Since 2006, a particular robin has flown into the Jerusalem Bird Observatory at about the same time in the morning of November 11. On Wednesday, the staff at the observatory behind the Knesset waited all day with baited breath for it to return. But it didn't. "The robin hasn't arrived yet," Amir Balaban, co-director of the observatory, told The Jerusalem Post on Wednesday evening. "I am very disappointed, but I'm still optimistic. Perhaps he will come tomorrow or Friday." Under the optimistic assessment, weather conditions may have delayed the robin's arrival, but Balaban could not rule out a more sinister fate. "Lots of things could have happened," he said. "He could have been eaten by a bird of prey, or injured on its flight here, or caught in Cyprus or Greece. We'll have to wait and see." It is not clear exactly where the robin flies to each year, and why it returned on the same date three years in a row. Millions of birds stop in Jerusalem on their twice-yearly flight between Europe and Africa. According to the observatory, over half a billion birds of more than 200 species cross over Israel in each direction every spring and autumn, many of them stopping to build nests - often in the capital. Over the years, the staff at the observatory, which also houses the national bird-ringing center, have ringed hundreds of birds, including the robin. Camouflaged black mist nets have been set up to catch the birds throughout the observatory. After its unique data are registered, each bird is fitted with a tiny, aluminum ring around its leg that has "Israel" or "Tel Aviv University" written on it as well as a serial number, and it is then set free. When a bird is netted by researchers elsewhere in the world, the observatory or university is usually informed where it was sighted and what its condition is. This allows the observatory to compile profiles of particular bird migration patterns. Such a study can also be meaningful to researchers monitoring global warming, which affects bird migration. The observatory is open to the public all day, seven days a week. Entrance is free.