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.