It is necessity of all life to feed, and so it is in ecosystm.  Flocks must, from time to time, feed.  To do so, they must feed on other flocks.  However, there are only certain flocks they can feed upon, and this is based on the individual flocks' relative performance. 

When a flock is fed upon, the individual birds become targets for consumption.  When you feed on a bird, the bird is effectively "sold" and the current value of the bird (its gene sequence) is added to the CASH resources. 

The poor bird is of course, no longer with us.  A flock can fed upon until there is only a single bird left.

It is quite clear which flocks you can feed upon, as they will appear to be as insects to you.  Bear in mind that flocks in the distance may seem smaller, and that tiny flocks can be passed right by if you are flying too fast.  The comparison is most easily made from a top or bottom viewpoint (see NAVIGATION) but with a little practice you should be able to distinguish from any angle.

When you need to feed (and you find some yummy buzzing insects you can eat)  press the keyboard "F" key to switch to feeding behavior.  Swoop in and feast!
 

 Each bird's physical size is a measure of its performance.  Birds can be very large or very small.  Any flock that is 5 times smaller than your flock can be fed upon.  Any flock that is 5 times larger than you, can feed upon you. 

When you create your flock, or when two flocks breed to produce offspring,  the flock has an initial performance value of 1.  If the total value of the flock increases (by summing the current values of the 32 unit gene sequence and multiplying by the number of birds in the flock), the performance value increases and your individual birds grow in size.  Please note that in ecosystm.1 (the currency ecosystem), the population of the flock grew, not the size of the individual birds.   In ecosystm.2 the population grows only under certain circumstances (see POPULATION).

Be warned!  As your performance value decreases, your potential to be food for another flock increases.