Yellow Warbler

The Yellow Warbler is the most strikingly yellow of North American wood warblers and within this species there is extensive morphological variation, more so than within any other wood warbler. It breeds throughout much of North America in habitats briefly categorized as wet, deciduous thickets. It winters in Central America and northern South America.

Adult Male (spring/summer)

Bright yellow head and throat. Eye is black. Bright yellow breast, belly and flanks all with reddish streaks. Undertail covers yellow. Yellow-green nape, back, rump and uppertail coverts. Yellow edged olive wings and yellow patches in tail.

Adult Female (spring/summer)

Similar to the adult male, but duller and more greenish above and paler yellow below. There is much less bold to no reddish streaking on the breast, sides and flanks.

Juvenile

Similar to adult female, but much more dull overall, and less obviously yellow.

General:

Medium size warbler. Length: 12-13cm. Wing: 16-20cm. Weight: 9-11grams.

Behaviour:

Yellow Warblers forage quickly and restlessly for insects, gleaning them from branches and leaves on shrubs, willows and small trees. They will also capture prey by making short flights or while hovering to reach leaves.

Habitat:

Common and widespread in any wet brushy habitat, such as willow thickets, field edges, stream edges. The most common feature of their habitat is the presence of various species of willows which dominate regions of high densities of Yellow Warblers, as in southern Canada, and regions where the species is sparse and local in distribution, as in the southwestern US.

Information:

The nest is a neat cup of grasses found in a willow bush or small tree, fastened to a fork in a branch just a few feet off the ground. Clutch is 1-7 grayish or green-white dark spotted eggs. The Brown-headed Cowbird frequently parasitizes the nests of the Yellow Warblers. The Warbler will often build another nest directly on top of the parasitized one, sometimes resulting in nest with up to 6 tiers.

Similar species:

Conservation Status:

Listed as Least Concern. Populations are stable.

Capture Rates:

A migrant species, a small number of Yellow Warblers breed in the riparian deciduous thickets of Colony Farm while the rest continue north to breed. Capture rates, starting in May and continuing through September/beginning of October, reflect the migrants passing through Colony Farm as well as the breeding population’s presence during the summer months of June and July. The peak during August reflects dispersal of both migrant and breeding individuals.

Molt Summary:

PF: HY partial; PB: AHY complete; PA: SY partial-incomplete, ASY partial
Preformative molt includes some to all med covs, 3 – 10 inner gr covs, usually the greater alula , and usually 1 – 3 terts, but no rects
1st PA includes 3 – 10 inner gr covs, usually 1 – 3 terts, sometimes s6 and occasionally s5, but no rects
Adult PA includes 8 – 10 inner gr covs and 2 – 3 terts, but no rects

Ageing and sexing wood warblers in the spring requires a complete understanding of the molt cycle of the individual species being studied. All of these birds will have undertaken a prebasic molt following the breeding season last year when adults replace all body and flight feathers. In species where the annual prebasic molt is the only molt occurring annually breeding occurs in basic plumage and the result when looking at an adult bird, which has undertaken this complete definitive prebasic molt, in the spring is that there are no discernible molt limits on the wing between replaced and retained feathers.

However, in many species there is a SECOND molt that occurs prior to the next prebasic molt called the prealternate molt which occurs in the late winter/early spring. This molt occurs in both adults and first year birds so that in the spring first year birds will show three generations of feathers but adults will also show molt limits but only two generations of feathers between adult prebasic and adult prealternate feathers.

Many Setophaga warblers have extensive prealternate molts in the late winter/early spring which results in adult birds having molt limits but unlike second year (SY) birds which show 3 generations of feathers, juvenal, formative and first alternate, adult birds show only two, adult basic and adult alternate.

The color of the lower mandible is pale (September – May) in HY/SY birds versus blackish in AHY/ASY birds. 

Many bright HY/SY females overlap in plumage with HY/SY males and should be left unsexed. Only the dullest HY/SY with indicative short wing chord should be sexed female as HY/SYs that lack red streaks are not necessarily female.

This SY in early May is a good example of the plumage overlap between HY/SY males and females. This bird is lacking any red streaking on the breast but is also very bright yellow.

The wing of the bird above is showing three distinct generations of feathers: retained juvenile primaries, secondaries, primary coverts, the main (lower) alula feathers (A2 & A3), and three outer greater coverts (GCs 1-3); formative lesser, median and inner greater coverts GCs 4-6, and first alternate inner greater coverts (GCs 7-10) and all three tertials (S7-S9).

