We know where they are, those millions of migratory birds that live among us each spring and summer, and we know what they’re doing.

Bright orange Baltimore orioles spend the winter scattered throughout Central America and northern South America, feasting on fruits, berries and tropical insects. They were already on their way northward in late February.

Over the winter they rubbed shoulders with stunning rose-breasted grosbeaks that winter in the same areas but consume a more varied diet that includes seeds and nuts. Grosbeaks aren’t in as much of a hurry to start out, however, and might linger in the tropics until April.

Ruby-throated hummingbirds are lapping up nectar in Belize and Costa Rica and those handsome black-throated green warblers will soon be leaving Mexico and the West Indies, heading our way.

Not all migratory birds spend the winter in Central or South America but those that do have enjoyed months of warm, very humid weather. They shared the rain forest and other habitats with brightly colored resident birds, huge butterflies and leaf-cutting ants. With plenty of fruit, seeds and insects to eat in their tropical paradises, the question arises: Why don’t they stay in those habitats? Why fly thousands of hazardous miles to breed and raise their young?

Not so lush

One of the most surprising things about Central and South America is that for all the apparent lushness, the region is not one huge rain forest cafe. Yes, fruit and berries seem abundant and to anyone who has visited there, insect life seems rife. But the tropics undergo periods of scarcity, especially during the dry season, and available food is widely dispersed in all seasons. So birds that live there year-round need a large feeding territory to be sure of having enough food for themselves and their broods.

As Steve Hilty notes in his “Birds of Tropical America,” local rain forest birds tend to hold a territory for life, so it must be large enough to ensure enough food over their lifetime. And, he notes, the rain forest is “teeming with diversity but not necessarily with great numbers of individuals.”

Migratory birds wedge themselves into southern climates for half of each year and since they won’t be breeding there, they require less space. They fly northward with the spring in order to find enough food to raise strong nestlings.

Insect ‘bloom’

The tropics simply don’t contain enough protein resources to nourish those billions of hungry young birds. But the temperate zone does, for a short period each year, just long enough for youngsters to grow up, hop out of nests and learn to fly. Neotropical migrants focus on the huge hatch of larval insects — essentially, caterpillars — that burst out of eggs laid last year on twigs and branches. Warblers, tanagers, orioles and vireos eagerly snap up the caterpillars they find chewing into spring’s young leaves high in the trees.

Parent birds hunt for hundreds of larvae and adult insects to bring back to their nests each and every day. And many birds, like robins, cardinals and bluebirds, raise two or more broods each summer, further helping keep insects in check.

Then these birds depart in the fall because their food — insects, berries and fruit — is on the wane and will soon disappear entirely. Essentially, then, our gorgeous songbirds fly from green season to green season in search of the best food buffet.

Late arrivals?

Migration is a gantlet of dangers (storms, lack of stopover sites, inadequate food, buildings and towers to run into, etc.) but this has been offset by the abundant food supply in our region. However, climate warming presents migratory birds with a new challenge. They leave their wintering grounds on about the same schedule each year, because they’d have no way of knowing if northern springs are early or late.

However, now that our spring is arriving several weeks earlier, the annual burst of insect life, which responds to local climate conditions, might occur before migrant birds reach our area. This could have a negative impact on birds as they enter the breeding season.