The vast majority of successful bird conservation projects in the world focus on habitat. Given appropriate habitat many bird populations have proven to be remarkably resilient in the face of disturbances, but without it the cliff grows ever closer for many. Migratory birds have it doubly bad, in that not only do they require habitat for breeding, but we’re increasingly finding out how critical over-wintering and stopover habitat is for these birds who spend so much time in transit.
If there’s a silver lining for many migratory birds in the Americas, it’s that the vast Amazon basin and the ample boreal forests of Canada are still in relatively good shape. With the many negative elements impacting bird populations – stopover habitat loss, window strikes, outdoor cats – it’s perhaps remarkable that many North American bird species are doing as well as they seem to be. Some of this can likely be chalked up to the relative health of the regions on both ends of their migration. That stands to change, however, as the effects of climate change start to bite South America.
Blackpoll Warblers undertake one of the longest migrations of any passerine in the Americas, photo by Steve Thompson via flickr
Rsearch, published last week by the Smithsonian Migratory Bird Center in the journal PLOS One, raises concerns that an increase in the frequency of the weather phenomenon known as El Niño, which is characterized by warmer water in the eastern Pacific and associated warmer and wetter air. During El Niño years, migratory birds tend to experience much drier conditions in South America, which limits food availability and in turn affects their ability to make those continent spanning migrations.
We found that migratory birds that over-wintered in South America experienced significantly drier environments during El Niño years, as reflected by reduced Normalized Difference Vegetation Index (NDVI) values, and arrived at stopover sites in reduced energetic condition during spring migration. During El Niño years migrants were also more likely to stopover immediately along the northern Gulf coast of the southeastern U.S. after crossing the Gulf of Mexico in small suboptimal forest patches where food resources are lower and migrant density often greater than larger more contiguous forests further inland.
As the effects of climate change really start to hit, we are predicted to see more El Niño years, which put more and more stress on those birds whose life-history already has them living on a knife’s edge.