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.
