During 
the last Christmas Bird Count, by the Mendoza river in Pipeline road (central Panama), I found an interesting nightjar perched atop a branch in the open, around 11:00 AM, probably a migrant Chuck-will's-widow (ChWW). All the team members (Venicio "Beny" Wilson, Rafael Luck and Osvaldo Quintero) were able to see the bird and to get photographs. The first thing we noticed was its posture, it was vertically perched like a potoo, showing well the undertail pattern but not as well the back nor the crown. We also noticed immediately its hugeness, looking definitively larger than the common nightjar usually found in Panama (Pauraque), approaching the size of a Common Potoo. Essentially, it was a cryptic patterned nightjar, mostly rufous-brown, with darker tones in the wing coverts and paler in the back. The underparts
were mostly dark brown, with many rufous spots and fine dark vermiculations. The closed wings almost reached the tail tip, and the primaries were black with rufous barring. More interesting, at least the two external rectrices showed pale buffy tips and buffy white inner webs all the way to the undertail coverts (which were rufous with black barring), with the external webs boldy barred in buff and dark brown. The bird didn't vocalize nor move during the ten minutes or so that we 
watched it. Osvaldo and Rafael used their flashes with the camera... my own photo (digiscoped with my DSLR camera) was with natural light (however, the day was cloudy and the lights conditions were not the best). The Common Pauraque (above photo, a male) can be easily ruled out by tail pattern and lack of conspicuous chestnut cheeks (and less patterned shoulders and wing coverts). The really hard task is to rule out the Rufous Nightjar (RNj). The RNj is one of our resident nightjars, closely related to the ChWW. It prefers drier habitats in the Pacific slope of the country (second growths, thickets and forest edges), so a bird found deep inside a mature. wet forest in the Caribbean slope as this one will hardly be a RNj... in the other hand, the ChWW is 
a boreal migrant recorded in a wide variety of habitats in Panama, including mature wet forest, but also the same habitats preferred by the RNj and even residential areas with big trees. So, habitat favors ChWW... but birds doesn't read books, and there are some records of RNj in the Caribbean slope of the Canal area (although in cleared areas), so habitat alone is not enough to separate these birds. Another thing is the size. The ChWW is our largest nightjar, but we all know that size appreciation in the field is very subjective, and depends of maaaaany factors, including posture. About field marks, these two species are almost impossible to tell apart in the field, specially because we do not hear the characteristic call of the ChWW in its wintering grounds. In the hand, the ChWW have lateral filaments to the rictal bristles, absent in the RNj. Enlarge my photo of a RNj (above, from Summit Ponds, just across the continental divide in the Pacific slope of central Panama) and you will see the long rictal bristles arising from the base of its mouth. Now, call me crazy, but enlarge Osvaldo's photo (the first of this post)... although the resolution is not the best due to the distance, I can see that the rictal bristles look like a brush! Photo artifact? Lateral filaments? OK, it is hard to tell, so lets forget it. What about plumage marks. Some references list differences in the overall coloration, the color of the crown, the back and the throat to separate these two species in the field, but I have found that these field marks are not reliable after checking LOTS of ChWW's photos in the internet, and many RNj's photos as well (there not so many published photos of this species). This probably is due to many contributing factors, like age, sex and race (ssp) of the bird, the photographic settings, the light conditions, etc... but one 
characteristic seemed useful to correctly separate these two: the male's undertail pattern. In the ChWW, the white (or buff) of the inner webs is very extensive, more than that of the RNj, almost reaching the base of the feather and only skipping the tips. You can see this in Nate's photo, of The Drinking Bird blog, of a preserved ChWW specimen. Compare it with our bird. In the RNj, the white (or buff) is shorter, occupying only the distal half or third part of the feather. This difference is listed and/or illustrated by Ridgely & Gwynne (Birds of Panama), Ridgely & Greenfield (Birds of Ecuador), Hilty & Brown
(Birds of Colombia, attached illustration), Garrigues & Dean (Birds of Costa Rica), just to mention a few! It is not exactly a field mark, because is very unlikely to watch this under normal field viewing conditions, so we were VERY lucky. I'm pretty convinced that our bird is a male Chuck-Will's-Widow based on tail pattern and habitat, and I want to know if you agree with me via the comments. Happy birding and Happy New Year by the way!
