The Effect of Weather Changes Upon Mallard Migration

Robert Gieszl
|7. December 1999

Ecology
Dr. Tietjen

Birds are a complex species that have developed slowly over time from the age of the dinosaurs. Their ability to fly and survive various environmental has led to their achievement of human respect. One fascinating aspect of the bird is their apparently innate instinct related to seasonal migration. This talent of migration will serve as the basis for the following introduction and research that follows.

Birds that breed in the northern hemisphere, especially those in regions with definite seasonal differences tend to migrate, even travelling down into the southern hemisphere. Birds who breed in the southern hemisphere also migrate. However very few, except perhaps sea birds, migrate from the southern to the northern hemisphere. The warm temperatures close to the Equator are the wintering home of many birds although many will choose a spot that is not as far but still warmer and has more food than where they left.

Fall migration allows birds to move to a different location so that they will continue to be able to find food. In the spring they return to the places where they will breed and raise their young. It may be the slant of the sun's rays, hormonal changes, the change of the weather or other factors that contribute to the birds' urge to migrate to their other home.

Hungry birds, however, do not set off on migrations. If their food supply diminishes in a region, they may move to find a better source of food resources. A bird needs a reserve of fat within its body to be able to travel the long distances, especially for those birds that attempt non-stop or very long flights. These fat deposits are partly due to hormonal changes that store the food necessary for the bird to survive migration. Those that migrate in shorter trips to the final destination are less dependant on adequate and abundant food prior to the trip and need not store the large fat reserves.

Many small birds may even double their body weight before migration while larger birds, such as the Canada Goose will gain proportionally less than this. These small birds store enough fat that many can fly non-stop for unbelievable lengths of time - sometimes up to 90 hours. In the best of conditions during migration, their body may only use less than a half gram of fat per hour.

However, weather and environmental factors need to be right for the birds to migrate otherwise they may experience difficulties. The winds must be blowing in the direction that the birds want to go. Birds will usually wait until the most favorable weather conditions then set off on their journey. Ornithologists watch the weather to help predict when large numbers of birds will be passing through their area. A strong wind in the right direction will speed the birds on their way. If there are strong headwinds, the birds' speed will be greatly reduced; therefore, it will need more fat reserves to travel the same distance. Many birds that encounter storms and strong headwinds perish into the seas. If these weary birds are over land, they will drop down and land, find food and rest before continuing.

The route that some birds instinctively take may seem strange until you look at the winds. For instance, many birds on the east coast of the U.S., head out to the Atlantic during a fall cold front. The northwest winds take them on a southeastern course over Bermuda and beyond. Then they meet the northeast trade winds and make it to South America. This unbelievable journey will take them over 1,800 miles of water and will last over 80 hours. But it is actually easier for them than the land route along the coast, down through Central America, and then onto their destination in South America. The winds are the key factor.

Flapping of the wings uses quite a bit of their stored energy. Therefore, many birds, especially the larger ones, will soar and glide as much as possible, using the winds and thermals. However, the journey will take longer than if the migration was made by more flapping flight. Spectacular kettles of hawks are seen at various places where thousands of hawks climb to the greatest height possible then glide and ride the thermals, soaring without flapping for great distances. A bird's lungs are different than human's so the colder air at these high altitudes actually helps the birds. However, most birds fly less than 3,000 feet in the air.

It is still not completely decided upon as to how birds navigate. However, various studies have been conducted that suggest that birds use the sun or stars as guides. Birds may also use the physical characteristics of the land below, following rivers, shorelines, foothills and valleys as guides. There is also the "homing" instinct that has been studied. There have been studies where birds have been taken in closed boxes to remote locations and released. The birds found their way back to their colonies. However, birds do get lost sometimes, thrown off course by huge storms, fierce winds, or other weather conditions. Young birds, perhaps those who fledged late in the season and unable to migrate with the older birds, may become confused and stray. However, time has shown that the majority of birds find their ultimate destinations.

This research is comprised of an attempt to link changes in weather to the migration of a species of bird that is rather common to the Louisville area. This species is the Mallard duck.

As a background on the mallard, it possesses the scientific name Anas platyrhynchos and is probably the most common duck found in North America. Their nickname is " the dabbling duck" for the simple fact that they feed on the surface of water. They have adapted to this feeding habit in that they have developed flat bills, which are rounded at the tips , in order to aid in the filtering of food.

They measure, on average, roughly twenty-two inches long and are omnivorous. Their diet consists mainly of small plants, weeds, and smaller invertebrates. However, they have no defenses other than swimming, flying, and camouflage and, in accordance with this disposition, they have many enemies, which include humans, small predators such as fox, and manmade complications such as chemicals.

The male is referred to as a drake and is distinguished from the female in that he possesses a green head, white neck band, and rust-colored breast. On the other hand, the female is much less obvious to predators for the simple fact that she will be sitting on the eggs and quite vulnerable to attack from predators. The female, also referred to as a hen, possesses feathers of various shades of brown—an excellent camouflage.

Now, to the actual research. A roughly one acre of streamside land was selected for this project. Three times a week, starting on the 1^st October 1999, the date, time, temperature, and number of mallard observed was recorded in the attempt to determine whether there was indeed a link between changes in weather, namely a transition to winter, is related to the migration of the mallard from the Louisville area.

Data: The following data is comprised of an Excel sheet of the actual observations as well as a graph of that data.

Excel Data Sheet:

Graph of Data: The graph of the data is found on the following page, as a full sheet.

Conclusion: The data collected in this research seems to strongly support the idea that the mallard does indeed migrate south for the winter (Excel data sheet and graph). This data collected is strongly supported by data collected by ecologists (normal and winter distributions). Therefore, the conclusion reached is that Louisville is certainly not far enough south to support a winter population of mallards.

Non-winter distribution of the mallard

Winter distribution of the mallard

Graph of Data