I didn't see any responses to this but thought it was interesting since I've sensed the same thing without thinking it out loud. A too-quick google brings up a few pointers that imply it's well known that bird behaviour changes with reducing pressure, though not so much agreement on how it changes. Most references (which might all be circular to a single originating misinformation) suggest they feed more at times of lowering pressure, which might equate to going quiet but I don't think so. Probably birds could have two different responses, one to feed more early on, then another to hide and sit tight (and go quiet) as storm comes closer. I’ve always thought that immediately after storms there is a surge in activity, a good time to look for bush birds.
Everybody agrees that birds can sense pressure changes with exquisite sensitivity, to around 10m altitude change, another amazing fact. Has Ian whimsied on this?
Journal Of Comparative Physiology A Neuroethology Sensory Neural And Behavioral Physiology (1974) Abstract
Homing pigeons were tested for their ability to detect air pressure changes in an otherwise constant environment chamber. Ten of 12 birds tested did respond to the pressure changes. The 50% threshold of detection was 10 mm H2O or less, which is approximately equivalent to a change in altitude of 10 m or less. Performance was better in a chamber with artificial background noise than in an abnormally quiet chamber
http://www.racerocks.com/racerock/abiotic/barometric/barometric.htm
However, it is to be noted that most birds possess a sensory organ that does respond to changing pressure. An organ in the middle ear called the Vitali organ or the Paratympanic Organ detects pressure changes, warning them to take cover before a storm arrives. Birds will also interrupt migration when there is a hurricane approaching. It is also obvious that changes in wind patterns, so closely tied to changing barometric pressure, influence foraging patterns, social interactions and timing of activity… Research elsewhere has shown that Bats are the only mammals that respond to pressure changes, because they also have a Vitali organ.
Below article mentions in passing that lower air pressure means more insects which would align to the feeding idea for birds. It also says that bats alter metabolic rate in response to air pressure changes, so maybe birds do similarly.
"BAT BAROMETERS SIGNAL MEAL TIME"
By Sean Henahan, Access Excellence
"CHAMPAIGN, Ill. - Bats appear to use a built in barometer to determine the chances of a good nights bug hunting, according to a study by Ken Paige of the University of Illinois Institute for Environmental Studies.
Paige spent nearly a year monitoring the dining habits of a colony of eastern pipistrelles, a cave-dwelling bat commonly seen in western Illinois. He found that the bats emerged in larger numbers when barometric pressures dropped and insects were numerous. He noted that the number of insects declined dramatically and that fewer bats left their roost when air pressure increased. Indeed, barometric pressure alone explained 87 percent of the variation in bat activity, he noted:
"My research strongly suggests that the eastern pipistrelle uses barometric pressure as a cue for predicting the relative abundance of aerial insect prey outside their roost. Barometric pressure is the only physical environmental cue available to a bat roosting deep within a cave. All other variables such as light, temperature, relative humidity and wind currents are virtually constant. When air pressure is low, aerial insects are abundant, and bats respond by leaving the roost to forage."
The bats built-in barometer is likely to be its Vitali organ, a middle-ear receptor that is thought to help birds measure air pressure. Bats are the only mammals to have such a sensory organ. Paige's bat-watching also showed that bats track barometric pressure metabolically. When the barometer fell, the bats slowed down their metabolisms, allowing them to conserve energy. By doing so, they can delay or eliminate the need for entering torpor, a sluggish state of dormancy that makes them susceptible to predators. In addition, the bats' tracking of metabolic pressure may function as a bet-hedging strategy, he said
"When pressure is low, insects are most abundant, except during heavy rain," he said. "Because it is unlikely that bats can detect the rain from deep inside a cave, they have to fly out of the roost to check. This is no problem, however, because at low pressure they already are running on an economy setting and will waste minimal energy if they find their trip to the outside is a waste of time."
Because they already are conserving energy, he added, they can extend the benefit of their previous meal. Thus barometric-pressure tracking can be viewed as an alternative evolutionary strategy to torpor, he explained.
This results of this research project appeared in the June issue of the British journal Functional Ecology.".
Julian
> -----Original Message-----
> From: David McDonald (personal) [
> Sent: 04 December 2011 21:16
> To: CanberraBirds
> Subject: [canberrabirds] Birds' activity levels and air pressure
>
> Greetings. Over the years I have often though that, when the air pressure falls -
> - a storm is on the way -- the bush birds go quiet. Less calling, less movement.
>
> What are your observations, thoughts and knowledge about this?
> - Have you observed that they go quiet when the glass is falling?
> - If so, do you have a hypothesis as to why this occurs?
> - Do you know of any scientific literature on the topic?
> - Or am I on a completely wrong track?
>
> Thanks - David
>
> --
> David McDonald
> 1004 Norton Road
> Wamboin NSW 2620
> Australia
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