There's a hole, there's a hole...

The photo above shows a small section of our yard.  The grass is starting to become green and grow, but what is really interesting to me are the hundreds of holes spaced every few inches throughout the entire lawn.

Here's a closer look.

So what's making these holes?

The holes are being made by birds, especially European Starlings (Sturnus vulgaris). 

Why are the birds doing this?  And how?

They are foraging for worms, grubs, and other invertebrates that are living in the soil.  They made the holes by pushing their beaks into the ground and then opening them slightly.  There are so many holes because the starlings will probe with their beak, take a few steps and then probe again.  A small flock of birds can quickly aerate an entire lawn.