In my last post, I discussed differences in the alarm calls of Gray Hawk (for the purposes of this post, Buteo plagiatus) and Gray-lined Hawk (here, Buteo nitidus — see the last post for taxonomic rationale).  Over the past few days, I have gone through the Gray Hawk sound collection at the Macaulay Library, which has given me some new information.

In my last post I said the alarm calls of Gray and Gray-lined Hawks tend to be “fairly stereotyped within populations.”  In light of what I’ve learned from the larger sample size, I think it’s worth retracting that statement in favor of something like, “the alarm calls of Gray and Gray-lined Hawks are variable within populations, and even within individuals, but are nonetheless consistently separable on several characters.”

Of the characters I mentioned in my last post, the most consistent seems to be energy spectrum; those differences seem consistent across the entire sample size.  Differences in frequency and pattern of inflection are also pretty reliable, although there are anomalous birds here and there.  The differences in duration are tricky, though, since several Gray-lined Hawk vocalizations from Venezuela clock in at around 1.75 sec in length.  It’s clear there is a lot of overlap in this character.

Here are some other ways in which the alarm calls vary:

1. The noise content of alarm calls in both Gray and Gray-lined Hawk is highly variable.  A clear tone quality (that is, without noise) is the norm in both taxa, but noisy calls are not infrequent, and some birds in both taxa may give noisy and clear calls on the same recording.
2. Within the context of the constant differences in energy spectrum, phase shifts (period doubling, period tripling, etc.) are common in both taxa, as they seem to be in many species of raptors.

I hope to publish more on Gray Hawks (and other bird sounds) soon, but as I’m in the middle of an unplanned trip, I may not get to it until next week.  Sorry!