We’ve been monitoring bird populations in the Guánica Dry Forest, southwest Puerto Rico since 1972, making it the longest continual monitoring study in the Neotropics. Guánica is a very special place: it’s an International Biosphere Reserve, protecting some of the best subtropical dry forest in the world. It’s also home to several unique and often endemic (indigenous) birds such as the Puerto Rican Lizard-Cuckoo (Coccyzus vieilloti) and Puerto Rican Tody (Todus mexicanus), and we’re fortunate to work there. We’ve learned a lot about the bird community over the past 40+ years, but there is still much more to understand.
Captures of migratory species declined sharply from 1983–1988, part of an overall downward trend from 1973 onwards. Their populations remained fairly stable over the next 10–15 years, with annual variation obscuring any trends that were present, but 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.
Most alarming, we have seen a 53% decline in Black-and-white Warblers (Mniotilta varia) 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). Other resident species, such as the Puerto Rican Tody, 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 another major research focus over the coming years.