (A1).
This ASY male in May is showing the same extensive glossy black cap extending to the back of the crown.
The wing of the bird immediately above is showing very worn primary coverts but there is little contrast between the alula covert (A1) and the lower alula feather (A2) and the remiges (visible primaries and secondaries) are truncate, high quality basic feathers with glossy rachises showing almost no wear having been replaced in the birds prebasic molt following the breeding season the previous year.
The rectrices of the same bird are relatively broad and truncate, again showing little wear to the tips.
A similar example of what definitive adult plumage with no discernible molt limits looks like in the spring. This ASY male in May is showing very fresh feathers with very little wear. Notice again the truncate outer primary coverts and lower main alula feathers with distinct olive-green edging not contrasting markedly in colour or wear with the greater coverts or greater alula covert (A1).
The tails of two adult (ASY) birds in May, again showing broad, truncate outer rectrices which are relatively fresh and showing little wear to the tips.
Tail shape should always be used with caution due to adventitious loss.