SPRING BIRD MIGRATION
By Leon Nawojchik, Manager, Ellanor C. Lawrence Park
Spring is here. The days are lengthening and the temperatures are warming. Now is an excellent time to see and study birds. Many of them are at a peak of activity, have their bright-est colors and are very vocal. You can visit natural park areas on your own or join a scheduled nature center hike. Several sites have bird checklists showing what you might expect to see. And during migration there is always the possibility of a surprise sighting.
Why do birds migrate? Some don’t migrate at all. Many species like backyard cardinals and chickadees are permanent residents, never trav-elling far from home. Other species winter here from higher latitudes or altitudes, enjoying our relatively warmer climate, and then leave in the spring. Some neotropical migrants may travel hundreds or thousands of miles to truly warm places.
The species’ choice of whether to migrate or not is a careful ecological and evolutionary “calculation” worked out over thousands of years to maximize success. Species that stay at home avoid the stresses and dangers of long flights but have to endure reduced food supplies and harsh weather. Active migrants may find abundant winter food by going south but suffer from predation, storms, accidents, navigational errors and physiological stress. Some long-distance fly-ers may lose up to one-third of their body weight during migration. Generally those that depend on food like caterpillars, fresh fruits or nectar, or on conditions like warm weather, must leave.
As day length increases incrementally in the spring, the bird registers changes in brain tissue that initiate hormonal changes. Increased hor-mone levels cause feather molt, sexual changes and the urge to travel (“migratory restlessness”).
How do they find their way? For thousands of years people have wondered about and admired bird flight and migration. Over many years researchers have uncovered several special avian abilities. Some species have been shown to be sensitive to the earth’s magnetic field. Others can use the nighttime star patterns to find their way, often back to the locality in which they hatched.
Many small species travel at night, possibly to avoid hawks. Some larger species like geese typically travel in daylight. Their V-shaped flocks provide aerodynamic efficiency and leadership from experienced front birds. These longer-lived geese make use of landmarks such as coastlines, mountain ranges and river valleys that can be remembered from past trips.
Often birds may wait for changing winds to assist their flights. At times they may be way off course. They may be subject to strong winds, storm fronts, individual errors or quirks. During migra-tion season, birders know that, if they are lucky, almost any species in the book may show up.
Many individual birds do not make the trip. They may perish from unexpected rain or ice storms. Some are caught by hawks or falcons. Others may be blown out to sea. Man-made structures also take their toll. Tall, lighted office buildings and lighthouses may be littered with the bodies of dead songbirds that were confused and drawn in to a sudden collision. Some locali-ties reduce lighting in tall structures during peak migration hours.
The populations of many spring songbirds are declining. Human-caused changes to the landscape such as development and forest fragmentation take their toll here. Slash-and-burn agricultural prac-tices and the clear-cutting of tropical forests are also reducing available wintering grounds.
Recent climate changes affected by global warming are causing new vegetation patterns, new weather patterns and changes in the dates of emergence for leaves and insects. Timing is important. Typically an increase in the length of the day is followed by the budding of young leaves, then the emergence of insects and finally by the arrival of birds. Some bird species will be able to adapt to changes in the timing of these natural events. Others, especially those that are isolated or limited in number, may not survive.
This spring many of us will make time to see and enjoy our colorful and varied songsters. It is also an important moment to pause and reflect on the complexities of migration and the con-servation needs of the future.