# How do birds avoid mid-air collision?

Across from me there is a large flick of Pigeons and Ravens that like to nest on the roof. They swoop and dive and fly all over. Sometimes the whole flock lifts off at once. My question is, how do they avoid running into each other? Certainly they can't see every bird around them, can they?

• I think, the reason would be the same as, when flocks of birds like starlings, etc. do not run into each other while making patterns. They can un-group and regroup with complete harmony. So, maybe the concept behaves the same with different types of birds as well. Or, maybe they have some kind of natural radio wave signaling due to which they are capable of preventing flying into each other. – MPG Aug 25 '15 at 21:57

They can't see all the birds that are around them as some are hidden behind the others. However, I don't think there is anything so extraordinary; they watch around, have good reflexes and can change direction very abruptly. There is no need for some kind of special sensing abilities to explain bird flocks.

3 simple rules

One can simulate a flock of birds with 3 simples rules:

• Adapt your speed on the speed of the other agents

$$\overrightarrow v_{i,1} = \frac{1}{K_L}\sum_j^K\overrightarrow v_j (t) - \overrightarrow v_i(t)$$

, where $K_L$ is the number of birds in the Large area of perception, $\overrightarrow v_j (t)$ is the speed of a given bird in the large area of perception and $\overrightarrow v_i (t)$ is the speed of the focal individual.

• Attraction

$$\overrightarrow v_{i,2} = \frac{1}{K_L}\sum_j^K\overrightarrow p_j (t) - \overrightarrow p_i(t)$$

, where $\overrightarrow p_i(t)$ and $\overrightarrow p_j(t)$ are the positions of the focal bird and of a bird present in the large area of perceptio, respectively.

• Repulsion

$$\overrightarrow v_{i,3} = -\frac{1}{K_s}\sum_j^K\overrightarrow p_j (t) - \overrightarrow p_i(t)$$

, where $K_s$ is the number of birds present in the Small area of perception.

Then, to update the speed of the birds you just sum up those 3 contributions:

$$\overrightarrow v_i(t+1) = \overrightarrow v_i(t) + \overrightarrow v_{i,1} + \overrightarrow v_{i,2} + \overrightarrow v_{i,3}$$

My point in descrbing this algorithm is that you can get the emergent behaviour of a bird flock out of very simple rules.

Bird Collision

If birds don't often collide each others (what I assume but honestly I don't know much), note that it is estimated that 100 millions birds that die every year after colliding a window (see here). Birds also collide with power lines (ref) and wind turbines (ref).

When they are flying together as a flock, they anticipate the flock's behavior and react accordingly. earthsky.org/earth/how-do-flocking-birds-move-in-unison

And when they are just flying around, but not in a flock, the concept is the same as how we humans don't collide into each other when we are walking. Although birds fly faster, they also see farther (eagles and other birds of prey can see four to five times farther than the average human can), which means that they have a longer distance to react.

• But how about when they fly together? They're really close as they fly. – SolarLunix Aug 27 '15 at 12:27
• When they are flying together as a flock, they anticipate the flock's behavior and react accordingly. earthsky.org/earth/how-do-flocking-birds-move-in-unison – Yvonne Liew Aug 28 '15 at 1:42
• @YvonneLiew I think the flying together part answers the question better, you should edit your answer to include that citation. – March Ho Sep 1 '15 at 5:07

Schooling and flocking behavior is actually very interesting stuff. The real answer- many of the things mentioned already (speed of processing, good reflexes) but there's still much to be found out. Computer analysis is helping enormously, and also helping to create robot flocks. Here's an article of scientists finding out more. Unfortunately, that article's from 2009, but I also read about it in "Complexity: a guided tour". Anyone have something more updated?

some neat flocking robots- there are many other similar projects lately

Sitting in stadium full of thousands of people looking around we notice that just about everyone is in our line of sight. Flocks of birds also always seem to follow a pattern involving maintaining this global line of sight. Even as the flock is flying straight ahead, changing direction, taking off or landing the the individual birds seem to move and adjust the pattern to maintain this line of sight to the maximum possible. The seem to follow this logic even while on the ground pecking grain.

• Can you add some references to this claim? – another 'Homo sapien' Oct 2 '16 at 9:05