Singing behaviour
Our general approach was to monitor male Bermuda Vireos intensively over a prolonged period to observe changes in their singing behaviour across multiple breeding stages of the breeding season as well as during the non-breeding season. We audio-recorded the daytime (0700–1200 h) singing behaviour of the 12 male birds throughout most of the subspecies’ breeding season (April 2019 – August 2019). Of these 12 males, 11 were relocated and recorded during a brief period of the following non-breeding season (December 2019 – January 2020). Researchers often record birds at or just before dawn (Bolsinger 2000, Dolan et al. 2007, MacDonald and Islam 2021) when passerine song typically peaks (Staicer et al. 1996, Dabelsteen and Mathevon 2002). We recorded vireos during the morning hours after sunrise for two reasons. First, Bermuda Vireos sing discrete and rambling songs starting at dawn, with song rate remaining high until ca. 1500 h (M. Mejías unpub. data). Second, the extremely dense vegetation created by exotic trees reduced visibility before sunrise and made it difficult to locate, follow, and record birds at that time.
As part of our sampling regime, we visited one of our two sites each day during favorable weather (i.e., no rain and little to no wind), alternating between sites each day. In total, we visited the Ferry Reach Park site 41 times during the breeding season and 5 times during the nonbreeding season. We visited the Spittal Pond site 44 times during the breeding season and 7 times during the nonbreeding season. While at a site, we recorded each male at the site during a separate 15-minute recording session throughout the morning. Our goal was to obtain unbiased estimates of singing behaviour from each male across multiple breeding stages. We therefore randomized the order in which we recorded subjects each day, thus reducing the risk of recording certain males or males at certain breeding stages at the same time each day.
Upon arriving at a subject’s territory, we searched for him for ≤ 15 min. If we found him, we waited 2 min before commencing recording. The 2-min delay was important because we sometimes located subjects by hearing them sing. Since our goal was to obtain unbiased estimates of singing behaviour, including estimates of daily song production, waiting for 2 min reduced the risk of biasing our recording sessions towards periods of time when the male was known to be singing. If we did not see or hear the focal male after 15 min, we stood in the approximate center of his territory, waited an additional 2 min, and commenced recording. Given the relatively small size of Bermuda Vireo territories (0.25 ha), their loud songs, and our familiarity with the song repertories of the 12 birds, we were confident that we would readily detect and locate the focal male anywhere in the territory if he began vocalizing after the start of the recording. If a vireo began singing from what we thought was the inside of his territory, we immediately approached him while recording. If we located the singing male and confirmed that he was our focal subject, we included in our analysis all the songs recorded throughout the 15-min session, including those acquired before visually locating him. In the rare instances when the singing male we located was not the focal subject (e.g., a neighbour), we aborted the recording session and repeated it later that day.
We recorded subjects throughout their 15-min session with a digital audio recorder (Marantz PMD661 MK II Professional recorder; WAVE format; 44.1 kHz; 16 bits) and a shotgun microphone (Sennheiser ME66 with K6 power module; super cardioid pickup pattern; 40−20,000 Hz frequency response (± 2.5 dB)) fitted with a foam windscreen. Recordings were made by following the subject no closer than 5 m while pointing the microphone directly at him (or towards the source of the songs if we had not yet located him). For each song produced while the subject was visible, we spoke into the microphone and visually estimated his song perch height above the ground (estimated accuracy ± 1 m); very few trees across Bermuda Vireo territories were > 10 m; all height estimates were made by the same person. We noted any periods in which we lost visual contact with the subject, but always continued recording until the 15-min session expired. After recording, if confirmed visually, we used a handheld GPS unit (Garmin eTrex® 0, ~3 m accuracy; Garmin International, Inc., Olathe, KS, USA) to mark general singing localities (separated by ≥ 5 m) of each male, per trial. We used ≥ 5 m because Bermuda Vireos move continuously through their territories with short (~0.3 − 1.0 m) flutter hops interspersed occasionally by longer (up to a few metres) loping flights (M. Mejías pers. observations).
In May 2021, we returned to our sites and measured the heights of the two tallest trees in each subject’s territory to allow comparisons between the heights of the song perches used by our subjects during recording and the heights of the tallest perches available to our subjects. We estimated maximum tree height by extending a Telescopic Fibreglass Mast Heavy Duty Pole (model MFJ-1916; maximum height = 10 m) alongside the selected tree and visually estimating (estimated accuracy ± 1m) any remaining height of the tree above the fully extended pole. Estimates of the heights of used song perches and the tallest trees were conducted by the same individual.