A Presumptive Rufous-capped Warbler X Common Yellowthroat Hybrid at Chalk Bluff Park, Texas.

Rufous-capped Warbler (Basileuterus rufifrons) is a widespread warbler species ranging from the southern fringe of the US to the northernmost bits of South America. Within that extensive range, populations form distinct subspecies groups. Northern birds (such as the ones that range occasionally into Arizona and Texas) are part of the white bellied northern group rufifrons. These populations (including the yellow bellied salvini of Guatemala and Belize) differ from more southerly birds in having a slimmer, longer-tailed structure and an entirely different vocal repertoire. These are important distinctions to make when comparing images and vocalization data available online. Birds from Pacific Guatemala south through Columbia and Venezuela are so distinct as to be considered a separate species at times. Rufous-capped Warblers that wander into Texas are believed to belong to the subspecies jouyi, which is part of the northern white bellied rufifrons subspecies group. Important distinctions for this subspecies include a strong rufous cap and rufous cheek patch; having yellow restricted to the throat and breast with remainder of the underparts pale grayish with browner sides and flanks. Importantly, the undertail coverts are grayish to off white. The song of Rufous-capped Warbler belonging to this subspecies group is variable, but is a series of somewhat staccato chips that often change pitch and interval during each song. Calls notes consist of somewhat hard, low pitched chips and much higher pitched tsik notes.

On Sunday, April 28th, I visited Chalk Bluff Park northwest of Ulvade, Texas seeking two Rufous-capped Warblers being reported there since first discovered by Tim Lentz on April 20th. These two birds hold territories well separated from one another. One, known as the “boat ramp” bird is located at 29 21.593, -99 59.078, just southeast of a boat ramp parking area at Chalk Bluff. The second individual holds a territory along the Pecan Trail which runs through a grove of trees. We relocated this bird at 29 21.659, -99 58.319, some 1200 meters east of the boat ramp bird.

The boat ramp bird is a classic northern type Rufous-capped Warbler in every respect. Its appearance is typical and it sings a standard northern Rufous-capped Warbler repertoire. The pecan trail bird, on the other hand, is distinctly different and has several attributes which suggest it is in fact a hybrid.

The first photo, taken in 2012, is of the bird that is occupying the boat ramp territory. Notice the distinct face pattern with a bright, sharply defined rufous cheek patch, overall slender shape and in particular, the plain coloration of the undertail coverts.

Chalk Bluff Rufous-capped Warbler

The next couple of photos represent the pecan trail bird. Notice the subtly different shape to the bird, a thinner, longer bill, the distinctly peculiar face pattern, and the clearly yellow undertail coverts. The muted rufous cheek patch with a pale whitish wrap around its rear border are features not characteristic of Rufous-capped Warbler.

Pecan Trail Warbler 2 FB Pecan Trail Warbler FB

Even more peculiar are its vocalizations. It sings a repertoire of Common Yellowthroat-like songs and was heard to give a rapid trill of chip notes indistinguishable from that of Common Yellowthroat. It was not heard to give any vocalizations typical of Rufous-capped Warbler.  A couple of song phrases are presented here.

      1. Chalk Bluff warbler

I’m sure more critical analysis would reveal further distinctions indicative of hybrid origin for this bird. Its voice, structure, coloration, and behavior all suggest Rufous-capped Warbler X Common Yellowthroat as the parentage. Birders visiting the area in the next few weeks have a great opportunity to document this presumptive hybrid.

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10 Responses to A Presumptive Rufous-capped Warbler X Common Yellowthroat Hybrid at Chalk Bluff Park, Texas.

  1. Nick Block says:

    I buy it! To me, particularly when viewed next to a “typical” jouyi Rufous-capped, it’s clearly not a pure Rufous-capped. Considering its vocalizations and plumage, it certainly makes sense that the other parent was a Common Yellowthroat. Perhaps the dad was a Common Yellowthroat and taught this guy his songs.

    At ~8.8% different at cyt-b (corrected), this is a pretty darn “old” hybrid combination for passerines! It would be awesome to get some feathers or blood to confirm the parents’ identities and write this up.

  2. Miranne Pellerin says:

    Well, how embarrassed I would be trying to find a Common Yellowthroat, after hearing that song! Great photos, thanks for sharing them.

  3. Tim Brush says:

    You should write it up!

  4. Dave Dolan says:

    When I saw the male last year he was running around with a male common yellowthroat.

  5. Keith Arnold says:

    Having just examined a series of nearly 30 specimens of Rufous-capped Warblers, I can clearly state that bill length and size and the intrusion of gray into the rufous cheek patch are little vale, as the species shows significant variation in these characters. In the series, I found only two individuals with buffy, not yellow, undertail coverts, with the remainder having coverts concolor with the lower abdomen, thus, this seems like a good feature to consider for hybridization. The wide superciliary strip also seems like a valid character, except that in Common Yellowthroats, this extends across the lores, while this individual clearly has the rufous cap covering the area.
    I am a bit reluctant to put too much emphasis on vocalizations, as the literature contains accounts of many different parulid species singing the song of another species. Indeed, a number of passerines are known to learn their vocalizations, based on their neighbors.
    Finally, while RCWA x COYE seems the most likely hybrid combination, I wonder why the black mask of the COYE is not evident in some way.

