Puerto Rican birds, why do they sing early?

This video from Puerto Rico says about itself:

7 April 2011

Episode 1: Adelaide’s warbler (reinita mariposera)

Warm-up benefit could explain morning birdsong

Dawn tune-up may improve sound for attracting mates

By Susan Milius

6:00am, August 25, 2016

WASHINGTON — Vocally warming up puts more dazzle into a bird’s singing for the day, a new test shows, perhaps helping to explain widespread outbursts of birdsong at dawn.

Males of Puerto Rico’s Adelaide’s warblers (Setophaga adelaidae) start trilling through their repertoires of 30 or so songs while it’s still pitch black. Tracking the songs of individual males showed that the order of performance had a strong effect on performance quality, behavioral ecologist David Logue said August 17 at the North American Ornithological Conference. In the early versions of particular songs, males didn’t quickly change pitch as well as they did later, Logue, of the University of Lethbridge in Canada, and colleagues found.

This was the first test for a warm-up effect for daily singing among birds, Logue said. To catch the full stretch of repetitions of songs, Orlando J. Medina (now with the U.S. Fish and Wildlife Service) had to beat the warblers at getting out of the nest in the morning. His recordings of each of nine males’ morning performance for four days allowed computer analysis of how fast a male swept through his trills.

Time of day alone didn’t explain the improvement in singing. So Logue and study coauthor Hannes Schraft, now at San Diego State University, don’t think that factors like increasing light or rising temperatures could explain the improvements. The robust effect of repetition leads Logue to propose what may be a new explanation for big dawn choruses: Males warming up sooner would fare better in competing for mates. Over time, a melodious arms race could have broken out as earlier warm-ups were beaten by even earlier ones.