So, what are the exceptions? Many species will interbreed. An extreme example - from a human perspective - involves the Golden-winged and Blue-winged Warblers. These two species look completely different:
|Blue-winged (left) and Golden-winged Warblers. Both were lifers for me|
last year, but there was no chance that I would confuse the two!
Another interesting group is the large white-headed gulls. A recent high-tech study of 17 species that breed in the Northern Hemisphere found that these species are nearly genetically indistinguishable! Yet a human observer can reliably distinguish a Herring Gull from a Great Black-backed Gull from a Glaucous Gull. However, large white-headed gulls are notorious for hybridizing amongst themselves. Some will even hybridize with black-headed gulls, producing offspring like this beautiful putative Ring-billed x Laughing Gull. The offspring are fertile - unfortunately for gullers, because second-generation hybrids are even trickier to identify! But we consider pure adults to be sufficiently distinguishable to be different species.
Except in the case of Iceland and Thayer's Gulls. These used to be considered two separate species. Iceland has white or pale gray wingtips; Thayer's has black (among other, more subtle differences). In those "pure" plumages, the two are easy to distinguish in the field (at least as adults). But the two interbreed extensively, so that individuals with intermediate plumage are at least as common as the pure birds and span the full spectrum from white to black wingtips. Last year, the American Ornithological Society (formerly AOU), which is the taxonomic authority for the U.S. and Canada, decided to lump the two into a single species. Thayer's Gull no longer exists - it's just another Iceland Gull.
So why would we consider Iceland/Thayer's to be a single species, but not other species pairs like Western/Glaucous-winged, which similarly hybridize extensively where their breeding ranges overlap? Sometimes the decisions are somewhat subjective; otherwise the committee might feel that there is insufficient evidence (thus far) to change the status quo. I've heard the species concept described as a mountain range: the individual mountains are species, and while it's easy to tell which peak belongs to which mountain, it's harder to assign the area between the peaks one way or another. Sometimes there seems to be a distinct valley until you get closer (like with a genetic analysis), and then the terrain gets lumpy and messy and you really don't know where one particular lump (or individual bird) should be placed relative to the two peaks (species) on either side.
Another proposal that AOS considered last year was to lump Common and Hoary Redpolls into a single species. There's scientific evidence for this - no one has yet found a way to distinguish these two species genetically. Identifying them in the field is also tricky, as the differences are subtle and - like Iceland/Thayer's Gulls - an observer can find individuals that span the full spectrum between the two. However, as mentioned above, hybridization alone is not sufficient reason to lump other species pairs; and genetic distinctions can be difficult to pin down. AOS decided to retain two North American species of redpolls, pending further information on how the two species interact on their breeding grounds. It was a close decision, though.
I lived in Alaska for a little over three years, and saw both species of redpolls breeding at Utqiaġvik (formerly Barrow):
|Hoary (left) and Common Redpolls, both nesting in large cages that were used decades ago for studying wolves.|
I frequently saw both species and didn't worry too much about identifying each individual, though if I were to go back now, that area would be a great place to study any segregation between the two - or whether they interbreed freely. Certainly the fact that both species breed there, while maintaining distinct characteristics, indicates that the differences between the two are not solely a result of the environmental conditions in which an individual hatched, as has been suggested given the apparent lack of genetic distinction between the two.
What does this have to do with Green Birding in Wisconsin in 2018? Well, Common Redpolls have been abundant in the area (and most of Wisconsin) this winter; I easily found a couple of flocks on January 1st. Hoaries are much less common here, but one or two show up in flocks of Commons every once in a while. The other day I heard that a pair of Hoaries has been visiting a feeder just 5 miles from my house. The homeowners put the word out to the local birding community and generously invited everyone to stop by and check them out. I biked up there today and enjoyed the convenient opportunity to sit on the front porch and watch and photograph the birds at the feeder from close range - perfect conditions for picking apart two similar species. The male wasn't around when I was there, but I had fun picking out the female as she came and went. The feeder was a riot of activity with perhaps 60 redpolls at a time on the feeder, ground, and nearby branches.
So, how Hoary is the female Hoary? Thanks to the excellent viewing conditions, I managed to get photos (not stellar photographic quality, but identifiable) of all the relevant field marks:
|Female Hoary at top. Note overall silvery color, limited streaking on sites, small bill (not entirely an artifact of the photo angle), small red patch on the poll, white rump, and white undertail coverts.|
|Another view of the white rump|
|White rump when perched, and white undertail coverts|
|Good view of the small bill and small red patch - compare to the female Common on the feeder|
So, this bird ticks all the boxes for pure Hoary, putting Hoary Redpoll on my list for #40. I also added Merlin (#41) later in the morning when I was biking around Brice Prairie (again hoping for Lapland Longspurs or Snow Buntings or a Northern Shrike, but no luck on any of those).
Of course, there may be no such thing as a Hoary Redpoll. Hopefully last year's AOS decision will spark further study of these birds on their breeding grounds and provide further evidence one way or the other. In the meantime, Hoary is still countable on the ABA Checklist (which follows AOS taxonomy), and thus for my green year. I'm really excited to add this species to my list, as there was certainly no guarantee that I would find it this year, especially given the difficulty in distinguishing Hoary from Common under most field conditions. I'm grateful to the homeowners for generously sharing their birds! They weren't home when I stopped by, but I left a bag of thistle seed as a thank-you gift, which will go to good use feeding this large, hungry flock.