|Paradise Tanager by Nathan Rupert|
| Young female Black-throated Green|
Warbler by Arthur Morris
The battle begins in the late summer, thousands of miles back in the North American boreal forest, when a hormonal shift causes the birds to feed frantically on anything they can find, and fatten up. After exponential growth in body weight, the bird, we'll say a young female Black-throated Green Warbler, gets the jitters, and when the right weather presents the opportunity, she instinctively rides the winds off to south (a miracle in itself). By September, she's survived the leap-frogging across patches of green over hundreds of American miles, and the long-haul, non-stop, over the Gulf of Mexico that make up her migration, and has made it to a decent forest stand in Central America. She is finally able to begin making a living in her first nonbreeding season.
Even before humans were around, first-year female songbirds did not have forest ecology in their favor. They are at the bottom of the passerine social hierarchy; in addition to competition between species for resources like food and territory, our female must compete with aggressive adult males, cunning adult females, and other youngsters as desperate and inexperienced as herself. As a result of this hierarchy, she ends up with some of the lowest quality forest available, pushed all the way to the dry and nutrient-deficient forest edge, a landscape quite alien to her. With the addition of human life, what was once a sprawl of rainforest that provided plenty of remaining territory for her has been replaced with a sprawl of farmland and rural communities. With so much of the necessary nonbreeding habitat destroyed, our female is now subject to new challenges to which she is not adapted. She experiences a sparsity of the food that would be easily found deeper in the jungle. Now in a more open area, she comes into contact with new predators in greater numbers, and is more vulnerable to them under the scattered cover. And possibly worst of all, she comes into contact with neurotoxic pesticides. Many countries in Central and South America do not have the bans on dangerous pesticides that we do in the United States. Organochlorides and other neurotoxins used to defend crops from insect infestation have wiped out unbelievable numbers of flocking migrants such as Swainson's Hawks and Dickcissels. Our young female, five inches long at .3 ounces, barely stands a chance. Even if she isn't taken by them, pesticides often disrupt migrants' navigational senses. More vagrants for birders, but not good news for birds.
Facing malnourishment, predation, and neurotoxic pesticides, young passerines especially face an enormous problem in their wintering grounds. Many of their original high quality wintering grounds no longer exist, so more birds from the North American breeding population must make due with the smaller amount of high quality habitat; it's no wonder that more and more of the population is being pushed to the margins of their habitat. In some highly fragmented places, only the strongest adult males are getting the highest quality habitat. This puts a worrysome percentage of the breeding population in quite a precarious position. It's no wonder that it's becoming increasingly difficult to find Acadian Flycatchers, Wood Thrushes, and Cerulean Warblers!
|Tropical Rainforests of the world. Rainforests|
yield the most biodiversity, but there are certainly
other biomes that house neotropical migrants.
So the next time you see a neotropical migrant, say, a young female Black-throated Green Warbler, think of the grand cycle and natural drama it has participated in. Think of the tens of thousands of miles it has flown, and all of the challenges it has faced. Think of how lucky that individual is to be living. It is sometimes said that 40% of the North American breeding songbird population has been lost since the 50's, with some species' populations nose-diving a frightening 80%. With the obstacles many young birds face in the tropics, it really is amazing that populations keep chugging along. So all I ask is not to take them for granted. We are blessed with the avifauna of our time, and it's a travesty to think that they may not exist for future generations.
Overall, the best we can do is to keep learning. Any observations of the birds are scientifically relevant; the more we know about them, the better we can protect them. So after we've endured the painful yet rewarding time that is winter birding and spring comes around, be sure to get out there and find some warblers! But then again, I don't have to tell you to do that, do I?
|To read more about this subject, I highly recommend Bridget Stutchbury's Silence of the Songbirds, as one blog post can only touch the tip of this iceberg. Her narrative ability makes the book a joy to read, and brings the reader to an incredibly intimate understanding of these neotropical migrants.|