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I have a friend, Tim, who has been in Hawaii about two years and enjoys the wildlife here. A couple of months ago, he saw a fairy tern chick in a tree and asked me some questions about these most charming of Hawaii's seabirds.

I was swamped with the chores of moving at the time and gave Tim a one-word answer with no explanation.

Soon after, I received this e-mail: "Dear Ms. Scott of Ocean Watch," it began. "I saw a fairy tern in Kapiolani Park yesterday sitting on her chick. I read about her in my Audubon bird guide and learned that she lays her egg on open branches, no nest. My question is: What keeps the egg from rolling off the branch or being blown off by the wind? (What a beautiful bird!)"

This question is as good as Tim's, I thought, and I saved it for a future column. I opened it today, and guess what? It is Tim's question. After my too-brief answer, he wrote for an "official" one, and I missed the joke.

OK, Tim (and everyone else interested in these remarkable birds), here is everything I know about the precarious positions of fairy tern eggs and chicks.

It's true that female fairy terns lay their eggs on branches without so much as a twig, feather or piece of mud to help keep it there.

These adaptable seabirds also like to lay eggs and raise offspring on human structures. In Hawaii's northwest wildlife refuge, I have seen both eggs and chicks sitting perilously on narrow window ledges, on the top of signposts and even on the flat, round valves of fuel tank spigots.

What keeps the eggs there? To restate my answer to Tim's original question: nothing. The only thing parent terns do to keep their eggs from rolling to the ground is sit on them. But still, eggs fall all the time. Only one-third to one-half of fairy tern eggs laid actually produce chicks.

And chicks fall, too. I once saw an egg roll off a windowsill while the chick was hatching from it.

A remote island biologist I once knew could not bear this heartbreaking quirk of nature. Figuring the researchers' barracks was disturbed territory anyway, he walked around the place with a hammer and chisel, chipping shallow dimples in the concrete ledges.

These hollows proved to be a great success. While working in the refuge, I had the great pleasure of watching a tern couple hatch an egg (which lay in the indentation) and raise their chick on the windowsill of my room.

This same tenderhearted biologist watched a tern couple repeatedly try to raise offspring on a small tree branch, but each chick fell to its death.

He decided to take action. After waiting for the unlucky tern couple to hatch yet another egg, he built a tiny Popsicle-stick enclosure and carefully slipped it beneath the baby bird.

Everyone laughed hard when we saw the results of this wooden "nest": The chick had hopped up on the narrow rail of the pen and teetered there precariously for its entire rearing. To everyone's delight (and surprise), the chick lived to fledge.

No one knows why fairy terns lay their eggs in such hazardous places, but it isn't done by accident. A mated male and female both choose an egg-laying site by walking back and forth over a branch or ledge repeatedly.

Whatever the reason for this curious nesting style, there is one thing for sure: A fairy tern chick that survives to fledge has remarkable balance.

And my friend has remarkable patience. Thanks, Tim, for asking a good question and waiting this long for a decent answer.