I just got back from an amazing 19 days spent in and around Rondeau Provincial Park leading guided hikes and generally birding like crazy. Expect a number of posts in the near future as I try to catch up.
My most interesting bird of the trip was in late morning on May 13, when I was birding a strip of road that was quite busy with migrant warblers. I spotted one bird that on first glance made me think Chestnut-sided, but it quickly became clear that it wasn't one.
I followed this individual for quite some time, and managed to get quite a few photos. You'll notice that it seem to be looking at me in many photos, as I had to pish it in to get clear views in the dense bushes. Here are all the photos I got that I think show some useful angle different than the rest:
This bird, although looking quite like a Chesnut-sided in some ways (and behaving, in that it kept its tail cocked up most of the time), is clearly not right for even an aberrant individual of that or any other warbler species.
Obviously a hybrid, but what is the other parent? My best guess is Magnolia - the tail pattern, yellow wash on underside, green and black back and white wing panel seem consistent with that species.
This bird is extremely similar to a bird banded at Long Point last year, although with a few fairly minor differences. This bird was confirmed to have a Chestnut-sided Warbler as a mother, and the father was thought to be almost certainly a Magnolia Warbler. A very odd combination that doesn't seem to have been previously recorded. I suspect that mine and the Long Point birds were born in the same nest.
While doing some research I realised that there are a lot more photos of bizarre warbler hybrids out there than I'd expected. I ended up spending a couple of hours compiling every photo I could find, as many of the photos were quite isolated and I thought others might appreciate seeing them.
Below is a list of every photo I could find of North American warbler hybrids. I didn't include a few combinations that are very well stuidied (Audubon's x Myrtle, Blue-winged x Golden-winged, Townsend's x Hermit, and MacGillivray's x Mourning), but otherwise I think it is reasonably comprehensive in terms of online photos. No doubt there are more I missed though so please let me know if you find any. This is by no no means comprehensive overall given how many hybrids have been recorded that exist only as specimens or descriptions.
Note that most of the combinations are only presumed, as only a few have been either been tested genetically or have known parentage.
Without further ado, and in no particular order, here's the list:
Townsend's x Audubon's Yellow-rumped
Townsend's x Myrtle Yellow-rumped Warbler, Another
Townsend's x Black-throated Green
Townsend's x Black-throated Gray (from Toronto!)
Myrtle Yellow-rumped x Black-and-white (in captivity)
Yellow-rumped? x ?
Black-and-White x ? (tons of pictures and sound recordings)
Magnolia x Black-throated Blue (sound recording only, pictures apparently exist but don't seem to be online)
Yellow-rumped x Northern Parula (PDF warning; black-and-white photos of a specimen but I'm including it for completion's sake)
Cerulean x Black-and-white (ditto the above link on both PDF and poor photos)
Northern Parula x Cerulean, photo 2, a different bird
Northern Parula x American Redstart, photo 2, photo 3 (I couldn't find any commentary on this bird; it doesn't seem to show many Redstart features)
Northern Parula x Yellow-throated Warbler
Bay-breasted x Blackpoll, photo 2 (looks weird but I don't have a better suggestion)
Bay-breasted x Blackpoll (spring female)
Connecticut x Mourning (Unfortunately, although this bird seems to have been well studied, all this information is locked behind a paywall on ABA's website. If any ABA members read this, the information is in the November/December 2013 issue, and I will edit this post if you let me know what it says)
MacGillivray's x Common Yellowthroat, another bird (very different and both really cool looking!)
Yellow x Common Yellowthroat
Prairie x Blue-winged (another really cool one)
Mourning x Kentucky, more info
Canada? x ?
Cape May x Townsends (Apparently there is a photo on page 192 of this issue of North American Birds. Unfortunately it is behind a paywall and I can't see it. There seem to be a few hybrids hidden in these publications over the years)
edit: I've recieved a few more photos and am adding them here:
Kentucky x Blue-winged (specimen)