Introduction: The Literature
Interest in “Lilian’s” Meadowlark has spiked with the publication of Barker et al. (2008), which found significant genetic differences between Lilian’s and other “Eastern” Meadowlarks and recommended that Lilian’s be elevated to species status. Naturally, I keyed right in on the following quote:
Cassell (2002) found significant differences in the songs of Eastern and Lilian’s meadowlarks. However, given the learned nature of song in this group (Lanyon 1957), and the sharing of unlearned call notes between these forms (Lanyon 1957, 1962), the relevance of this observation with regard to species limits remains an open question.
A couple of months back I ordered Cassell’s (2002) thesis through interlibrary loan, to judge the evidence for myself. Overall, I found some aspects of the work very useful, but other aspects quite questionable. In particular, I have two complaints:
- In the introduction, Cassell makes the argument that differences in song, as an important isolating mechanism, are important to species boundaries. This is certainly true, but she makes the claim about meadowlarks and then supports it with discussion of nightjars, manakins and antbirds, none of which learn their songs. Meadowlarks not only learn their songs, but individuals have complex repertoires and populations show geographic dialectal differences. In addition, where their ranges overlap, Eastern and Western Meadowlarks defend territories against one another and will respond to playback of one another’s songs. Thus, on the one hand, the two species of meadowlarks recognize each other’s songs as similar enough to pose a territorial threat, a threat usually only posed by conspecifics. On the other hand, the two meadowlarks very rarely interbreed, and hybrids are infertile, so some kind of strong isolating mechanisms are clearly at work. The ways in which meadowlarks recognize songs must be much more complex than a simple evaluation of “same” or “different,” and so in my opinion the role that song plays in isolating these taxa needs more study.
- Cassell groups songs by number of “syllables” (2, 3, 4 or 5). She then compares, e.g., all two-syllabled songs of Lilian’s to all two-syllabled songs of Eastern. She doesn’t justify this methodology, and it seems to me that it makes an unwarranted assumption that all same-syllabled songs are homologous (that is, that songtypes with the same number of syllables share a closer evolutionary history with each other than with songtypes of a different number of syllables). To her credit, Cassell does cover her bases a little bit by adding a different analysis, comparing all long syllables (>0.20 sec) to all short syllables (<0.20 sec). But here again is a questionable assumption of homology. I would have found the whole argument more convincing if she had performed her analysis on the level of the whole song rather than the syllable.
Despite these issues, Cassell does convince me that Lilian’s songs average significantly lower in frequency than Eastern songs, in agreement with the descriptions in Sibley’s field guide. Cassell says:
In the field, the astute listener may recognize the primary song of S. m. lilianae by its overall lower pitch than that of S. m. magna.
So far, my experience indicates that this statement holds true in most (but not all) cases. Lilian’s song is frequently, but not always, low-pitched enough that it sounds like Western Meadowlark in tone quality (although the pattern of each song is still much more like Eastern than Western). In addition, it seems to me that Eastern Meadowlarks frequently end on a drawn-out, nearly monotone clear whistle, while Lilian’s tend to end on a whistle that is much more downslurred. There is a lot of overlap in the final note type, so caution is warranted, but I think the combination of terminal inflection and, especially, overall pitch (perceived largely as tone quality) should identify most Lilian’s Meadowlark songs.
Let’s Test This Hypothesis
A few weeks ago in Arizona I was able to record a few Lilian’s Meadowlark songs:
The first songtype, with its low pitch and its complex middle section, is particularly reminiscent of Western Meadowlark, but all three above seem fairly typical of Lilian’s. Note the downslurred endings.
For comparison, here are three typical songs of Eastern Meadowlark. Note the more “ringing” monotone endings (characterized by more horizontal lines on the spectrogram) and the relatively high pitch (lowest frequencies about 3 kHz, as opposed to 2 kHz for Lilian’s):
If all songs of both forms fit these patterns, field identification would be pretty easy, but sometimes the birds will throw you for a loop. Here’s a Lilian’s that sounds more like an Eastern on account of its high pitch and monotone ending:
And here’s a Eastern that sounds something like a Lilian’s on account of its relatively low pitch:
And, of course, not all Easterns have the ringing monotone ending:
For a little more ear practice, check out this wonderful repertoire remix of 19 different Lilian’s Meadowlark songtypes, all recorded by Andrew Spencer on 6/1/2009 from one individual bird in Eddy County, New Mexico. You’ll notice that about half the songtypes end in a ringing monotone whistle, which seems to be a slightly higher percentage than among the birds I recorded in Arizona.
All right, let’s find out how well this works. Here’s a remix of three meadowlark songs. Which ones are Eastern and which are Lilian’s? Leave your guesses in the comments field, and I’ll post the answers in a couple of days.