I think most birders know by now that Red Crossbills in North America sort into a number of different call types, each of which may constitute a cryptic species.  Identifying these types in the field promises to be a bugaboo of legendary proportions.  Where does one begin?

In this post I’ve collected links to some online resources that can get you started: a sort of Crossbill Q & A, if you will.

1) What are the crossbill types?  How did they evolve?  Are they for real?

A great source for beginning to answer these questions is Craig Benkman’s introduction to crossbill types in the July 2007 issue of Colorado Birds. Although it doesn’t discuss identification, the article’s overview of the research on crossbills is extremely valuable to anyone in North America, in spite of its nominal focus on Colorado.

When you’re ready to dig a little deeper, head to Benkman’s home page and look at the list of publications he has posted in PDF form.   The man is a scholarly publication machine.   Every important paper on crossbill types that has been published to date can either be downloaded from Benkman’s page or can be found referenced in the bibliographies of one or more of his works.

2) Where can I listen to online recordings of Red Crossbill types?

Your first stop should be Jeff Groth’s old page on the website of the American Museum of Natural History.   Although Benkman may be the current guru, Groth was the original discoverer of the call types.  The good news is that his page has audio files of the flight calls, excitement calls (“toops”), and alarm calls of types 1-7.  This makes it the most comprehensive collection of crossbill vocalizations on the web.   It also has range maps and basic natural history information for each of these types.

The bad news is that Groth’s site was, believe it or not, last updated in 1996, back when the Internet was in primary school.  Not all of the links still work.  There’s only a single audio file for each vocalization, and worst of all, because the files date from those dark days when 50 kB took a long time to download, they’ve been cut so short that they don’t sound like they do in the field–instead they all sound too much like each other.  Appreciate Groth’s site for what it has, but don’t let it intimidate you.  Crossbill identification is very hard, but it’s not hopeless.

Here are a couple of less comprehensive but more user-friendly resources:

1. Matt Young’s crossbill identification paper on the eBird website.  This is the best identification article published to date.  It deals only with Red Crossbill types 1-4 (flight calls only) and White-winged Crossbill (which, interestingly, does not have multiple populations defined by call type).  These are the birds you are most likely to encounter if you live in the eastern United States.  If you live in the West, you are most likely to encounter Red Crossbill types 2-5, so this article will still be of great use to you.  It has much higher-quality (i.e. longer) recordings than Groth, including scrolling spectrograms in .mov format.  However, it has only a single example of each type, which makes it more difficult to get an idea of the variation within types.
2. My Colorado Birds article on types 2, 4, and 5 in Colorado.  This was published along with Benkman’s overview in July 2007, and already I’m a little ashamed of the poor quality of some of the accompanying recordings (which you can listen to here: Fig. 1a, 1b, 1c, 2a, 2b, 2c).  Several people have pointed out that my spectrograms should have been zoomed in a little more, too.  Sorry about that.  On the bright side, this piece is a good supplement to Matt’s article, especially because it discusses Type 5, which is widespread in the West.
3. Xeno-Canto’s Red Crossbill collection.  As of this writing you can listen to and download recordings of types 2, 3, 4, 5, 9 and 10.  I have posted a few mediocre recordings of Type 9, the South Hills Crossbill, which Benkman et al. 2009 have proposed as a separate species, Loxia sinesciuris.

In addition to the resources I’ve mentioned, there’s always the Macaulay Library, which is my favorite collection of online bird sounds.  In an upcoming post I’ll try to put together an index to the types heard on the Macaulay recordings.

More on crossbills soon!