October 2008

Autumn changes at the Refuge are subtle since only a few species of trees, like cypress and red maple, are deciduous rather than evergreen. They have leaves that get fall color and then are dropped.

Red Maple, one of the few Refuge species to show fall color

More obvious signs of autumn at the Refuge are the arrival of "snow birds." Both people and birds know that winter is coming. Fall migrants have started to arrive at the Refuge. More people have come and sales at our Friends' store have increased. Blue-winged teals and ring-necked ducks are joining resident mottled ducks and wood ducks. Coots have joined resident moorhens. And huge flocks of sandpipers and tree swallows have begun to pass through on their way further south. Most of the warblers and vireos have already arrived at their Caribbean and South American wintering grounds.

Why do some birds migrate and others remain in one place as residents? To answer this question we need to consider advantages, i.e. benefits, and disadvantages, i.e. risks. Benefits and risks are mainly related to reproductive rates and mortality rates. Let's consider two hypotheses to explain the evolution of migration.

At the beginning of food chains we have green plants. Surprisingly few birds specialize on eating underwater green plants. Migrant waterfowl that eat green plants and come to the Refuge include blue-winged teal, ring-necked ducks, and coots. Hunters like to eat these these herbivores because they generally taste better than fish eaters like mergansers and other diving ducks. Many of the migrant ducks would find it difficult to get to underwater green plants in their northern homes because the freshwater lakes and ponds where they nest in summer freeze over in winter.

Continuing up the food chain, many animals, especially immature insects, eat green plants. Whether or not they  eat plants, very few insects are active in winter. So we might predict that birds that specialize on eating insects would have to migrate south. Swallows, swifts, nighthawks, and flycatchers always migrate. Even warblers and vireos depend so heavily on insects for food that they all migrate. Robins can switch from worms and insects to fruit and even seeds; only in severe winters do they migrate far south.


Hypothesis 2. Birds migrate North to Raise More Young

The other hypothesis posits that birds migrate north to take advantage of the spring pulse of plant and insect growth and reproduction. Presumably, with more food, birds can lay more eggs and raise more young than if they were to stay in the south to breed.

Birds find much more food up north in the spring for two interrelated reasons.

1. Because the days are longer, plants grow faster. Thus the plant fruits, seeds, and insect-eating insects are more plentiful as food for birds.

 

2. Birds have longer to find food each day. At the equator, a day's length is 12 hours year around, but at the North Pole day length increases in summer to 24 hours.

As we go north in summer, plant biomass, including leaves, fruit, and seeds is more abundant. Birds may eat the seeds and fruit, or they eat insects that eat plant leaves, fruit, and seeds. Plants grow faster up north because days are longer and nights are shorter and cooler. At night plants do not photosynthesize, but they do continue to respire. Thus the shorter the night, the lower the respiratory loss. And, the cooler the night, the lower the rate of respiratory loss.

Longer days also give birds, which are overwhelmingly day-active, longer to find food and feed young in the nest.

Predictions from the hypothesis that migrating north provides a reproductive advantage are supported by natural observations on the number of eggs laid. On average in the north temperate zone, songbirds lay 4 - 6 eggs per nest and in the tropics only 2 - 3 eggs per nest. These data are convincing because such differences are the same for all species studied.

Not finding adequate food during migration is a problem because the metabolic costs of long, continuous flights are very high. Birds store fat each day before migrating because it is the most efficient storage product. Compared to carbohydrate and protein of the same weight, metabolism of fat yields twice as much caloric energy and twice as much water. Those ruby-throated hummingbirds that fly south across the Gulf of Mexico put on 2 grams of fat the day before they leave and increase their weight by 70 percent!!

Of course the above hypotheses are not mutually exclusive. All could be sources of mortality during migration.

Surprisingly few data show the relative mortality rates of birds during migration vs. on their breeding or wintering grounds, but the few data are convincing. Indirect data come from predation by domestic cats. In the past, when we let our cat go outside, she frequently caught migrant thrushes and warblers but never resident robins and house sparrows. Direct data come from multiple recaptures of banded birds on their breeding ground within and between breeding seasons. For example, black-throated blue warblers have a 99 - 100 % survival rate on their breeding ground but only 77 - 81 % survival between breeding seasons. The much lower mortality on the breeding ground is most likely because birds get to know the good places to hide from predators, escape routes, and location of food patches in their territories.

Some Birds do not Migrate. Why?

Especially here in south Florida, most bird species do not migrate; they are permanent residents. Examples at the Refuge include anhingas and moorhens that stay in the same local marshes all year. Great horned owls and red-shouldered hawks, both predators, and cardinals and red-winged blackbirds, both generalist feeders, are also permanent residents.

At times some resident species move locally but this is not migration. Various herons and egrets move daily to favorable feeding sites and move seasonally to nesting sites on or near the Refuge. As with northward migration these local movements help to increase reproductive success. When feeding nestlings, wading birds search widely to find local concentrations of prey.

Bird species that are Refuge residents avoid the high mortality rates of migration. Of course they also lay fewer eggs than the same species farther north where spring is distinct. Exceptions that may prove the rule are moorhens and purple gallinules, that are mostly resident but migrate part way northward with the spring in some years. Those that do migrate lay more eggs and raise more young than those that stay in south Florida. This occasional migration could lead to yearly migration over evolutionary time.

Whatever a bird species does, its behavior must provide more benefits than risks or the species would go extinct. Migrants have very high reproductive benefits and moderate risks of mortality during migration. Resident birds have low reproductive benefits but also low risks of mortality.

No Mammals Migrate to or from Florida: Why?

I leave you with this mystery to stimulate your thinking. We will discuss it in November's column that will compare similarities and differences in mammal and bird adaptations. In the meantime, what hypotheses can you develop to explain why mammals do not migrate? Here are some questions to help your thinking. What different senses are dominant in birds and mammals? What is the main difference in mode of locomotion in birds and mammals? And finally why do so many more mammals than birds eat plants and what are the digestive specializations that allow this?

 

REVIEW QUESTIONS  

a. Small birds that eat seeds must migrate south in winter because they find it difficult to find enough to eat during the short days to survive the long, cold nights.
b. Birds that specialize on eating insects always migrate south for the winter.
c. Birds that migrate north to breed lay twice as many eggs as resident bird species in the tropics.
d. There must not be any great risk to migration since so many species of birds migrate north in spring and south in fall.