Flock Flying Tires Pigeons

Ben -   A new analysis of the way that pigeons flock suggests that it actually costs them energy to do so.  It's actually more efficient for them to fly on their own.  To find out what this could tell us about bird behaviour I spoke to Dr. Jim Usherwood from the Royal Veterinary College...

Jim -   Birds fly around in flocks quite a lot.  That's something we know about birds.  They like to be together.  The question has been there for awhile - what are they getting from it?  They could easily not fly around in flocks. In the background of bird world we think of lots of rather large birds flying around in Vs.  Things like pelicans and geese fly around in nicely ordered Vs.  Previous work has shown some kind of benefit from flying in the V.  You flap at a lower rate and your heart rate goes down.  Indeed, if you fly airplanes in a nicely ordered V they reduce fuel consumption.  So, you can fly in such a way that you save energy.  Now most birds don't fly around in well structured Vs.  Pigeons are a good example of a normal flock termed a 'cluster flock'. Why might they be flying around as a group?  We simply didn't know whether they're getting any kind of aerodynamic or energetic benefit from it.  What we went about doing is putting, in effect, a Satnav and some internal sensors on every pigeon.  A lot of the same kit as you get on an iPhone on every pigeon of the flock. We then got them to fly around we worked out that these birds are probably getting some kind of energetic cost from flying close together. This leads the biologists in us to look for why on earth would you fly in a flock? We need to look for different answers to that now.

Ben -   So what were the conditions under which they were flying?  Could they have been flocking in response to something that they saw in the environment?

Jim -   That's the beauty of these new sensors now, you can leave them on animals. They can wake up when the animals are doing something, and you can let the animals roam free, doing exactly what they would normally be doing.  These were put on my flock of racing pigeons which were allowed to do whatever they wanted for 3 days.  They're flying around sometimes at 6 o'clock in the morning when nobody was around here.  They take off, fly around in flocks for 5, 10 minutes, tens of thousands of flaps, pulling 2G as they go around in circles. They are then  sitting down and having a bit more breakfast.  They're doing this quite spontaneously.  It's very much like a wild flock would do.  They are in free flight, but we can take the effects of going up and accelerating, and going around corners into account because we've got so many flaps.

Ben -   So how many pigeons were you following and how many wing beats did you actually manage to capture?

Jim -   We wired-up up to 20.  Not all 20 flew all the time.  We're talking about a quarter million of flaps, 400 pigeon kilometres, that we measured. This is just so awesome.  It's the sort of data volumes that you would just dream of.  If you were trying to do this in a wind tunnel just imagine trying to do that.

Ben -   The other question of course is, for all of these pigeons, they have a bit of technology strapped to their back that they wouldn't normally have.  Is it possible that the way that you're measuring this is actually having an impact?  Is that leading to the fact that they need to fly with greater energy?

Jim -   Anything you add to an animal is likely to influence it a bit, especially when it comes to aerodynamics, little bumps sticking up could do untold things with drag.  Having said that, they were flying voluntarily and watching from a ground point of view, they were flying around in a flock like they always do.  So we've got no particular reason to think they're doing something completely out of the ordinary.  Also whatever the effect of having a logger on is presumably the same for all of them.  I don't say it's a huge issue but of course, there's always a call for lighter, smoother, better loggers.

Ben -   Why are you looking at pigeons in particular when we have other birds that have this glorious V formation or those incredible flocks of starlings that seem almost liquid in the air?  Why pigeons?

Jim -   Pigeons are pretty useful in that they represent typical flocks.  They also have a great advantage that they can carry a fair old payload.  These pigeons are quite happy to fly carrying a payload of 30 grams and they come back. So we can just walk up to them in the shed afterwards, pull out an SD card and get a gigabyte of data off them. That makes things a lot easier.

Ben -   And what impact did flying in a flock actually have compared to a bird just flying free on its own away from other birds?

Jim -   It's actually quite a large difference.  If you think of a pigeon flapping along at 8 hertz then as it gets into a flock you've got to 8.1-ish hertz.  That doesn't sound much, but if you compare that with how much it changes to flying uphill.  If it's flying uphill, it's 4 metres a second.  That's a similar kind of change in flap frequency.

Ben -   What advantages do you think there may be in flying in a flock?  We were saying there's strength in numbers, but then if that strength is counteracted by the fact that you need to put more energy into flight, then it seems that it would be something that would be selected against.

Jim -   Yes, so we've got this slight issue that they're presumably primarily flying in order to take exercise here because they're not going anywhere in these flights.  They're getting up from their loft, they stream around, and then they're going back for more breakfast.  So, is it necessarily a bad thing that they flap a bit harder?  Then the prime thing that people will always think of when considering flocks is some kind of a protection from attack.  The evidence for that is fairly strong in that when there's a sparrow hawk or something like that around, they tend to bunch in a much greater flock.  So, there's probably some advantage of being in a tight flock in terms of being difficult to catch.

Ben -   Is this information only really interesting to biologists?  Is it only useful for people studying birds or studying flocks?

Jim -   Well of course, I'm coming at it from the biologist point of view. There's more and more interest in these autonomous or unmanned air vehicles drones. Flocks of them are becoming more and more useful now. They're always interested in making things more efficient.  If you can get a little bit more out of your drone then that's very useful. This would point to not flying your drones around as a flock of pigeons, but keeping in a goose-like structure if you can.

Ben -   Jim Usherwood from the Structure and Motion Lab at the Royal Veterinary College in Hertfordshire. You can read about that work in the journal Nature this week.