I can usually tell what sort of weather conditions are coming using my “bird barometer”, which consists primarily of monitoring the intensity with which birds are feeding at the various feeders scattered around the backyard. About 24 hours before a big thunderstorm or a dreary day with steady rainfall, those feeders are humming with activities of multiple individuals of a variety of species. For example, yesterday was cloudy but mild, but dozens of finches, sparrows, and chickadees crowded in or around the feeders for several hours in the morning.
These ground-feeding birds were reaping the benefits of all the spilled seed from the multiple gangs of finches, chickadees and nuthatches vigorously attacking the feeder above them.
Today (just 24 hours after the feeding frenzy) is rainy and cold, and the feeders are quiet. An occasional Blue Jay visits, but even the Chickadees are absent. How do birds predict such changes in the weather? Do they really have an internal barometer?
In fact, they do. A group of hair cells in the Vitali Organ, or Paratympanic Organ (named for Giovanni Vitali who discovered them about 100 years ago) in the middle ears of birds is thought to be responsible for detecting changes in atmospheric pressure of just 10-20 mm of water (about 0.75-1.5 mm of mercury). Such exquisite sensitivity allows birds to maintain their elevation while flying long distances within 10-20 feet of desired altitude — essential for navigation, but it also allows birds to sense changes in barometric pressure at ground level, especially the declining barometric pressures that signal approaching storm fronts. Input from these specialized cells due to barometric pressure changes then causes abrupt and marked changes in bird behavior, as they prepare for and protect themselves from changes in the weather.
Are birds the only animals with this unique ability? Hardly. Most animals can sense impending changes in the weather, using a variety of other cues (infrasound, daylight, smell (ozone?), static electricity, etc.). But only one group of mammals may possess the type of middle ear barometric pressure detectors present in birds. Can you guess what group that might be?
An interesting reference on this subject can be found here: Von Bertheld and Gianessi, 2011. The Paratympanic Organ: A barometer and altimeter in the middle ear of birds? Journal of Experimental Zoology 316 (6): 402-408.