Dan Kunkle, director of the Lehigh Gap Nature Center, said the speaker series is bringing in some expert speakers. Scott Weidensaul, the Sept. 29 speaker, has been a naturalist since birth. Kunkle thinks that is close to accurate. He is also an outstanding author who has published two-dozen books, plus magazine and newspaper articles. One book earned him a Pulitzer-finalist designation.

Nancy Taras has chosen his book, "Mountains of the Heart," as the January discussion book for the Cabin Fever Book Club at the Center.Weidensaul's topic is "Living on the Wind - the World of Migrating Birds.""This man is a conservationist," said Kunkle.Weidensaul said it was a pleasure to be at the Center. "You guys are doing amazing work."Members of the Center are not the only ones he considers amazing. Several times throughout the evening he said, "Birds are amazing."He began by saying, "As we sit here birds are going overhead. Get away from the noise and you hear them."Weidensaul had the opportunity to sit inside a radar van and could "see" the thousands of birds overhead - "five or six going across at any moment up to 6,000 feet above us."That's 1.75 million an hour and the peak had passed at the end of November when he had been watching on radar. In early January golden eagles and goshawks are still leaving, a constant flow up and down thousands of feet, he said.People think of migrants as going only north and south but some travel 10s of thousands of miles a year drawn to one spot where there is a seasonal supply of food.Just after dark there are so many on the radar it looks like a fighter scramble in World War II, but it is birds.People have only begun to understand migration in the last 100 years. In the mid '90s the migrations were followed from Alaska to the tip of South America.The blackpoll warbler, weighing one-half ounce, leaves Alaska, crosses to Nova Scotia then to New Jersey where it waits for a wind. Traveling on the northwind, it flies non-stop to the Amazon basin over the Atlantic.It was believed the Arctic tern flew 25,000 miles on its migration, but now that there are ways to measure the distance, it is 47,000 miles per year.Alaska is a phenomenal place to find out about migration - the birds fan out over three-fourths of the world.The bar-tailed godwit, a shorebird, feeds in Bristol Bay, Alaska, on marsh invertebrates, binge feeding to more than double their weight. One-half is body fat when they are ready to move. As the gut empties and shrivels the muscles develop, After its 7,200 mile flight of seven to nine days nonstop across the Pacific Ocean the gut regrows preparing for food. When returning, it goes north to the Yellow Sea, 5,000 miles, and an additional 2,500 miles across to Alaska.There are 800 species of birds in the United States but not all are migratory.The ruby-throated hummingbird goes only to southern Mexico. There are partial migrators such as the bald eagle. Some migrate and some just move south a short distance. Great horned owls are entirely nonmigratory.Eruptive migrants such as finches and owls live in the north and stay summer and winter as long as there is a food supply. Last year there was plenty of food, said Weidensaul, but this year there is less so they will move south looking for food.One hundred twenty species can be found in Cape Churchill, known for its polar bears. Some of the species are not seen anywhere else. All but 20 species leave in winter.Migration is dangerous. Millions of birds die each time they migrate. There are insects in the southern United States. They don't have to go to South America, but many are tropical birds that evolved in the southern hemisphere. After the ice age they moved northward and follow the path back to the homelands during migration.Weidensaul is asked how they know where they are going. He said ducks, geese, swans and cranes learn specific routes by following older birds.The Swainson's thrush does not say, "'I think I'll go.' It is compelled to fly. The trigger is daylight. The photo period gives an overriding command to fly. There may be some exploratory flights, but one night they blast off.The house finch can see the magnetic field and use it for navigation even though we cannot see it."Birds are amazing things," said Weidensaul.