I’ve noticed that the frenzied feeding activity at bird feeders (see yesterday’s post on “finch feeding frenzy“) usually coincides with a precipitous drop in temperature the next day, but I’ve wondered what enables birds to predict that occurrence.
Sure enough, the day after I photographed the finch feeding frenzy, the overnight temperature dropped from 20 F above zero to -10 F below. Moderate winds added to the chill making the effective temperature for us humans about -30 F! No birds visited this feeder the next morning until later in the afternoon when the sun had warmed the air up to just about zero degrees F.
Cardinals, Blue Jays, Nuthatches, Chickadees, Slate-colored Juncos, House Finches, and Goldfinches all made an appearance at the bird feeder during the morning hours the day before the frigid temperature drop.
Mr. Cardinal didn’t stay long — the finch mob must have scared him off.
Nuthatches zoom into the feeder, pick out a peanut treat, and zoom off, while the finches just sit and gobble up sunflower seeds.
But it’s not the temperature drop the birds are predicting, it’s the drop in barometric pressure. Low barometric pressure in one area means there will be air movement from a higher pressure area, and in the winter, that usually means Minnesota will receive a big blast of frigid Canadian air.
And the data bear this out: barometric pressure reached a low point at noon on the day of frenzied finch feeding and increased almost 30 mm by midnight the next day, bringing with it bright sunny weather but a 30 degree drop in air temperature.
It seems that birds are the only land vertebrates (with a couple of exceptions) that possess a paratympanic (i.e., next to the ear drum in the middle ear) barometric sense organ, and it is derived from the same hair cells in sharks and their relatives that provide those fish with information about their jaw movement relative to the prey they intend to gobble up.
Chickadees and Nuthatches go for the high energy peanut treats in preparation for a blast of cold weather.
So, is a 30 mm change in barometric pressure (a little over 1 inch) enough to trigger such a feeding response in my backyard finches? Yes, it is. Experimental data on White-throated Sparrows showed that they could detect a pressure change of as little as 10 mm: when barometric pressure was decreased artificially in a chamber holding migratory sparrows, they immediately began feeding when the lights were turned on; when the pressure was higher (and normal), they became active, preening and hopping around in the chamber, but not feeding.
And why is it just the birds that have evolved this magnificently sensitive sensor? In addition to predicting weather fronts, the barometric sensor is most useful for maintaining level flight at a particular altitude during migration.
What about mammals, especially humans? Can we detect changes in barometric pressure?
Gray Squirrels were out in numbers looking for seed scattered in the snow by the finch mob. Did they know what was coming the next day?
Only one species of bat possesses a physical sense organ that detects barometric pressure, but mammals, including humans, can sense changes in pressure in their ears, sinuses, sometimes joints, but there is no specific receptor for the sensation. Rather it is changes in pressure within a confined cavity that elicits the sensation, and not in everyone.