Okay, folks, here's what the Cornell Lab of Ornithology has to say in their Home Study Course. They kindly gave me a couple of relevant pages in response to my inquiry about "dawn chorusing," but they were unable to confirm or deny <g> any CDs out there yet, esp. for flycatcher song comparisons. Anyhoo, here's what I found out, as promised:
Katy
Dawn Chorus
By Donald E. Kroodsma
(c) Cornell Lab of Ornithology
Home Study Course in Bird Biology, Ch. 7 "Vocal Behavior," pgs. 75-76
(References to track #s correspond to the CD-ROM for the course)
"Some of the most remarkable singing occurs at dawn, especially in temperate zones (Staicer et al. 1996). Most birds are silent the entire night, presumably roosting quietly somewhere on their territory. But beginning an hour or so before sunrise, males begin their dawn chorus. Each species chimes in at a slightly different light level, often beginning with American Robins, and other species join in a rather regular sequence. The quantity and often the quality of singing during this time differ markedly from singing during the rest of the day (Track 53, Track 54).
"Species vary considerably in how they behave at dawn. During daytime singing, an Eastern Towhee sings one song form (say, A) over and over, and then introduces another (say, B), then another, and perhaps another, until he has delivered his entire repertoire of three to eight song types over a period of an hour or so. His singing pattern might be illustrated as AAAA... BBBB... CCCC... and so on. The pattern is one of "eventual variety," in which a male repeats one song type many times before "eventually" proceeding to the next type. At dawn, however, the towhee sings with "immediate variety," perhaps delivering all of his song types in 20 to 30 seconds: ABCABCDEDE. The singing is far more energized and dramatic than it is during the daytime.
"Chipping Sparrows also sing differently at dawn (Fig. 7-17). A typical daytime song is about two seconds long, and consists of perhaps 20 repetitions of a single song element. The male pauses 10 or so seconds between successive songs (Track 55). While singing, the male typically sits on an exposed perch high in a tree. At dawn, however, a male often sits on the ground near a male from a neighboring territory and delivers bursts of song as if they were shot from a machine gun. He sings two or three elements of the song, pauses briefly, sings another burst followed by a pause, and so on (Track 56). As with the towhee, the vocal display seems highly energized, even frenetic.
"Other species use entirely different songs at dawn than during the day. Certain warblers that have two song categories use their aggressive songs for the first 30 to 60 minutes of the morning, after which they lapse into a slower-paced delivery of their other song. A male Yellow Warbler uses about 12 songs delivered with immediate variety during his dawn chorus, and after half an hour or so he switches to his single daytime song type. The male reverts to songs of his aggressive dozen during daytime encounters with other males, too, but the singing is never as energized as at dawn. American Redstarts behave similarly, although songs of their aggressive repertoire more typically number only three or four. Some flycatchers, too, such as the Eastern Wood-Peewee (Fig. 7-58), use qualitatively different songs at dawn (Craig 1943) (Track 57, Track 58).
"The drama at dawn is truly extraordinary, as if so much pent-up energy is unleashed. But why? Why such a burst of energy at dawn, and again, why does that energy reveal itself in such diverse ways?
"Researchers have proposed many ideas for why dawn singing is so dramatic (Staicer et al. 1996). Perhaps males sing at dawn because that is the best time to attract females, especially those who have migrated and arrived overnight. Conditions at dawn are often calm, too, so song at that time carries the maximum distance. Furthermore, during the dawn chorus conditions are often too dark to forage, so singing then is an efficient use of time. Or perhaps singing at dawn is especially important for territory defense; dawn follows the longest period of inactivity, and predation occurs at night, too, so dawn might be an important time for a bird to proclaim "I am still alive" and "This territory is still mine."
"The energized displays and often dramatic interactions, such as those of Chipping Sparrows, suggest that during the dawn chorus is when patterns of social dominance are established and daily reconfirmed. Little is known about dominance patterns among males on adjacent territories, but females may attend to such displays and make mating decisions, both intrapair (between mates) and extrapair (with a bird other than the mate), based on male singing exchanges. So many extrapair fertilizations take place in some species, such as the Indigo Bunting and Red-winged Blackbird, that any kind of ritualized display that establishes male hierarchies and broadcasts information about them would benefit listening females. A female Black-capped Chickadee, for example, will mate with males other than her social partner, and her extrapair gambit tends to be with a male higher in the dominance hierarchy than her own mate (S.M. Smith 1988). She could base her decision on her knowledge of dominance hierarchies established during the winter flocks, but that knowledge is probably reinforced by information she gleans from the dawn performances of males in the population. For many species, perhaps singing at dawn is a formal way to establish and monitor social relationships and hierarchies, giving individuals in complex societies the information they need to make wise decisions.
"The dawn chorus probably serves multiple purposes, with the emphases undoubtedly varying from species to species, just as social environments and management differ among species. Our failure to understand the full implications of the dawn chorus, however, in no way diminishes the drama played out each morning in these extraordinary singing displays (Fig. 7-59)."
