Study site and study species
Bermuda is a remote island (32°18’N, 64°47’W) formed from fossilized, calcareous shell-sand (Verrill 1902). It lies in the western North Atlantic Ocean, approximately 1048 km from its closest landmass, Cape Hatteras, North Carolina. The island is low-lying, yet hilly (0 m – 76 m, mean: 38 m) with a subtropical climate (18 – 27.5ºC). Sunshine and light winds occur from April-September, whereas rain and gales are more prevalent during the winter months (Amos 1991). We collected data at Spittal Pond Nature Reserve (60 acres) and Ferry Reach Park (64 acres) (Figure 1). Present-day wooded habitat in Bermuda is dominated by invasive secondary forest (Mejías and Nol 2020, Mejías and Mejías 2020). Trees in our study sites included introduced Brazilian pepper (Schinus terebinthifolius ), casuarina (Casuarina equisetifolia ), and fiddlewood (Citharexylum spinosum ), and a few native trees including southern hackberry (Celtis laevigata) , bay grape (Coccoloba uvifera ), and Bermuda cedar (Juniperus bermudiana ).
In May 2018, January 2019, and April 2019, we captured 10 male vireos and 4 female vireos. Birds were captured along walking trails at Spittal Pond and Ferry Reach Park by luring them into mist nests with playback of conspecific song. We determined their sex by observing whether vireos sang discrete song (known only in males; Bradley 1980) as they approached the mist net or during follow-up observations (see details below). Female response to playbacks was weak and we were unable to capture the remaining females associated with captured males. We fitted captured birds with an aluminum Porzana identification band on one leg and either one or two plastic colour bands on the other leg. We released banded birds at their point of capture within 10 min. To increase our sample size, we also included 4 previously colour-banded vireos (2 males and 2 females) living at Ferry Point Park. In total, we had 12 colour-banded males that served as our focal subjects (Spittal Pond: 7 males; Ferry Point Park: 5 males). All 6 colour-banded females were mates to focal males.
Throughout the data collection periods of the breeding season (April 2019 August 2019) and the non-breeding season (December 2019 January 2020) , we estimated territorial boundaries by opportunistically following each subject for 1 2 hours per day on several days and taking GPS coordinates of used perches. We marked the GPS coordinates of several perches with a handheld GPS unit (model: Garmin eTrex® 10, approximately 3 m accuracy; Garmin International, Inc., Olathe, KS, U.S.A). In general, territories birds were captured in during the 2018 2019 banding period were the same as during the 2019 2020 data collection period. Two exceptions occurred: (1) one male we captured in January 2019, for which we had yet to map his original territory, had, by April 2019, taken over a territory previously occupied by another colour-banded male, independent of our study, approximately 100 m away, and (2) a male we banded in December 2018 was present for the entire breeding season study had disappeared by December 2019, thus reducing our sample to 11 males for analyses involving the non-breeding season.