Temperatures Inside the Chimney


What's the big attraction?  Why do ten thousand small swifts seem compelled to stuff themselves in an old brick chimney for the evening?  Here are some ideas.

1.  If the chimney has a small enough opening, predators will not be able to "get" them. Vaux's Happening has documented a Cooper's Hawk going a short distance down the Monroe Wagner chimney attempting to snag a swift. Our security cameras have captured this event at least twice. A Barn Owl was seen flying into and catching swifts at the biggest holed chimney of the three Mcnear Brick and Block roosts. But once they get in the chimney and down a ways, as far as we know, the swifts don't need to worry about predators.

2.  The joints between common bricks have just the right spacing, (2.25 inches) for the 4.5 inch long swifts to cling to while overlapping the swift hanging onto the next joint up.

3. Inside the chimney they will be out of the wind.

4. Inside the chimney they will be out of the rain. This is not a sure thing. The swifts will prefer the chimney wall that faces the direction the wind is blowing.  The bird above them may also catch some precipitation headed their way. If the chimney has a taper, that may make it more difficult to cling to, but all walls could be out of the rain if there's no wind.

So inside the chimney the Vaux's Swifts are safe and dry and...........WARM?

5. The birds can share their body heat. The more swifts the better if it's heat you want. Do the birds snuggle together?  Our inside the Monroe Wagner chimney camera has confirmed that they do. They also might make a pile, swifts clinging to swifts that are clinging to swifts clinging to bricks.

But might the evening air be warmer inside a brick roost site, even with no birds, than it is outside? In April of 2012 Vaux's Happening committed to answering that question.

We briefly considered sticking a $10 residential outdoor thermometer down inside the Monroe Wagner chimney where our inside camera could see it, but we went with a more sophisticated option, LogTag electronic temperature recorders. Vaux's Happening ordered three of these fat credit card sized units along with a computer linked docking station. Their performance has been flawless, recording the temperature to the nearest 1/10 of a ℉ every 15 minutes. After nearly 11 months and four deployments they are still functional and blinking that they are ready for more.  Here's their website.

We hung one encased logger one meter down and another four meters down the chimney.  The white blob is the back of the inside the chimney security cam.

April 10 we got up on the Wagner roof and climbed up our 40 foot ladder.

 We located the third logger outside, two meters up the west chimney wall.  We retrieved the units 29 days later and downloaded the measurements.  This graph shows temperatures recorded on April 13-14.    ➜

For this 24 hour period the outside temperature ranged from 32 to 92℉, a difference of 60 degrees. Inside the low was 51 and the warmest recorded temperature was 64, just a 13 degree range. The high and uneven outside recorded temperatures are probably the result of direct sunlight falling on the logger. The greatest temperature difference between outside and inside was 26 degrees in the middle of the night. Stone, concrete, and brick are materials with a high thermal mass. They absorb the sun's energy (heat) during the day and then slowly release it after the sun goes down and the outside air cools. Twenty six degrees is a lot.  I was expecting maybe five. Inside that chimney is a great place for a swift, or ten thousand swifts, to spend the night.

It seems there is little difference in being inside at four or just one meter down, no more than two degrees.  Here is how a few thousand swifts affect that. It’s easy to see what time the flock came to roost.   ➜

Rain really changes things. Looks like the swifts went to bed early.  ➜

Is there a relationship between how many swifts are in the chimney and how warm it is inside? One would expect so, but do our measurements confirm that? Kind of. The problem is that the heat is where the swifts are parked and that might not be too close to a temperature recorder.The chart below compares how much warmer it was five meters down the Wagner chimney than outside for six different evenings with increasing numbers of swifts. The temps are averages from the times the swifts were in the chimney.  The chart shows a less than perfect relationship, but it certainly suggests it.   ➜

Apparently the farther down the chimney you go the warmer it gets.  Perhaps .7 ℉ for each meter below one meter. So as the evening progresses and the air temperature drops the swifts move down the chimney.  On the cool nights they pile up. This is an image taken by our IR cam at 6:00AM, 9/10/12. There are about 5,000 Vaux's swifts in that pile.  ➜

April 10 we got up on the Wagner roof and climbed up our 40 foot ladder.

Two hour later things are warming up outside. The swifts have unpiled, spread out, and are heading for the exit.  It will soon be time for breakfast.  ➜

All chimneys are not created equal.  So far (Feb 2013) we've only put our loggers into three chimneys.  Besides Wagner we have measurements from the Rio Lindo CA roost site during the 2013 southbound migration, and we recorded 30 days of swiftless early winter temperatures from the Monroe School District Offices alternate roost site. This chart compares these two sites with the Wagner site, broken down into three separate divisions. One is all 35 of the days we obtained temperatures five meters down measurements in 2012. A second is just those evenings that only had less than 200 swifts. The third averages the nine evenings with over 2500 birds. ➜

Study the little bits of colorful spread sheet below and it should make more sense to you.

The swifts might actually be warmer than the inside air temperatures. They could be pressed against a brick that's slowly releasing heat and totally surrounded by hundreds of their warm blooded friends. The biggest difference in temperatures for the Wagner northbound migration was over 41 degrees on 5/11/12 when 5000 swifts got the inside temperature up over 80 degrees for most of the evening.