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This radar image from the National Weather Service's Key West facility shows a massive flock of migratory birds is seen moving north earlier this week. The green/yellow objects are biological objects flying north over the Keys. The darker blue objects indicate rain. 
 Radar Detects Massive 'Fowl' Weather System Over Florida Keys (Video)
​Our sportfishing friends like to call a particularly well tuned radar “bird radar.” Finding birds is important because birds in the air often means fish below. Recently the Florida Keys saw this phenomenon in an extreme form thanks to government weather radar Here’s what the Tampa Bay Times reported over the weekend:
A natural phenomenon that usually goes unseen and unnoticed was seen and noticed over Key West on Monday: the massive nocturnal migration of birds was captured by the National Weather Service’s radar stationed there.

The field of birds was first detected around midnight and stretched over the island city, according to the Weather Service. Meteorologists said the radius of the flock measured at least 90 miles out from the center — but the actual size of the migration could have been much bigger.
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The migration is typical this time of year as more than 118 species of birds start returning to North America after wintering in Central and South America or the Caribbean.
This animation from the National Weather Service shows the migration of 118 species of terrestrial bird populations in the Western Hemisphere. Each dot represents the estimated location of the center of each species’ population for each day of the year. These estimations come from millions of observations from the eBird citizen-science database. eBird is a real-time, online checklist program, launched in 2002 by the Cornell Lab of Ornithology and National Audubon Society, that allows birdwatchers to enter their observations.

The colors in this dataset represent the average daily air temperature at the surface, calculated from observations from 1981-2010. The yellow and red colors represent warmer air, while blues show areas below freezing.

A recent study that explored these trajectories, published in the Proceedings of the Royal Society B, found broad similarities in the routes used by specific groups of species. A key finding from the study is that bird species that head out over the Atlantic Ocean during autumn migration to spend winter in the Caribbean and South America follow a clockwise looped trajectory and take a path farther inland on their return journey in the spring.

Species that follow this broad pattern include Bobolinks, Yellow and Black-billed cuckoos, Connecticut and Cape May warblers, Bicknell’s Thrush, and shorebirds, such as the American Golden Plover. These looped pathways help the birds take advantage of more favorable winds during spring and autumn migration. For species that do not fly over the open ocean, the study finds that many use the same migration routes in the spring and fall.
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Geographic features shaping this pattern include mountain chains or isthmuses that funnel migrants along narrow routes. Additional details on the 118 species can be found here.

Notable Features
  • Each dot represents the estimated location of the center of each species’ population for each day of the year for 118 terrestrial bird species in the Western Hemisphere.
  • The bird data comes from a citizen science project called eBird, launched by Cornell Lab of Ornithology.
  • The air temperature colored data represent the average daily air temperature at the surface, calculated from observations from 1981-2010.
  • Seasonal migration patterns, typically leaving in the fall and returning is the spring, are distinctly noticeable.