Zeep calls
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Discussion:
The "zeep group" is one of the larger complexes of similar-sounding flight calls in eastern North America. The members of this group include Yellow, Magnolia, Blackburnian, Bay-breasted, Blackpoll, Cerulean, Worm-eating, Kentucky, and Connecticut Warblers, and Northern and Louisiana Waterthrushes. The calls are short, typically between 1/20th and 1/10th of a second (50-100 mS) in duration, and high-pitched, typically between 6-10 kHz. The calls have an audible modulation that gives them a ringing, buzzy, or sometimes trilled quality. It should be emphasized that not only are these calls extremely similar, but each species also shows variation in their calls, making auditory identification of an unseen bird problematic. Playing the calls in the table above will reveal just how similar-sounding many of these species are. However, with practice, subtle differences between calls can be detected in a variety of parameters which, in combination, may allow for species identification of some individuals. Such parameters include the length of the call, degree of buzziness, distribution of loudness, abruptness of the call, pitch of the call, whether the call is rising, descending, or monotone, and subtle overtones such as dryness, sweetness, and sibilance.
In many ways, Blackpoll Warbler gives the most "average" flight call of the zeep group, making it a good standard for comparison. For example, compared to Blackpoll Warbler, Yellow has a slightly lower, louder call, Blackburnian has a shorter, higher-pitched, and sweeter call, Worm-eating has a higher-pitched and drier call, and Magnolia has a longer, lazier-sounding call. Another useful parameter to consider with these zeep calls is the rate of modulation. When viewing spectrograms, measuring the time between modulations can reveal consistent differences between some species such as the slower modulations of Bay-breasted Warbler compared to the faster modulations of Blackpoll Warbler.
In the table above we have tried to include calls that reveal the range of variation for each species. However, for some species we have only recorded calls from one individual (e.g., Kentucky Warbler) so the examples are more or less incomplete. Another caveat here is that some species typically give rising calls (e.g., Northern Waterthrush) while other species give rising calls only in certain behavioral situations such as when they take off for morning flight. Examples 3 & 4 of Magnolia Warbler illustrate such rising flight calls. Bay-breasted Warbler seems to frequently give a rising call in morning flight but predominantly gives a much different-sounding descending flight call in nocturnal migration. Bay-breasted examples 1-3 are diurnal calls while example 4 is a nocturnal call presumed to be Bay-breasted.