The tail is showing very worn central rectrices but with extensive yellow to the inner webs of the outer rectrices (R5 & R6) diagnostic of males; HY females generally average relatively little yellow to the inner webs of these feathers.

This HY male Yellow Warbler in July has already completed its preformative molt and is showing very fresh plumage. On first glance it could be fairly easy to age this bird as an adult given the very fresh appearance of the wing, the edged primary coverts and fairly truncate remiges (primaries and secondaries). However once the wing is examined under magnification (close-up photo below) it is easy to see the difference between the replaced lesser, median, greater coverts and carpal covert and the retained juvenal primary coverts and alula, the molt limit shown with the red arrow between the outer GC and inner PC. The replaced feathers are better quality feathers, more darkly pigmented with darker rachis and more interlocking barbules and, therefore, tighter barbs, giving them a more solidly structured appearance compared to the adjacent retained juvenal feathers. Notice too the subtle difference between the replaced greater alula covert A1 (red arrow) and retained lower alula feathers A2 and A3.

For this reason, especially after adults of these same species begin to show signs of approaching the end of their complete prebasic molt banders should always confirm age by additional criteria such as skull ossification.

Under magnification the contrast can be seen between the distinctly yellow edged replaced greater alula covert (A1) and more lightly pigmented retained juvenal main alula feathers and lightly pigmented retained juvenal primary coverts with indistinct yellow edging and glossier replaced greater coverts and carpal covert with distinct yellow edging.

Notice also the difference in quality of retained versus replaced feathers, the barbs more closely compacted together on the replaced feathers versus more distantly spaced on the retained juvenal feathers.

This HY male in August has replaced lesser, median, greater coverts, carpal covert and tertials, the molt limits indicated with red arrows. Notice the narrow, tapered outer primary coverts and main alula feathers with indistinct yellow edging contrasting with the distinctly yellow edged replaced greater coverts and greater alula covert (A1).

This HY male also in August has replaced lesser, median, greater coverts, carpal covert and tertials, the molt limits indicated with red arrows. Notice the narrow, tapered outer primary coverts with indistinct yellow edging contrasting with the distinctly yellow edged replaced greater coverts. Notice in this example that the greater alula covert (A1) was not replaced and that there is no contrast between the 3 alula feathers.

This HY male in August has replaced lesser, median, 9 inner greater coverts, carpal covert and tertials, the red arrow pointing to the molt limit within the greater coverts. Notice the contrast between the retained outer greater covert (GC1) and adjacent greater covert (GC2) and carpal covert. Notice the narrow, tapered outer primary coverts and main alula feathers with indistinct yellow edging contrasting with the distinctly yellow edged replaced greater coverts and greater alula covert (A1).

This HY female in September has replaced lesser, median, greater coverts and carpal covert but no tertials, the red arrow pointing to the molt limit between the outer greater covert and inner primary covert. Notice the very narrow, tapered outer primary coverts with virtually no yellow edging contrasting with the distinctly yellow edged replaced greater coverts.

This HY female in August has replaced lesser, median, greater coverts, carpal covert and tertials, the red arrow pointing to the molt limit between the outer greater covert and inner primary covert. Notice again the very narrow, tapered outer primary coverts with virtually no yellow edging contrasting with the distinctly yellow edged replaced greater coverts.

Tail shape should always be used with caution due to a) individual variation, b) the possibility of accidental loss and replacement (adventitious molt), and c) in Setophaga warblers all age classes can show rounded retrices .
This HY male in August is showing tapered and sharply pointed rectrices with extensive yellow to the inner webs of the outer rectrices (R5 & R6) diagnostic of males; HY females generally average relatively little yellow to the inner webs of these feathers.

This SY male in May is showing 3 generations of feathers. In its preformative molt following the breeding season last year it replaced lesser, median and greater coverts and carpal covert, the preformative molt limit shown between the outer greater covert and inner primary covert (red arrow). In its 1st prealternate molt in the late winter early spring it replaced 8 inner greater coverts and all 3 tertials (blue arrows); primaries, secondaries, primary coverts and all 3 alula feathers are all retained juvenal feathers.

Notice the narrow and tapered outer primary coverts and alula feathers with little or no yellow edging contrasting with the distinctly yellow edged formative and broadly edged first alternate feathers.