the last Christmas Bird Count, by the Mendoza river in Pipeline road (central Panama), I found an interesting nightjar perched atop a branch in the open, around 11:00 AM, probably a migrant Chuck-will's-widow (ChWW). All the team members (Venicio "Beny" Wilson, Rafael Luck and Osvaldo Quintero) were able to see the bird and to get photographs. The first thing we noticed was its posture, it was vertically perched like a potoo, showing well the undertail pattern but not as well the back nor the crown. We also noticed immediately its hugeness, looking definitively larger than the common nightjar usually found in Panama (Pauraque), approaching the size of a Common Potoo. Essentially, it was a cryptic patterned nightjar, mostly rufous-brown, with darker tones in the wing coverts and paler in the back. The underparts
were mostly dark brown, with many rufous spots and fine dark vermiculations. The closed wings almost reached the tail tip, and the primaries were black with rufous barring. More interesting, at least the two external rectrices showed pale buffy tips and buffy white inner webs all the way to the undertail coverts (which were rufous with black barring), with the external webs boldy barred in buff and dark brown. The bird didn't vocalize nor move during the ten minutes or so that we 
watched it. Osvaldo and Rafael used their flashes with the camera... my own photo (digiscoped with my DSLR camera) was with natural light (however, the day was cloudy and the lights conditions were not the best). The Common Pauraque (above photo, a male) can be easily ruled out by tail pattern and lack of conspicuous chestnut cheeks (and less patterned shoulders and wing coverts). The really hard task is to rule out the Rufous Nightjar (RNj). The RNj is one of our resident nightjars, closely related to the ChWW. It prefers drier habitats in the Pacific slope of the country (second growths, thickets and forest edges), so a bird found deep inside a mature. wet forest in the Caribbean slope as this one will hardly be a RNj... in the other hand, the ChWW is 
a boreal migrant recorded in a wide variety of habitats in Panama, including mature wet forest, but also the same habitats preferred by the RNj and even residential areas with big trees. So, habitat favors ChWW... but birds doesn't read books, and there are some records of RNj in the Caribbean slope of the Canal area (although in cleared areas), so habitat alone is not enough to separate these birds. Another thing is the size. The ChWW is our largest nightjar, but we all know that size appreciation in the field is very subjective, and depends of maaaaany factors, including posture. About field marks, these two species are almost impossible to tell apart in the field, specially because we do not hear the characteristic call of the ChWW in its wintering grounds. In the hand, the ChWW have lateral filaments to the rictal bristles, absent in the RNj. Enlarge my photo of a RNj (above, from Summit Ponds, just across the continental divide in the Pacific slope of central Panama) and you will see the long rictal bristles arising from the base of its mouth. Now, call me crazy, but enlarge Osvaldo's photo (the first of this post)... although the resolution is not the best due to the distance, I can see that the rictal bristles look like a brush! Photo artifact? Lateral filaments? OK, it is hard to tell, so lets forget it. What about plumage marks. Some references list differences in the overall coloration, the color of the crown, the back and the throat to separate these two species in the field, but I have found that these field marks are not reliable after checking LOTS of ChWW's photos in the internet, and many RNj's photos as well (there not so many published photos of this species). This probably is due to many contributing factors, like age, sex and race (ssp) of the bird, the photographic settings, the light conditions, etc... but one 
characteristic seemed useful to correctly separate these two: the male's undertail pattern. In the ChWW, the white (or buff) of the inner webs is very extensive, more than that of the RNj, almost reaching the base of the feather and only skipping the tips. You can see this in Nate's photo, of The Drinking Bird blog, of a preserved ChWW specimen. Compare it with our bird. In the RNj, the white (or buff) is shorter, occupying only the distal half or third part of the feather. This difference is listed and/or illustrated by Ridgely & Gwynne (Birds of Panama), Ridgely & Greenfield (Birds of Ecuador), Hilty & Brown
(Birds of Colombia, attached illustration), Garrigues & Dean (Birds of Costa Rica), just to mention a few! It is not exactly a field mark, because is very unlikely to watch this under normal field viewing conditions, so we were VERY lucky. I'm pretty convinced that our bird is a male Chuck-Will's-Widow based on tail pattern and habitat, and I want to know if you agree with me via the comments. Happy birding and Happy New Year by the way!
Posts
Yup, Chuck-Will's-Widow, a very wet one. Good thing it was a male.
ReplyDeleteWe were lucky in that aspect!
ReplyDeleteCongrat's, it looks good for Chuck-will's-widow. However, while I have yet to see a Rufous Nightjar where the patch on the inner webs of the outer rectrices (=the ones visible from below when the tail is closed) actually appeared to "touch" (or nearly so) the undertail coverts, beware that there is quite some variation in this species: It is not true that it "only [occupies] the distal half or third part of the feather". As seen in the field, it most commonly occupies the distal 2/3 of the inner web of the feather with only a relatively narrow, but nevertheless clearly visible, dark-and-rufous barred section at the basal 1/3. However, as noted there is a level of variation, and individuals with smaller patches, only around 1/2 the length of the tail feather, do occur. A typical male RN:
ReplyDeletehttp://www.birdforum.net/opus/images/e/e2/Rufous_Nightjar12.jpg
Certainly different than that seen on the your photos, but the difference is less distinct than one might believe based on the illustration in many field guides.
A relatively upright manner of perching (at 45 degrees, or sometimes even more) is not unusual for singing male RN encountered during the night, but I don't recall seeing one perched like that during the day. I do not know the CWW well enough to comment on possible variation in its tail pattern or its typical manner of perching when singing.
NeoBirder: thank you very much for your solid analisys. I still need to see this mark in the field, despite I have seen many Rufous Nightjars. The photo (of your link) is very illustrative
ReplyDelete