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What is migration?

What is migration?
What are Neotropical Songbirds?
Why is it important to know more about Neotropical Migrants?
Diurnal vs. Nocturnal Migration


     It's 8:00 p.m. on an autumn night and a cold front is moving through the region.  Brisk northwesterly winds signal a change in the season.  High above our neighborhoods the wings of hundreds of thousands of songbirds strain under the force of flight.  Airborne and on the move to points south, these nomads soon must land . . . hopefully where there is food and shelter.  The annual migrations of songbirds are a high stakes, life and death drama that has been played out for thousands of years and for countless millions of birds.  The birds know what to do, but we are just beginning to understand the spectacular nature of nocturnal bird migration.  Although we can't see nocturnal migration with our eyes, the fact that it is happening is being documented as we quietly sleep through the night.  

WHAT IS MIGRATION?

     Migration is movement. It is a process. It is an observable phenomenon for those willing to see it, and is almost always evocative when witnessed. Migration applies to animals as well as plants. It occurs worldwide to populations as well as to species within a microclimate. The very existence of migration drives scientists to study it as surely as it inspires artists to render it and poets to capture its essence in words.

     There are many different definitions of migration, but all include these essential qualities:
  • Migration exhibits predictable movement of an animal from one location and climate to another location and climate.
  • Typically these movements are linked to resource availability, seasonal changes and reproduction.

     New Jersey's official bird list (all of the birds documented in the state) stands at 445 and about 50% of these are migratory. 

     Some of these birds migrate during the day.  These diurnal migrants include shorebirds, hawks, ducks and geese, as well as PURPLE MARTIN by KEVIN KARLSON some songbirds.  But the bulk of the songbirds, though, migrate at night.  These nocturnal migrants such as warblers, vireos, thrushes, and sparrows leave their daytime habitats just after dusk and spend the next eight to ten hours flying.  Near dawn they descend to another site along their migratory route, and spend the day or next few days feeding and resting until they can continue the journey. Each species employs its own strategy for migration, but all require two important things during their migration - food and rest.  New Jersey's habitats provide both of these necessities.

     Think of migration like this: 
     You are planning a car trip along the eastern seaboard from Florida to Maine.  Along the way, you have to stop and refuel the car.  If you follow Interstate 95 the entire way, there are numerous places for you to do that, but if you take another route, you mayAMERICAN ROBIN by KEVIN KARLSON have to be more diligent in finding gas stations so that you don't run out of fuel somewhere along the way.  At some point you will also want to rest so you can complete the journey safely.
      Birds do essentially the same thing, although they don't have maps and can't plan ahead.  They rely upon the fact there is appropriate habitat along their route to provide both food and resting areas.

WHAT ARE NEOTROPICAL SONGBIRDS?

 

     Neotropical songbirds are those species that spend approximately eight months of the year wintering in Central and South America and the remaining months on their breeding grounds in North America's temperate latitudes.  Currently, the U.S. Fish and Wildlife Service classifies 11 of the 96 Neotropical songbird species (12%) as endangered, threatened, or being of management concern and another 65 Neotropical songbird species (68%) show measurable population declines (Degraaf and Rappole, 1995).


WHY IS IT IMPORTANT TO KNOW MORE ABOUT NEOTROPICAL MIGRANTS?

     Neotropical songbirds are truly birds of two worlds. They rely on the resources and conservation efforts throughout the Western Hemisphere. Although widespread population decline in Neotropical migratory songbirds is one of the most critical issues in avian conservation, the concern does not end there. 
     Since the early days of mining, people have used the phrase "canary in the mineshaft" to describe how birds can act as barometers for human health. Ornithologists, ecologists, and conservationists see this reduction in species biodiversity as the canary for larger quality of life issues. Because these birds are of two worlds the quality of life issues that we are looking at are not limited in scope but transcend traditional political boundaries and stretch throughout the New World.


DIURNAL VS. NOCTURNAL MIGRATION

     The benefits of diurnal migration exhibit themselves clearly in the largest group of daytime migrants, the hawks. These migrants depend heavily on riding rising air currents called thermals. As these air currents heat up during the day and rise in the atmosphere, birds take advantage of this lift. Many species thermal hop - they rise to the top of one thermal, set their wings in the direction they want to travel, coast to the next thermal, and then repeat the process. The simple fact is that thermals do not occur at night in such a quantity to allow these birds to utilize them successfully. Other thermal riding or soaring birds include storks, pelicans, and cranes. Some Neotropical songbird species, like kingbirds, swallows, and blackbirds will migrate during the day but the vast majority are nocturnal migrants.

    
Imagine also that you will be doing some traveling at night, and all you have to help you find your way is the sun, the stars, and your five senses. Amazingly, this is essentially what hundreds of millions of young birds experience when they undertake their first migration south. For adult birds, the task of finding their way between wintering and breeding grounds is slightly less difficult, but no less incredible.

     As a rule most birds (excluding owls, night-herons, goatsuckers, and some other species) are typically diurnal during most of the year, but they migrate only at night. Nocturnal migrants tend to be birds that have long distances to fly and do so in powered flight. At night the atmospheric structure is much more stable. It is cooler and smoother than during the day. The coolness helps birds to maintain healthy body temperature without large water losses, while the smoothness of the air allows for a straight level course without expending energy correcting and maintaining a course in turbulent air. (Kerlinger, 1995.) The cover of night is also a great way to avoid predation.