Dubrovnik, Dubrovnik, Dubrovnik. Did I mention Dubrovnik? You don’t need to go anywhere else in Croatia, just go straight to Dubrovnik.

Again, I walked across the border where I managed to find a taxi. On the road to Dubrovnik, we saw a broken-down rental car, so we stopped. The man, who was Scottish, said the car wouldn’t start. The taxi driver, who didn’t speak any English, pointed to the car and said “diesel!” Upon hearing this, the Scot realized he’d put regular gas into his diesel rental car, which is why it wasn’t starting. His wife looked at him like this was his 800th mistake of their trip. My taxi driver said “no problem” and drove away. Then he got on his cell phone and called someone. I heard him say “Blah blah blah benzina, blah blah blah diesel, blah blah blah kaput.” Then he nodded at me, indicating they would be rescued.

I spent four lovely days there, bumped into a few thousand Americans (they’re buying up all the real estate), went sea kayaking around an island, took a day and went to Bosnia and enjoyed hanging out in this wonderful old city. Felt a bit like being back in Jerusalem. I stayed with a lovely oldish woman named Mellie, who rented me a room, blew cigarette smoke in my face, and told me stories of the war. Eighty percent of the town was shelled – almost all the roofs are new. Even though it’s overrun by Americans and the restaurants are mediocre, it’s a very special place.

Like most of the Dalmatian coast, barn swallows fly incessantly above the town from dawn until dark. They fly very quickly, flitting here and there, eating tiny flying bugs that are smaller than ants. You never see them stop or stand on anything (rooftops, wires, stones), because barn swallows will never stop in an exposed place – they always dart into holes. And they presented a puzzle to me that I think I solved.

During the day, they squeal as they fly. Not all of them squeal, but about 40 percent of them do. But then, as evening falls and the light fades, the squeals turn to a high-pitched chatter. Why do barn swallows squeal smoothly during the day and chatter at night?

The answer, I believe, is that they are telling each other where they are so they don’t hit each other. A certain percentag of swallows squeal to let others know they are coming. They don’t all need to squeal (in fact, if all swallows squealed it would be way too confusing), just enough of them to keep the others informed where they are.

The key to solving the puzzle is that a swallow going sideways is easy to see and avoid, but one coming right at you is like a dart – very hard to see and very dangerous. During the day, they can see each other, so the squeal is all that’s necessary. But as it gets darker, it’s much harder to see an incoming bird, so they need to communicate better, especially to oncoming traffic. And that’s where the chatter comes in. The chatter generates a Doppler effect, so the oncoming bird tells anyone in front how close it is and how fast it’s approaching. The chatter rate increases to about 60 percent at night. It takes more energy to generate the chatter, which is why the squeal is good enough during the day, but the chatter is more accurate and necessary for survival during those active feeding times around dusk.

To test my theory, I listened one morning early before the sun was completely up and, sure enough, they were chattering in the early morning light as they snatched their aerial breakfast and avoided incoming flying darts. But as soon as the sun came over the mountain, they started squealing again.

- by David Siegel