Big Trees, Kentucky Warblers, and Site Fidelity

La Reserva Natural Absoluta Cabo Blanco -- the Cabo Blanco Absolute Nature Reserve -- was Costa Rica's first national park.  The Reserve celebrated its 50th anniversary last year.  Before the land was preserved, the area was a hodgepodge of private farms and cattle ranches.  Now the 3,000 acres of forest protected at Cabo Blanco are a testament to the impressive speed at which tropical habitats can regenerate: its youth is masked by the presence of gloriously massive trees, including fast-growing giants such as Guanacaste (Enterolobium cyclocarpum) and Ceiba (the genus to which the mighty Kapok belongs).

This forest is the winter home of a number of neotropical migratory songbird species, which is the reason we find ourselves in this wonderful place.  Among them are several species whose numbers are apparently in acutely rapid decline, such as Kentucky Warbler, Worm-eating Warbler, Ovenbird, Wood Thrush, and Swainson's Thrush.  This week we captured all of these species except Ovenbird (although we did manage to capture one at Finca Pura Vida last week).  We also captured and banded two other neotropical migrant warblers: Chestnut-sided and Tennessee.

The most exciting capture of the week was a Kentucky Warbler, not only because of my personal annual sagas trying to spot them in New Jersey, or because they are threatened in the state, but because one of the Kentuckys banded late last year by my team at Cabo Blanco was just recaptured again on Monday, one year and eight days (and about two thousand miles) later.  It was an immature bird at the time of capture, meaning it celebrated its first birthday this past summer, after its first (hopefully successful) breeding season in North America.

Fortunately I keep my photos organized well enough to have located a photo I took of this very bird during last year's research expedition:

Kentucky Warbler when it was originally banded on
December 14, 2013 (© Tyler Christensen)

Same individual as pictured above when it was recaptured
on December 22, 2014 (© Tyler Christensen)

Birds banded in a previous season and re-encountered in a later one are called between-season recaptures, and are not rare occurrences here (at this point about one in ten birds we capture was banded by the team in previous seasons), but we capture Kentuckeys infrequently enough that this was our first between-season recapture of this species.

Recaptures such as this demonstrate winter site fidelity, or the tendency of a bird to return to the same winter territory year after year.  Many of the migrants we capture are site faithful, and recapture rates among our most common migratory species from one year to the next are roughly 18% for Tennessee Warblers, 20% for Prothonotary Warblers, and a whopping 26% for our Northern Waterthrushes!

As several recent studies have shown, winter site fidelity and winter territoriality (which are related) are, in at least certain species, linked to habitat quality - higher-quality habitats often have repeat customers who stake territorial claims within them.

Evidently the forests of the Cabo Blanco nature reserve are agreeable to at least one Kentucky Warbler, who elected to return to spend another winter.  This group of bird banders happens to have made a similar assessment.