John Faaborg began monitoring bird
populations in Southwest Puerto Rico in 1972, joined by Wayne Arendt in 1981. The original netline was discontinued in
1980, due to severe habitat change following drought in
the 1970's. However, another netline has been monitored
every year since 1973 (except in 1977 and 1979). Each netline consists of 16 nets (each 12
m long), strung end-to-end along a trail. Nets are open
from dawn to dusk for three consecutive days, so the
monitoring effort is constant from year-to-year.
When we
capture a bird, we put an individually-numbered band on
its leg, which allows us to track that bird over time.
Captures of migratory species declined sharply from
1983–1988, part of an overall downward trend from 1973
onwards. In response, we expanded our monitoring effort,
adding six netlines in 1989, one in 1990 and a ninth in
1991. The same nine netlines have been monitored each
year since then.
Populations
of migratory species remained fairly stable over the
next 10–15 years, with annual variation obscuring any
trends that were present. However, dramatic declines in
these populations were seen starting about 2001. Our
project has now documented significant declines in both
the number of species, and the abundance of the most
common species (see graphs below, from our recent
publication in Biodiversity and Conservation). Most
alarming, we have seen a 53% decline in Black-and-white
Warblers (Mniotilta varia; left) from 1989–2011. Survival rates have remained high
for all migrants, suggesting the declines are occurring
because fewer individuals are settling in Guánica. We do
not yet understand why that might be occurring, making
it one of our top research priorities over the next few
years.

We have also seen declines in some of the
resident species over this time, such as the Red-legged
Thrush (Turdus plumbeus; right). Other resident
species, such as the Puerto Rican Tody (Todus
mexicanus), appear to have stable populations. In
the past, population fluctuations of many residents were
linked to rainfall patterns. However, it appears that
their populations may now be responding to other
factors. Trying to understand why some species are
declining while others are stable is our second major
research focus over the coming years.
It is not easy
to carry on a project over these time scales, and we
could not have done it without assistance from many
sources. This research has been funded by the International
Institute of Tropical Forestry (US Forest Service),
and is conducted through the University of Missouri, in
co-operation with the University of Puerto Rico. This
research is approved by the Puerto Rican Department of
Natural Resources and the Environment (Departamento de
Recursos Naturales y Ambientales de Puerto Rico), the
USGS bird banding lab, and the MU Animal Care and Use
Committee. We wish to particularly thank Miguel Canals
for his logistical help over the years, and the many
volunteer field assistants who have helped with the
project.
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