Background
Double brooding in birds, in the words of the authors, "is the initiation of a second clutch of eggs after successfully raising young from the first clutch," and in this regard they note that tree swallows (Tachycineta bicolor) "are insectivorous migratory birds that breed in the northern half of North America" and "are generally classified as single brooded," but that "rare instances have been documented in which a female laid and raised two clutches of eggs in the same season."

What was done
After observing two second clutches in a newly established population of tree swallows in the Shenandoah Valley of Virgina (USA), Monroe et al. monitored all late nests in the following two breeding seasons to see what they could learn about the phenomenon.

What was learned
The four researchers from the College of William & Mary's Institute for Integrative Bird Behavior Studies found that "among all females nesting in the early breeding rounds of 2006 and 2007, 5% of birds with successful first clutches later laid second clutches." In addition, they report that the mean productivity for double-brooded females for 2006-2007 was 4.4 ± 1.3 fledglings from first clutches and 3.4 ± 0.8 from second clutches, so that "double-brooded females significantly increased their total annual productivity compared to birds nesting only in the early rounds of breeding." In fact, the productivity of the double-brooded females was approximately 75% greater than that of the single-brooded females.

What it means
In summing up the discussion of their findings in the concluding paragraph of their paper, Monroe et al. say that, "in general, late summer and fall nesting among North American birds is underappreciated and may be increasing due to global warming," citing the work of Koenig and Stahl (2007). If this be the case, it would appear that the ongoing increase in annual vegetative productivity that is manifesting itself throughout the plant world -- see Greening of the Earth in our Subject Index -- is beginning to work its way up the trophic ladder to the animal kingdom as well.

Reference
Koenig, W.D. and Stahl, J.T. 2007. Late summer and fall nesting in the Acorn Woodpecker and other North American terrestrial birds. The Condor 109: 334-350.