  6. John Berner says:

    I have no knowledge of this species but not sure about the vocalizations indicating a COYE x RCWA hybrid. Here’s a link to a recording from Colombia of a presumed Rufous-crowned Warbler. Note the similarity of this linked recording to the one on this site from Chalk Bluff of the presumed hybrid. (Thanks to Drew D for discovering this). I also think Dr. Keith Arnold raises some concerns above.


  7. admin says:

    Hi John,

    Thanks for your thoughtful responses to my blog post. Rufous-capped Warbler identification is a complex matter, complicated in large part due to the fact that there are some very distinctive forms that occur in different parts of its range. A good primer would be to read through the taxonomic information that I presented in my original post. All of the links that you supplied in your responses are those of the southernmost forms delattrii and mesochrysus. These southern subspecies are perhaps best considered a different species, Chestnut-capped Warbler. Their structure, plumage, calls, and songs are all different to those of northern birds. The morphologic differences were discussed in my original post. Vocal differences are equally striking and most birds can be quickly assigned to northern or southern type vocalizations. Looking at the cuts available on xeno-canto, those from Columbia, Panama, Costa Rica, and Nicaragua are all birds of the southern delatrii/mesochrysus clade. Their songs are slower, more musical jumbles of notes, quite Canada Warbler-like to my ear, and reminiscent of a yellowthroat. The contact notes are also different from northern birds. XC76601 and XC76599 from Guatemala are also Chestnut-capped Warblers, based on call type and song type, despite being labeled otherwise. Guatemala is the area where three distinct forms come into close proximity. Recordings from Mexico are typical of the northern forms, with rapid staccato notes shifting in pitch or tonality during the song bout. The contact note of northern birds is an extremely brief tick note not heard from southern birds. Finally, I should reiterate that the Chalk Bluff bird was also heard to give a Geothypis-like trill or rattle, something not given by any form of Rufous-capped Warbler. An example can be heard with XC80620. I believe that these contact calls are likely innate, even in oscine parulids, in contrast to their more elaborate songs which are learned. Thus a Rufous-capped Warbler giving a yellowthroat-like trill is cause for alarm.

    I suspect a modern review of the systematics would result in northern and southern Rufous-capped Warblers being considered separate species. So how is that relevant to the Chalk Bluff bird? Well, I believe that the only relevant comparison is one made with northern Rufous-capped Warblers since those are the ones that occur in Texas. Once the Chestnut-capped forms are weeded out of the comparison, the peculiarity of the Chalk Bluff bird becomes apparent.

  8. John Berner says:

    Thanks for your detailed response. I agree the southern form has yellow on the lower belly and under the tail. Have you ruled out the Chalk Bluff bird being an inter-grade between the northern and southern forms of rufous-capped warbler. Wouldn’t that theory also cover all that was observed. Southern song and call note, southern undertail coverts, northern-type breast/belly (white). Also the fact that there seems to be no evidence of male common yellowthroat plumage would seen to be a problem for the hybrid theory. Maybe this is a bird originally from Guatemala. Dr. Arnold feels the songs are variable and so is bill size. Just stirring the pot…. (p.s. I was supposed to go to Chalk Bluff tonight but a family health issue prevented it–i have seen/heard neither bird.

  9. admin says:

    Hi John,

    Thanks again for your thoughts. Rufous-capped Warbler Intergrades, if they truly exist at all, would occur in areas where they meet, which would be in Guatemala, some 1100 miles from Chalk Bluff Park. Dr. Arnold provided some thought provoking comments on the variability of the rufous ear covert pattern and bill structure. Without examining the specimens myself, I cannot refute or corroborate his assessment. However, I would indeed be surprised to examine a specimen showing the characters of ear covert pattern and bill structure found on the Chalk Bluff bird. I am very familiar with both the northern and southern forms of Rufous-capped Warbler and was immediately struck by the odd plumage, shape, behavior, and voice of the Chalk Bluff bird. Rufous-capped and Chestnut-capped warblers do not utter yellowthroat-like trills/rattles. They just don’t. Ever. If the bird was somehow an intergrade, I would expect to hear some vocalizations typical of either parental form, which I did not. Again, I would encourage you to analyze the songs in greater detail. You will find that the similarity of the southern Rufous-capped/Chestnut-capped warblers is superficial. Common Yellowthroats and the Chalk Bluff bird songs contain repeating elements, the familiar witchity-witchity-witchity. The Chestnut-capped Warbler songs usually have some introductory chips followed by a jumble of more complex elements, which do not form repeating “phrases”. So one does not hear a rollicking pattern repeated as with Common Yellowthroat and the Chalk Bluff bird.

    I do appreciate contemplating alternative viewpoints and I appreciate your thoughtfulness in this regard. It would be wonderful to acquire some blood or tissue in order to carry out some DNA analysis that might corroborate the findings. My own busy schedule and those potential contributors has thus far preventing moving forward. I hope your family health issue resolved itself with a good outcome. If you do make it out to Chalk Bluff, I would spend some time trying to locate the other Rufous-capped Warbler that was at least formerly hanging out at the boat ramp area. Seeing the two within a short time frame would reinforce their differences.

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