This SY male in June is showing the same 3 generations of feathers. In its preformative molt following the breeding season last year it replaced lesser, median and greater coverts and carpal covert, the preformative molt limit shown between the outer greater covert and inner primary covert (red arrow). In its 1st prealternate molt in the late winter/early spring it replaced 6 inner greater coverts and all 3 tertials (blue arrows); primaries, secondaries, primary coverts and main alula feathers are all retained juvenal feathers.

Notice the lightly pigmented and narrow, tapered outer primary coverts and main alula feathers with little or no yellow edging contrasting with the distinctly yellow edged formative and broadly edged first alternate feathers.

This SY female in June is again showing 3 generations of feathers. During its preformative molt last fall, this bird replaced 8 inner greater coverts, the carpal covert and greater alula covert, then stopped molting. The preformative molt limits (indicated with red arrows) are the molt limits between GCs 2 and 3, between the carpal covert (CC) and retained juvenal primary coverts and between the greater alula covert (A1) and the adjacent larger alula feather (A2) all replaced in the preformative molt following the breeding season last year.

Greater coverts 5-10 were molted this spring i.e. the prealternate molt limit is between GC 5 and 6 (blue arrow). Thus the three generations of feathers are: GC 5-10 which are first alternate feathers i.e. the most recently molted feathers from the prealternate molt this spring; GC 1 and 2, the carpal covert and greater alula covert which are formative feathers i.e. retained feathers from the preformative molt last fall; and finally, the two larger alula feathers, primary coverts, and primaries and visible secondaries (not including tertials) which are retained juvenal feathers.

Note also how the retained juvenal feathers, particularly the primaries are very worn and abraded now being close to 12 months old and having been worn for one long Neotropical migration and for an entire breeding season.

A complete understanding of the timing, sequence and extent of molt is essential to the accurate ageing and sexing of birds in the hand always remembering that young males look a lot like adult females.
An understanding of the sequence of molt often allows adults in heavy prebasic to still be identified as SY or ASY.

This SY male in August is in the midst (towards the end) of its first definitive (adult) prebasic molt and being a species with a partial preformative molt (which doesn’t include primaries and secondaries) could simply be aged after hatch year (AHY).

In the complete adult prebasic molt all body and flight feathers are replaced and during the sequence of feather replacement the alula are molted during the molt of the primaries and are among the last feathers to be replaced (as can be seen here).

Notice the obvious contrast between the greater alula covert (A1) and as-yet-unmolted and very worn retained juvenal lower alula feathers (A2/A3). An ASY would show little or no contrast among the as-yet-unmolted alula feathers and molted greater alula covert (A1).

This bird can therefore still be aged SY for another week or so until the alula feathers are lost at which time it would simply be aged AHY.

A similar example of a second year (SY) bird in July, again in the midst of its definitive prebasic molt replacing all body and flight feathers. Again, notice the obvious contrast between the greater alula covert (A1) and as-yet-unmolted and very worn and tapered retained juvenal lower alula feathers (A2/A3). 

This SY male in June is showing abraded rectrices with extensive yellow to the inner webs of the outer rectrices (R5 & R6) diagnostic of males; SY females generally average less yellow to the inner webs of these feathers.

This AHY female in August has replaced all body and flight feathers in its definitive, complete adult prebasic molt following the breeding season. The wing is a prime example of what definitive adult plumage, with no discernible molt limits among the coverts or alula, looks like in fall. Compare this with the HY birds above.

This ASY male in May is showing a prealternate molt limit (blue arrows) between the outer 3 greater coverts replaced as part of the birds definitive adult prebasic molt last year and inner greater coverts replaced as part of its adult prealternate molt this spring together with the two innermost tertials (S8 & S9). Thus there are only two generations of feathers, adult prebasic and adult prealternate.

Notice also the distinct yellow edging to the primary coverts and alula and lack of contrast within and between feather groups.

Oddly this bird had one shorter primary (p2 illustrated with the black arrow) and although this feather was not in sheath (e.g. not growing) it was also not symmetrical and likely just an anomaly.

This ASY male in May is showing very truncate outer rectrices with a corner to the inner web and extensive yellow to the inner webs diagnostic of males; ASY females generally average moderately to extensive yellow but less than ASY males. Compare this with the first year (HY/SY) birds above.