[CT Birds] Interior Nelson's
elphick at sbcglobal.net
Fri Oct 21 17:04:35 EDT 2011
I wanted to clarify a few points about subspecies identification of Nelson’s
sparrows that came up in Paul’s recent email. The bottom line is that the issue
is complex, so (again) I find myself apologizing for the length of my email.
First, it is important to know that there are three named subspecies of
Nelson’s, not two. The Atlantic coast form is called subvirgatus. The
subspecies that breeds in the upper Midwest is the “nominate” form nelsoni, and
is what I suspect Paul is referring to as the interior form. These two
subspecies are pretty different and should be separable in the field with good
views (see your Sibley guide for details). I would also agree with Paul that
nelsoni is probably more likely to be confused with saltmarsh sparrow than
subvirgatus (though both confusions are possible without really good views).
In between those two groups there is a third named subspecies, alterus, which is
found along the coast of Hudson Bay. These birds are intermediate in appearance
between the other two forms and are the main reason I have reservations about
some subspecies identifications. These reservations are exacerbated by the fact
that (based on geography) alterus is the more likely of the two interior
subspecies to occur in CT. [Note that the Sibley Guide does not address
subspecies identifications directly and only deals with “field identifiable
forms”; consequently in the Nelson's account it lumps alterus into the
“Interior” form (though I'm pretty sure the illustrations all show typical
Several features have been suggested as good indicators of alterus, but these
are mostly useful for a bird in the hand. Also, I am not convinced (others, who
have thought about the issue more carefully, may disagree) that we have a good
handle on how much variation in plumage there is within subspecies. In part, I
think this is a concern because most places where alterus occurs are places that
no birders go to. Even if “average” birds are distinguishable, I suspect there
could be a lot of overlap between subspecies. It also seems possible that there
could be a cline (gradual change) across the species range, rather than three
(or two) distinct groups.
Current thinking acknowledges that some birds will be impossible to name to
subspecies, even in the hand (especially when it comes to nelsoni vs alterus).
If plumage variation within subspecies is greater than we think then even more
birds may be in the “overlap” zone.
I lack the field experience with alterus and nelsoni to say much more, but I
think they would be tough to distinguish in the field unless you had really good
views and/or killer photos. If you want to know more about the differences
there is a nice photo essay on the topic in a recent issue of North American
Birds (by Fletcher Smith). There is also a more technical paper that the photo
essay is based on (by Jon Greenlaw and Glenn Woolfenden).
Two other issues complicate this problem even further. First, as Patrick
mentioned to Paul, feather wear can really change the appearance of both
sharp-tailed sparrows. Young birds and adults also look pretty different. For
these reasons an adult saltmarsh sparrow in May looks very different from one in
late August. And an adult saltmarsh sparrow in September, looks really
different from a young bird at the same time. Trying to identify these birds in
terms of general colour (e.g., how “orange” or “bright” it appears) is,
therefore, tricky unless you are familiar with how the general colours vary.
Better, in my experience, is to focus on things like the colour and location of
streaking, bill size, etc.
The second complication is that saltmarsh and subvirgatus Nelson’s hybridize
from northern MA to southern ME. Some hybrids are obvious (there are some
pictures in Fletcher Smith’s article), but, again, I’m not convinced we know the
full range of variation in hybrids. We do know – based on a study by Jen Walsh
at the University of New Hampshire – that birds identified as normal saltmarsh
sparrows can have DNA that came from a Nelson’s. This tells us that there must
be some hybridization in their history. As you get close to the hybrid zone,
this finding is not uncommon – 8% of over 400 birds sampled in Jen’s study,
including one that bred as far south as Rhode Island.
Returning to the point of the Paul’s post, I would not hesitate to put a name to
a Nelson’s that looked typical for subvirgatus (i.e., pale grey, blurry
flank/breast streaks, no dark streaks in eyebrow, small bill, etc.). I would
also expect the vast majority of birds in CT to be this subspecies – every
Nelson’s I’ve seen in the state has been subvirgatus (although I have banded a
couple that I think could have been hybrids).
I would, however, be much more hesitant to label a bird that showed features of
an interior Nelson’s as anything more than nelsoni/alterus. Even then I would
want to spend some time considering the possibility that the bird could be a
hybrid. I should add that this equivocation may just be due to my inexperience
with the two interior subspecies - people who catch a lot of these birds in
winter do routinely identify at least a moderate proportion of the birds they
catch to subspecies … but even they do not distinguish them all.
Fletcher Smith's article is here: http://www.aba.org/nab/v65n2sparrows.pdf
The other research I mentioned is all cited in Smith.
I hope that this helps,
elphick at sbcglobal.net
More information about the CTBirds