A record number of sandhill cranes gathered in Nebraska in mid-March during their annual spring migration. Matt Urbanski / Crane TrustHundreds of thousands of sandhill cranes are once again flocking to central Nebraska during their annual migrationand so far, the leggy birds appear to be unaffected by the highly contagious bird flu strain H5N1.Wildlife biologists in Nebraska remain concerned about the possibility of the deadly virus spreading through the huge numbers of the gray birds with red caps thatcongregate on the Platte River every spring.But, as they reach the likely peak of their migration this year, the birds seem to be happy and healthy. No crane deaths have been reported in Nebraska so far, reports NTV News Jack Bartlett.Bird flu aside, Nebraska biologists have another reason to celebrate: During their weekly aerial crane survey on March 17, the fifth week of the migration, they counted roughly 736,000 sandhill cranes between the towns of Chapman and Overton, Nebraska, which are roughly 80 miles apart. Thats an all-time, record-high number of cranes counted in the region at onceand its probably an underestimate. The lanky birds, which stand betwen roughly three and four feet tall, are known for their loud, rattling bugle calls. Kylee Warren / Crane TrustSandhill cranes have shown us that theyve been super resilient to lots of different things that weve thrown at them, says Bethany Ostrom, a wildlife biologist with the nonprofit Crane Trust who conducts the aerial counts, to Nebraska Public Medias Arthur Jones and Macy Byars.The number dropped to 705,000 on March 25, during the sixth week of migration, but thats to be expected as the birds start to head north for breeding. Biologists expect the numbers to continue decreasing as the cranes take advantage of favorable wind conditions.5 Record-Breaking Bird MigrationsWatch on Earlier this year, at least 1,500 sandhill cranes were found dead in Indiana from H5N1, raising concerns about the health of migrating flocks. But those birds were part of the eastern population of sandhill cranes, which is separate from the mid-continental group that migrates through Nebraska every year, according to Nebraska Public Media. Experts say the two groups rarely cross paths.Still, the bird deaths in Indiana had Nebraska biologists on high alert. The virus can be spread by migrating waterfowl, and sandhill cranes passing through central Nebraska often share their Platte River habitat with geese, swans, ducks and more than 20 other bird species.In addition, since so many sandhill cranes gather in Nebraska, the highly contagious virus could quickly and easily affect large numbers of the birds. An estimated 80 percent of all sandhill cranes in North America flock to a 75-mile stretch of the Platte River during their annual journey to their northern breeding grounds, according to the Nature Conservancy. Sandhill cranes spend their days eating leftover field corn before retreating to the safety of the braided Platte River at night. Kylee Warren / Crane TrustThe migration takes place from February through April, with individual birds staggering their arrivals and departures. Each bird typically spends about a month in central Nebraska, fattening up on kernels of corn left behind during the last falls harvest. Then, they take to the skies and continue north to Alaska, Canada and eastern Siberia.There is nothing else like it in the world, says Marcos Stoltzfus, director of the Iain Nicolson Audubon Center at Rowe Sanctuary in Gibbon, Nebraska, to News Channel Nebraskas Peter Rice.The birds have been flying through central Nebraska for at least nine million yearslong before the Platte River formed some 10,000 years ago, according to the Nebraska Game and Parks Commission. But, today, the cranes relish the relative safety of the braided river, which helps protect them from predators when they roost each evening. Individual sandhill cranes spend roughly a month fueling up in central Nebraska. Kylee Warren / Crane TrustWhat makes the central Platte River valley attractive to sandhill cranes is the river that we help manage, says Matt Urbanski, a spokesman for the Crane Trust, to KSNBs Madison Smith. We will make sure that theres not a ton of vegetation choking the river out. Well make sure that it can widen, so the sandhill cranes have six to eight inches of water to sit in during the nighttime.With their lanky legs and slender necks, sandhill cranes stand roughly three to four feet tall. Their bodies are covered in grayish-brown feathers, except for a patch of crimson on their heads. Sandhill cranes make a distinctive call that ornithologists describe as a loud, rattling bugle. Sandhill cranes have been flying through Nebraska for at least nine million yearslong before the formation of the Platte River roughly 10,000 years ago. Kylee Warren / Crane TrustYou have the roar of thousands and thousands of them all taking off or coming in to land, says Dusty Barner, who offers sandhill crane viewing tours through his company Dusty Trails, to KNOPs Tristen Winder.Endangered whooping cranes also migrate through central Nebraska, and an H5N1 outbreak could be potentially devastating to the species. Only an estimated 800 whooping cranes remain in existence, a number that includes birds living in the wild and those in captivity.So far this spring, at least two whooping cranes have been spotted among their sandhill cousins in central Nebraska, reports the Omaha World-Heralds Marjie Ducey.Its always a special moment to catch them along with the beautiful skies of Nebraska in spring, Brice Krohn, CEO and president of the Crane Trust, tells the publication.Get the latest stories in your inbox every weekday.Filed Under: Animals, Biology, Birds, Disease, Disease and Illnesses, Endangered Species, Land Birds, Microbes, Bacteria, Viruses, Nature, Outdoor Travel, Tourism, Travel, Viruses, Water, Wildlife, Zoology