A Baltimore Oriole, a bird that nests in Maine, is spotted in Costa Rica. Many of Maine's summer birds fly down to the tropical country to spend the winter. Credit: Courtesy of Bob Duchesne

I think I may have let slip in a previous column that I was in Costa Rica last month. Yes, there were many unusual and exotic birds there. I also encountered a lot of old friends. They drove me nuts.

Costa Rica is smaller than Maine, yet it has as many birds as all of North America. Why? The geography is truly weird. Costa Rica has two coasts. Only 75 miles separate the Caribbean Sea from the Pacific Ocean. Between them stands a still-active volcanic region, bisecting the country from top to bottom. Lowland jungles along both coasts quickly rise to foothills, then to peaks twice as tall as Katahdin. Costa Rica packs an entire continent’s worth of habitats into a country the size of a postage stamp.

Then there is the Goldilocks effect. Despite its tropical location, the temperature is “just right.” That is, on the same day that it reaches 90 degrees in the lowlands, it may not exceed 50 degrees in the cloud forest just 30 miles away. The lowland forest may be full of palms and bananas. The peaks may support little more than stunted oak and bamboo. There is a comfortable temperature for every bird that wants one.

There are also dramatic rainfall variations. The Caribbean trade winds sweep warm, moist ocean air over the mainland, where rising elevations and cooler temperatures squeeze the air like a sponge. The Caribbean side of the country is awesomely wet. By the time those breezes clear the 14,000-foot peaks, the air has been wrung dry. The Pacific coast has a significant dry season, making a big difference in the vegetation and birds to be found there.

[Also read: Maine bird expert puts knowledge to the test in Costa Rica, a birder’s paradise]

Lastly, the influx of wintering birds helps to swell the species population. Migration is mostly from the north, but there are some South American birds that do what their North American counterparts do: they fly to Costa Rica for the off season.

Our little tour group kept running into our own backyard Maine birds in Costa Rica. It was such a constant that it became a running joke. The chief culprit? Chestnut-sided warblers. They were everywhere in the tropical areas. Picture this: We’d be scanning mixed flocks of exotic birds, when someone would mutter, “Darn it, another chestnut-sided!” They were a major distraction and a pain, especially because they were in a nondescript winter plumage that Mainers don’t get to see often.

Tennessee warblers were nearly as abundant. Wilson’s warblers were, too. Both are uncommon nesters across northern Maine. Both were common in Costa Rica, to the point of being annoying.

Philadelphia vireos are hard to find in Maine. They nest in clusters in the northern part of the state. They enjoy the woods around Roaring Brook campground in Baxter State Park, the forest around Sugarloaf and the Appalachian Trail parking lot in Maine’s Grafton Notch State Park. In Costa Rica, they were backyard birds at our first inn.

Even songbirds from the southern states were annoyingly common. I’d only seen two Kentucky warblers in my life, both in Nashville. That is, until I pointed one out to the group along a waterfall trail in the Costa Rica highlands. I’ve only seen two prothonotary warblers in the U.S, but I’ve now seen 20 in Costa Rica. I’ve seen just one golden-winged warbler here. I’ve now seen one there.

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Baltimore orioles were everywhere. Rose-breasted grosbeaks were only a little less common. Every imaginable species of flycatcher is in Costa Rica, including possibly the very same great-crested flycatchers that nested in my yard last summer. Black-throated green warblers are likely Maine’s most abundant warblers. They were present. So were black-and-white warblers and yellow warblers. There are over 50 species of hummingbirds in Costa Rica. One of them is Maine’s ruby-throated hummingbird.

It makes sense that we would see so many Maine birds. There are 9,540,000 square miles of land in North America. There are only 202,230 square miles of wintering habitat in all of Central America, and only 19,730 in Costa Rica. When North American birds fly south for the winter, many get squeezed into an area smaller than Texas. If you’ve ever wondered why scientists worry when scarce tropical wildlife habitat is deforested for cattle grazing, banana groves, coffee plantations and palm oil production, this is why.

Come meet the birds and the country. I’ll present a Costa Rica travelogue Friday night, Feb. 14 at 7 p.m., at Fields Pond Audubon Center in Holden. It’s free.

Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.