It's Raining Frogs and Fish
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Drop a frog off a building, and unless it's extremely small, it goes splat; so undoubtedly, what people think they're seeing in these stories can't be what's actually going on. If the frog didn't come from the sky, could it have come from somewhere else?
Frogs do swarm naturally on occasion. It happens frequently enough that people start to correlate these events with other things that happened: Storms, earthquakes, celebrity deaths, what have you. It's been reported that frog swarms were correlated with both the 1989 Loma Prieta earthquake in California, and the 2008 Sichuan earthquake in China. Shortly thereafter, when frogs swarmed in Bakersfield, California, some called for earthquake preparedness. Needless to say, there was no earthquake; just a random population explosion of frogs. Sometimes these explosions can be dramatic. In 2004, four hurricanes hit Florida, making that state about the wettest it's ever been. The local species of frogs and toads all had a banner year, described by the Florida Museum of Natural History as a carpet.
Every spring and fall, frogs migrate between shallow breeding ponds and deeper lakes. Because they're amphibians and need to keep their skin moist, they migrate most often during rainstorms. In many cases, the day with the right conditions will come, and the whole frog population will move cross-country en masse, across roads, across properties, wherever it needs to go. When you look outside during a heavy rainstorm and you see thousands of frogs jumping everywhere all over the ground, the illusion that they're falling from the sky and bouncing can be quite convincing. A swarm of frogs looks like ping pong balls bouncing in a lottery machine. The fact that there usually aren't frogs here adds credibility to the illusion. Throw in a healthy dose of confirmation bias and some exaggerated second, third, and fourth hand reports, and you automatically end up with every imaginable detail like the frogs were choking rain gutters on top of buildings.
Although this explanation might satisfy the stories of frogs falling from the sky, what about fish? You don't find mass migrations of fish crossing overland, do you? Well, maybe not mass migrations, but believe it or not, there are fish species that occasionally take to the ground in search of better waters. There are many species of "walking fish" in the world. Mudskippers are probably the best known variety. In Florida in 2008, a school of about 30 walking catfish emerged from the sewer during a heavy storm and went slithering around on the street. The northern snakehead is another fish that can wriggle its way around on dry land. Throughout Africa and Asia are 36 species of climbing perches. They have a special organ that allows them to breathe air, and are able to walk using their gill plates, fins and tail, despite looking completely fishlike with no obvious ambulatory limbs. None of these fish move gracefully or even look like they have the ability. To the average witness, it's a live fish flopping around on the ground where no fish has any business being, and having fallen from the sky seems as good an explanation as any.
More:
http://skeptoid.com/episodes/4170