Thoughts on Migration
We are in the midst of Spring migration here in New England. Millions of birds are streaming in from their Southerly wintering grounds, hoping to arrive and establish breeding territories earlier than their rivals. The mating game is afoot and making efficient choices on the timing of migration and where and when to stop and refuel is critically important. A bird's reproductive success is largely dictated by their overwintering biology and migratory physiology. A male that arrives on breeding grounds early and in great physical condition will procure a high-quality territory and this, in turn, will attract a female and ensure a mating opportunity.
Our understanding of migration biology still has many gaping holes, but biologists have come to the realization that, in order to truly understand the energetics of a bird, one must consider every aspect of its life...not just the portion that plays out in our backyards. Full-annual cycle (FAC) Models are increasingly being used to bolster our understanding of the costs of being a bird. These models take into consideration the overwintering and migration biology of a species, two aspects of bird biology that, until recently, were relatively under-studied. As we peer into these periods in the lives of birds, we are beginning to better appreciate their habitat needs on a year-round basis.
For instance, we can now pair on-the-ground data on species occurrence, abundance and prey availability with NEXRAD radar to determine how birds use habitats during migration. The stopover duration, coupled with prey availability and other habitat characteristics can be used to determine the relative importance of multiple locations for migrating birds. This is all very cool...but why am I bringing this up? Well, for starters, technologies such as these demonstrate just how little we know about birds, but they also show how far we've come towards arming ourselves with the knowledge to make the biggest impact in avian conservation. And, in a world with fewer in-tact habitats available to birds, this is very important.
Let's talk about some specifics from the tropics. If you are a resident bird in the tropics, you fall into one of two broad categories: you can be territorial and spend your time defending a patch of rainforest that supplies the resources you need for yourself and your offspring, or you can be relatively nomadic, following foraging resources through the landscape and breeding "on the fly". Ant-following birds fall into these categories nicely and we know that some species, such as the Spotted Antbird, hold and defend territories and only opportunistically follow raiding army ants.
As a result of holding a territory, these birds become very familiar with the resources the territory holds. In particular, hiding places from predators are committed to memory and used as needed to avoid danger. A downside to holding the territory is that you need to work harder to find food (as opposed to following army ants and allowing them to do the work for you). A big benefit, however, is that the familiarity with your territory increases your chances of survival. The average Spotted Antbird will live to be 10-years of age and some may live to the ripe old age of 14...pretty old for any bird.
Bicolored Antbirds, on the other hand, are obligate ant-following birds and instead of holding and gaining familiarity with a territory, they roam the rainforest following ant swarms. While they benefit from the foraging efficiency of ant-following, their lack of knowledge on where the best spots are to hide from predators make it harder for them to avoid meeting their end at a much younger age.
So those are examples of birds that live in the tropics year-round. Life can be tough even for residents. But consider for a moment the neo-tropical migrants. These are birds that spend 4-5 months out of the year in the tropics and the rest in transit during migration or on breeding grounds. These birds need to be familiar with many, many different habitats. Not only do they need to find enough energy to propel them on multi-thousand mile migrations, but they need enough to complete the act of breeding as well (which is inclusive of territorial defense). In addition, as they move through these varied habitats throughout the year, they need to be efficient at avoiding predators. In the tropics, migratory birds are relegated to marginal habitat (as the prime habitat is already spoken for by resident species) where food resources may not be as high and where predation rates may be even higher.
This picture of a Northern Waterthrush was taken in El Valle de Anton in Panama during a recent tour. The bird was foraging in the ditch behind a coffee shop as it made its way North.
The ability of such a small bird that is marginalized to the periphery of suitable habitat to find enough energy to fuel its very expensive lifestyle is amazing. It's job is far more difficult than that of the Bicolored Antbird and yet, every year these amazing birds show up in my backyard in New England, establish territories, sing their hearts out, mate and then head South. A cycle that has been repeating for millions of years.
So, the next time you go birding and watch migrants returning to your backyard or picking through the leaf litter in Panama while en route to some far away destination, take a moment and recognize the shear magnitude of what you are witnessing.