Vocalizations are an important, and sometimes critical, aspect of identifying flycatchers of the genus Empidonax. Alder and Willow Flycatchers, for instance, are so similar morphologically that many individuals cannot even be separated in the hand, but their vocalizations are diagnostic. Mostly it is the song that birders learn, but oftentimes the call notes are equally important to learn - and in the case of Dusky vs. Hammond's Flycatchers, the call notes are more easily distinguished than the songs.
But Empidonax flycatchers have more vocalizations than are typically described in a field guide, and the line between what is a "song" and what are "calls" is unclear in this genus. In most Empidonax species, and in fact many other flycatchers (e.g. pewees and phoebes), the male sings a "dawn song", given only at first light, that is a longer and more complicated version of its daytime vocalizations. Sometimes a vocalization that is referred to as a song (e.g. the "FITZ-bew" of the Willow Flycatcher) may actually be analogous to another species' "position note" (e.g. the two-note call of the Cordilleran Flycatcher): in both cases, these are calls uttered throughout the day, typically by males. However, males and females are identical in all Empidonax flycatchers, so exceptions to these rules go unnoticed unless one is studying a group of color-banded birds, whose sexes have been determined in-hand. Below, I will use my experience with a color-banded breeding population of Cordilleran Flycatchers in Colorado to describe each vocalization, its context, and which sex uses it and when.
1. Dawn Song or Primary SongPlay Recording
The Cordilleran Flycatcher has a three-part song, used only by the male during the breeding season. Early in the season this may be sung during the daytime, presumably by males that have not yet attracted a mate. Later in the season, mated males sing it only for a few minutes at dawn.
2. "Male Position Note"Play Recording
This is the common call heard throughout the day in summer, and during migration and winter, is the only way to distinguish the Cordilleran Flycatcher from its closely-related look-alike Pacific Slope Flycatcher. In the literature, this two-note call is referred to as the "male position note". I use quotes here, because although I have found that the male uses this call consistently, I have also observed females using it. Sometimes the female utters a higher-pitched, slurred version; other times it is identical to the male's call. The most frequent context seems to be territorial advertisement - saying "I'm here and this is my territory". The male utters this call non-stop throughout the day early in the season, when territorial boundaries are being established. He then goes quiet much of the time while the female is incubating and while he and his female are feeding nestlings. Later in the season though, I've noticed that the male resumes his near-constant use of this call throughout the day; this coincides with two things: 1) fledging, which leads to a breakdown in established territories as the fledglings wander around; and 2) arrival of early migrant Cordilleran Flycatchers, most likely failed breeders from nearby who, with no time left for a second nesting attempt, have abandoned their territories and begun pre-migratory wandering. These forces put pressure on the pair to defend their territory, and it is during this time that I have seen the most frequent use of the "male position note" by females.
Nothing yet is known about the Cordilleran Flycatcher's behavior on the wintering grounds, but stay tuned - I have received some funding to work on this species in Mexico, and I bet you anything that both males and females defend winter territories using this call. We'll see!
3. "Female Position Note"Play Recording
Again with the quotes, because while the female uses this vocalization far more frequently, I've seen males use it on occasion. This is a single, high-pitched call, and although I suspect it is used in multiple contexts including communication with the male, most often it seems to be given in alarm. Anytime I approach a nest or fledglings, the female gives this call repeatedly, accompanied by nervous tail-flicking. Her fledglings know what it means, too - they go quiet and still when they hear this call.
After two seasons of close observation, I've noticed slight variations in this call, even within individual females - sometimes the note is flatter, sometimes sharper, and I strongly suspect this variation is related to context. I'd love to get some high-quality recording equipment and study this.
4. "Gurgle Call" or "Interaction Calls"Play Recording
This is a call I've never seen mentioned in any field guides. It's a soft series of rolling or gurgling notes, used by Cordilleran Flycatchers when in close proximity to each other. Pairs use it when approaching each other, such as passing one another while feeding nestlings, and strangers use it while chasing each other. Thus is it often used in aggressive contexts (both among strangers/neighbors and within the pair), but might also have a non-aggressive context within the pair. It is used by both males and females.
5. "Soft Interaction Calls"Play Recording
This is another type of vocalization used in close interaction between individuals. It is soft, lower-pitched than the position notes but not as low and growling as the aggressive "gurgle call". Both males and females use this, and the most frequent context I've noted is around fledglings, especially when the parents are trying to gather up a scattered brood.
It's a joy to be able to study a species intensively, to learn the context of its various vocalizations and to come to know calls that are not described in most field guides. I want to acknowledge Xeno-Canto (www.xeno-canto.org) as the source of the vocalizations used in this post - this is a fantastic repository for bird vocalizations. Someday I hope to purchase some nice recording equipment and contribute my own recordings to this site.