Spot-mapping:
As in the original study, the primary census method was spot-mapping. We
spot-mapped nearly every morning from August 10th to
November 8th, 2018 and replicated the original
technique of estimating distance and direction to all birds detected
perpendicular to the nearest trail markers found at 25 m intervals along
all trails. Whenever possible, we recorded the position of
counter-singing territorial neighbors, which were then used as boundary
indicators for delimiting territorial boundaries. On each day, routes
were randomized in their assignment to observers who rotated their
starting and ending positions on each route to provide even coverage of
the plot. Most routes were covered in the early morning hours, starting
pre-dawn and ending two to three hours later. Routes were covered after
dusk in order to census nocturnal species. Opportunistic detections at
other parts of the day were also noted and observed. Over the 2018 field
season, we logged 18,430 spot-map registrations of 224 species on the
plot. These observations were digitized and corrected for observer
sampling error (see Supplementary Information) and then used for several
methods of comparison between census and the re-census.
After correcting for distance, we wrote a user-friendly, graphically
interactive R program to detect and plot all counter-calling events and
then produce automatically the map of each species recorded in 2018.
These maps were inspected independently by AEM, SKR and JT to determine
the number of territories within the plot. Knowledge of the natural
history of each species, and, in particular, the number of individuals
per territory/group, then resulted in a density estimate within the
97-ha plot, following the methodology used in Terborgh et al(1990). To carry out the Kernel Density Estimation (KDE) and analytical
comparison of the geo-referenced observations in 1982 and 2018, we
manually digitalized and geo-referenced the maps for 247 species
produced in the original study. Producing these 247 maps involved
writing and designing software-specific tools which are presented as
supplementary material.
Mist-netting :
We replicated the sampling effort for each of the original mist-net
lanes designated in the original study. We captured and banded birds as
part of a standardized mist-netting regime, and also while
target-netting understory mixed-species flocks throughout the entirety
of the plot. We conducted standardized netting in seven locations on six
trails, with lines of 18 mist-nets (12 x 3.2 m, 36 mm mesh) along
250-350 m of trail in each location, and operated the nets from dawn to
dusk on three consecutive days when weather allowed, as in Terborgh et
al (1990). During standardized netting we captured 360 different
individuals of 79 species using the same methodology as the original
study. When target-netting understory mixed-species flocks, we operated
the nets in conjunction with playback from dawn until either we had
captured a significant portion of individuals in the flock or until bird
activity had slowed between the hours of 09:30 and 11:00. Target
mist-netting was not conducted near any standardized mist-netting lanes.
Core flock species and some additional species strongly associated with
flocks (e.g., Isleria hauxwelli , Myrmoborus myotherinus )
were color-banded in order to facilitate following flocks and mapping
out flock territories.
We were unable to band captured hummingbirds due to not having
appropriate bands for the family, and, despite past use of the
technique, we did not clip rectrices due to the high number of species
that incorporate the tail in courtship displays. However, we
photographed the right wing, tail, and body of all captured individuals,
and with recent advances in the understanding of identifying individual
birds through the use of digital photography (Pyle & Sullivan 2010;
Nelson & Pyle 2013), we feel that a combination of age, sex,
measurements, and molt pattern allowed us to identify all captured
hummingbirds and to account for recaptures.
Colonial birds :
One observer surveyed the entire study plot for active nesting colonies
of icterids. Once all colonies were found, the observer counted the
number of active nests on the plot. The number of individuals of each
species was pro-rated on the plot after taking into consideration
foraging ecology and natural history observations compiled by one of the
authors (Robinson 1985a, b; Robinson & Terborgh 1995).