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Bicknell's Thrush / Grive de Bicknell (Catharus bicknelli) |
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Introductory notes:
Bicknell's Thrush is quite similar to Gray-cheeked Thrush, and should be identified with care in areas where both may occur. Bicknell's generally shows more yellow on the lower mandible, and a greater contrast between the back and a rustier tail. As with other Catharus thrushes, ageing Bicknell's Thrush is easiest via the greater coverts, but can be difficult by spring. |
QUICK TIPS:
| 1) Look
at the greater coverts - the presence of buffy-white
tips or shaft streaks indicates a HY/SY bird, however the
absence of buffy-white shaft streaks is NOT an
entirely reliable indicator of AHY/ASY, as some HY/SY
birds may lack these markings
2) Consider the shape of the rectrices - they are narrow and relatively tapered on HY/SY birds, but broad and relatively rounded on AHY/ASY birds
3) Examine
the outermost primary (p10) - on HY/SY birds it
is rounded and 0-6 mm shorter than the primary
coverts, while on AHY/ASY birds it is narrow,
pointed, and 4-10 mm shorter than the primary coverts
Species account updated February 2011 |
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Ageing and sexing overview:
Ageing and sexing details:
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JAN - JUL: after-second-year
unknown |
Bicknell's Thrushes vary little by age in terms of overall appearance, and sex can be determined only be checking for a brood patch or cloacal protuberance.
A typical Bicknell's Thrush; note the extensive yellow on the lower mandible.
Photo by Marcel Gahbauer,
McGill Bird Observatory (QC), May 2005
ASY Bicknell's Thrushes have a uniformly brown wing, without pale tips on any greater coverts. The outermost primary is narrow and 4-10 mm shorter than the adjacent primary coverts.
This closeup unfortunately does not show p10, nor most of the greater coverts, but
the few greater coverts that are visible lack pale tips or shaft streaks.
Photo by Marcel Gahbauer,
McGill Bird Observatory (QC), May 2005
ASY Bicknell's Thrushes have relatively broad rectrices.
Photo by Marcel Gahbauer,
McGill Bird Observatory (QC), May 2005
RETURN TO AGE/SEX
OVERVIEW
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JAN - JUL: second-year
unknown |
Overall plumage is rarely helpful in ageing Bicknell's Thrushes, but in some cases SY individuals have readily visible pale tips to their retained juvenile greater coverts, which are a diagnostic feature.
SY Bicknell's Thrushes retain at least half of their juvenile greater coverts, which usually have a pale tip or shaft streak. Also, p10 is broader and more rounded than on ASY birds, and although usually also shorter than the primary coverts, the difference in length is usually smaller (0-6 mm).
SY Bicknell's Thrushes have relatively narrow rectrices, that often show show somewhat more wear in spring than those of ASY birds.
RETURN TO AGE/SEX
OVERVIEW
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JUL - DEC: after-hatch-year
unknown |
Overall appearance usually cannot be used to identify AHY Bicknell's Thrushes, since the pale tips to the greater coverts that may be used to distinguish HY individuals are not always readily visible.
AHY Bicknell's Thrushes have uniformly brown wings, without pale tips or shaft streaks on any of the greater coverts. Additionally, p10 is narrow, pointed, and substantially shorter (by 4-10 mm) than the primary coverts.
AHY Bicknell's Thrushes have broad rectrices with relatively blunt tips, although there can be a slight point right at the end of the shaft.
RETURN TO AGE/SEX
OVERVIEW
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JUL - DEC: hatch-year
unknown |
Overall plumage is rarely helpful in ageing Bicknell's Thrushes, but with a good view of the wing of a perched individual, it may be a possible to spot the pale tips to their retained juvenile greater coverts, which are a diagnostic feature.
Photo by James Junda,
McGill Bird Observatory (QC), October 2008
HY Bicknell's Thrushes usually have a molt limit among the greater coverts, with up to several inner coverts replaced that are slightly darker and lack any pale markings, contrasting with the retained outer juvenile greater coverts that have pale tips or shaft streaks. Additionally, p10 is relatively broad, rounded, and only slightly shorter (by 0-6 mm) than the adjacent primary coverts..
In this case, only the innermost greater covert has been replaced, and is noticeably
darker and also longer than the remaining juvenile greater coverts.
Photo by James Junda,
McGill Bird Observatory (QC), October 2008
HY Bicknell's Thrushes have narrow and relatively pointed rectrices.
This photo shows the typical narrow and pointed shape of the rectrices. It appears the
two central rectrices are being replaced, which is not known to occur as part of the
preformative molt, but could simply be an example of adventitious replacement.
Photo by James Junda,
McGill Bird Observatory (QC), October 2008
RETURN TO AGE/SEX
OVERVIEW
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