They have been dubbed the birds of the Western Wall. They are mentioned in the Book of Jeremiah, and are believed to have come through Jerusalem as spring approaches for thousands of years. And when the small black-brown birds known as Common Swifts arrive in the holy city at the same time every year, they do not pick just any home for their breeding ground and resting place; they choose the retaining wall of Judaism's holiest site. Their decision to stop at the Western Wall is no small matter: the migratory birds spend most of their seven-year lives in the air, eating insects, drinking - and even sleeping - aloft for years at a time. The Swifts stop flying only for nesting, which they don't do in treetops like other birds but in cracks of buildings and structures. For the birds, who spend most of the year in the warmth of southern Africa, Israel is the first stop on their post-winter tour before heading to Europe. Typically, they arrive in Israel at the end of February and spend 100 days in the country, some mating and laying eggs while the younger ones explore the area; they can also be found outside Jerusalem, including in Tel Aviv, Haifa and as far south as Beersheba. The Western Wall is an old and tall structure, where large crowds gather, creating a current of hot air moving upwards toward them, making it an ideal place for mating, nesting and laying eggs. The birds' decision to nest in the crevices of the Western Wall is also attributed to the site's easy access, and the fact that the 17-cm.-long birds are able to find room - and warmth - inside the structure, said Ulrich Tigges, a German expert on Common Swifts who carried out a study on the Swifts of the Wall in 2002. After receiving an e-mail from American-born Rabbi Yosef Cornfeld, who had noticed the birds for years during morning services at the Wall, Tigges came to Jerusalem and discovered nearly 90 nests in the Wall's crevices, mostly in the middle on the men's side of the Wall. He said 120 to 130 Common Swifts come to the Western Wall every spring, with nearly half the nests used by pairs of birds for breeding, while others prepared the site for breeding the following year. "They need a covered location and the Western Wall provides the ideal nesting place," Tigges said. "The author of the Bible must have been a bird-watcher," he said. Thousands of Common Swifts pass through Israel each spring, said Dr. Yossi Leshem, a bird expert at Tel Aviv University. After the first Common Swifts were sighted last week, the Jerusalem Municipality organized a nature event at the Western Wall plaza marking their arrival. The event took place Tuesday evening and was coordinated with the Society for the Protection of Nature in Israel. Rabbi of the Western Wall Shmuel Rabinovitch read from the Book of Jeremiah, and Tel Aviv Deputy Mayor Yael Dayan gave a more secular reading on the birds' annual homecoming. "The Jerusalem Municipality welcomes the Common Swifts on their return to the Western Wall," a sign read in Hebrew. The birds seem to know a thing or two about timing, choosing to arrive at the Western Wall about half an hour before sunset after spending the day flying outside the city. "Their flight is full of joy and their swiftness is breathtaking," said Jerusalem resident Shira Twersky-Cassel, a poet and bird lover who is a ninth-generation descendant of the Ba'al Shem Tov. Then, as the sun started to set, the birds - first one, then another, then a third and finally a group of couples - suddenly appeared in the sky just above the Western Wall, sometimes calling to one another and circling before speedily and elegantly descending into their nests in the crevices of the golden-white stones and falling out of sight.