Swainson’s Thrush

The Swainson’s Thrush is a secretive bird of forests and woodlands across the northern portion of North America. It’s beautiful flutelike, upwardly spiraling song is common during spring and summer there. Swainson’s Thrushes breed principally across the boreal forest from Alaska to Newfoundland, but also south through the Rocky and Coast Mountains. The winter range is southern Mexico, Central America, and northwestern South America south to north central Argentina.

Adult Male (spring/summer)

Cap is olive-brown. Auricular is lighter olive-brown with faint lighter lines. A buffy eye ring extends to the front of the eye creating a ‘spectacled’ look. Submoustachial strip cream, malar stripe brown and chin cream-white. Throat and breast cream-white with brown spotting. Off white belly often smudged pale rusty. Flanks are rufous tinged. Undertail coverts are white. Nape, back, rump and uppertail coverts olive-brown. Wings, tail olive-brown and underwing coverts buffy yellow.

Adult Female (spring/summer)

Similar to adult male.

Juvenile

Similar to adults but spotted and streaked on back.

General:

Medium size thrush with slender body and long wings. Length: 16-19cm. Wing: 29-31. Weight: 23-45grams.

Behaviour:

Generally inconspicuous on the forest floor or shrub layer. Forages by kicking up leaf litter and will more frequently fly-catch from shrubs than other thrushes. Usually hops along the ground, with its tail held upright, occasionally pumping its tail.

Habitat:

Closed canopy forest, including deciduous, mixed or coniferous. During the breeding season they are usually found in coniferous forest, typically spruce-fir forest, although they will use riparian forest in more southerly populations. In winter they frequent rainforest and second growth areas. During migration they are known to use a variety of habitats, but require a dense understory for protective cover.

Information:

Swainson’s Thrushes, while primarily insect eaters during the breeding season, utilize late summer and fall berry crops to increase fat loads for migration and to provide additional energy during cold winter weather. Birds will frequent open or disturbed areas, such as bogs, clear cuts and burns to take advantage of high berry production in these areas. Frequently found in mixed species flocks during migration. Swainson’s Thrushes breed from April to June and typically have one brood per year, however two have been reported in some areas. Most clutches are 1-5 blue, greenish-blue speckled with reddish or brown eggs. During the breeding season most Catharus thrushes in BC segregate themselves by elevation or habitat. Swainson’s Thrushes are found primarily in lower elevation forest and Hermit Thrushes in upper elevation forest in BC. Veerys, while found in low-land forest, are found in more riparian areas than Swainson’s Thrushes.

Similar species:

Gray-cheeked Thrush, Hermit Thrush, Varied Thrush, Wood Thrush.

Conservation Status:

Listed as Least Concern but gradually declining across its range with steepest declines along the West Coast and in parts of the Northeast.

Capture Rates:

Capture of Swainson’s Thrush begin in April as these long distance migrants arrive at Colony Farm from Central and South America. Capture rates peak in August as young disperse from breeding areas of the mixed forest and brushy riparian habitat of Colony Farm.

Molt Summary:

PF: HY partial; PB: AHY complete; PA absent
Preformative molt includes some to all med covs and 0 – 5 inner gr covs but no terts or rects.

This HY in August has completed it’s preformative molt and is showing a clear molt limit (indicated with the red arrow) between the replaced inner 3 greater coverts and the retained 7 outer greater coverts. Notice the buffy tipping or ‘tear-drops’ on the retained outer greater coverts and the olive-greenish edging on the replaced inner coverts. Notice also the replaced feathers are longer than the retained ones producing the visible ‘step-in’ between replaced and retained coverts typical of Catharus thrushes.

This HY bird in August has also completed its preformative molt but the molt limit here is much more subtle – the replaced inner GCs are longer with olive-brown edging and there is still the ‘step’ in between replaced and retained feathers but the outer retained GCs lack the buffy tear drops of the bird above.

This photo shows a close up of the under alula feathers of a hatch year (HY) in August. By sliding the large alula feathers (A2/A3) to one side the buffy shaft streaks of the retained under alula coverts can be seen (red arrow) contrasting with the adjacent replaced feather (blue arrow).

This HY in August is replacing inner greater coverts (GCs) as part of its partial preformative molt, the molt limit in the making between the 4 retained outer GCs and 6 inner GCs in sheath (red arrow).

This HY in August has completed it’s preformative molt showing a clear molt limit (indicated with the red arrow) between the replaced inner 3 greater coverts and the retained 7 outer greater coverts.

Notice again the buffy tipping or ‘tear-drops’ on the retained outer greater coverts and the chocolate brown edging on the replaced inner coverts.

Notice again the replaced feathers are longer than the retained ones producing the visible ‘step-in’ between replaced and retained coverts mentioned above.

As with other Catharus thrushes, the shape of the small, vestigial outer primary (P10) is helpful in determining age, this feather being broader, more rounded and longer in first year birds, often extending beyond the primary coverts as in the photo below.

P10 is narrower, more tapered and shorter than the primary coverts in adult birds as in the photo below of an ASY bird in May.

Another characteristic of first year (HY/SY) birds is the presence of fault bars which can be see on the rectrices (tail feathers) of the hatch year (HY) Swainson’s Thrush in August below.

The conspicuous growth bars near the tip of this young bird’s tail tell us that this bird probably was not well fed for two consecutive days, perhaps the first two days after it fledged from its nest.

These so called ‘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.

This after hatch year (AHY) in July is undergoing its definitive adult prebasic molt when all body and flight feathers will be replaced.

This sequence starts with the lesser and median coverts, then tertials and innermost primaries, primary replacement proceeding distally towards the outermost primary (P10). Primary coverts are molted with the corresponding primary. Greater coverts commence after the start of the primaries and proceed proximally towards the birds body with the innermost GC10 the last replaced. The carpal covert is replaced when the GCs complete. After the tertials the remainder of the secondaries will be replaced beginning with the outermost and proceeding proximally towards the birds body with the innermost secondary (S6) being the last flight feather replaced.
The alula covert is molted at the same time as the median coverts, however the alula feathers are molted during molt of the primaries and are among the last feathers to be replaced.

During heavy prebasic molt like this adult birds can still be aged second year or after second year based on the contrast between the molted alula covert and as yet unmolted alula. Here all are worn but too brightly coloured to be retained juvenal feathers especially given this species is a long distance migrant and retained juvenal feathers would be close to 12 months old.

This combined with the shape of the outermost (as yet unmolted) primary (P10) could therefore still allow this bird to be aged ASY (i.e. an adult bird going through at least its second definitive adult prebasic molt) for another week or so until the alula feathers and outermost primary are lost at which time it would simply be aged AHY.

This AHY in August is at the very start of its definitive adult prebasic molt with innermost primaries (P1 & P2) and tertials in sheath. Again, this is likely an ASY (i.e. an adult bird going through at least its second definitive adult prebasic molt) given the lack of wear to the as yet unmolted alula feathers and secondaries.

The tail of the AHY immediately above is as yet unmolted and showing extreme wear having been worn for one long distance migration and an entire breeding season. Note however the relatively rounded and truncate shape to the outer left rectrices.