With warm days still hanging on, it’s hard to remember that summer is waning. Though it may not be that obvious, September is a month of change. Autumn doesn’t officially start until Sept. 21, but nature is already preparing for it, transforming and migrating.

In the forests, trees and other plants are beginning to alter their physiology. Some trees have already begun to change color, hinting at the inevitable falling of leaves.

This leaf-shedding process, known as abscission, has already started. For several weeks now, cells at the point where the leaf stem is attached to the tree toughen and begin to form a protective waterproof scar. The cells in the leaf stem swell, weaken and degenerate.

This process interferes with the flow of moisture and nutrients into the leaf, reducing the production of chlorophyll, which gives leaves their green color.

Leaves contain other pigments but these colors are hidden most of the year by the abundance of chlorophyll. With less chlorophyll, other pigments are unmasked and leaves begin to reveal their autumn colors.

Xanthophyll produces the color yellow while carotene, which is also found in carrots, produces yellow-orange. Sunny days and cool nights can produce a sugar-related pigment, anthocyanin, which produces fiery reds. Other chemicals and breakdown products produce bronze, purple and crimson.

Trees aren’t the only ones making preparations for the changing of the seasons. The monarch butterfly, one of the few butterfly species that migrate, is on the move. This large butterfly is easily recognized by its dark orange and black wings. As the days grow shorter, millions of monarchs make their way south. Monarchs in the West migrate to southern California, while those found in the central and eastern parts of North America overwinter in the Gulf states and remote mountain valleys of south-central Mexico.

From the first week of September to the third week of October, the monarch butterfly, that familiar orange and black beauty, is making its way south through the Chesapeake Bay watershed.

The watershed also lies within a major migration path for birds known as the Atlantic Flyway. Mountain chains to the west and coastal shorelines to the east serve as geographic boundaries that channel millions of migrating birds through
the Bay region. These include raptors, a group that includes eagles, falcons and hawks.

Raptors in Canada and northern United States begin their annual southward migration just before the fall foliage color change in early September. The earliest of migrants may go unnoticed. But as the first autumn cold front passes through the area, the skies fill with more and more raptors.

As they approach the Chesapeake Bay, some raptors funnel along the coast, while the others are steered along the mountains. At the same time, songbirds and shorebirds are preparing for long flights from the northern breeding grounds to tropical wintering areas. Songbirds are not as obvious to spot as the many male songbirds now sport duller colored feathers.

The shortening of days signals waterfowl to begin moving south along the Atlantic Flyway. Instead of just passing by, though, the Chesapeake Bay serves as the wintering ground for swans, geese and ducks.

One of the most familiar of waterfowl is the Canada goose. Migrating flocks are most noted for their distinctive “V” flying pattern. Loud honking signals their arrival to southern wintering grounds. Few species mark the changing of the seasons as distinctively as the celebrated Canada goose.

Meanwhile, in the rivers, juvenile American shad spawned in freshwater last spring prepare to leave their natal homes.

By autumn, the young shad gather in schools and swim toward the ocean where they will remain for the next three to six years until they are sexually mature. Then, they return to freshwater to complete their life cycle.

Other juvenile fish that were born in freshwater but mature in brackish water or saltwater are beginning their migration toward the sea. Some of these species include alewife, blueback herring and hickory shad.

So even though the trees may still be green and the days warm and sunny, look a little closer and you’ll notice subtle changes around you. A reddening sumac tree, a broad-winged hawk flying south, monarch butterflies flitting through your yard are all signs that autumn is just around the corner.