Many years ago, when I lived in Tennessee, I used to have a pet 3 foot long black rat snake. On one day with sunny weather, I carried it out of its terrarium and took it out for a walk to a little park across the street from my house. I figured an occasional exposure to nature would be good for "Rufus X". (Why do we name our pets, anyway? they don't recognize the concept, certainly not the cultural references. But's that a topic for another post...).

At any rate, the park was ideal for this because there was no shrubbery where the snake could go to ground and escape. But there were a few scattered saplings, and I soon learned the rat snake family were excellent climbers. Rufus moved up the trunk, very deftly taking advantage of every wrinkle and crevice in the bark to leverage itself up quickly. It was still moving too slowly to get high enough to escape from me, so I allowed it to climb, fascinated by how expertly it exploited the texture of the bark to move up off the ground.

Suddenly, from all over the park, a flock of song birds showed up;
jays, robins, thrushes; attacking the snake, pecking at it, doing their best to peck at it and drive it off the tree. I tried to wave them off, I was afraid they could hurt my pet, but they didn't seem to even notice I was there. They were determined to make life miserable for that snake. Eventually, I had to pull it off the tree and carry it back to my house, followed by a haze of angry birds who seemed to have no fear of me whatsoever.

I've learned since that tree-climbing snakes are a threat to nests, eggs and chicks, and the birds have evolved an instinctive hatred for them, and are ready to suspend their own territorial issues and cooperate to drive them away from their turf.

I know that in Florida, the corn snake (a close relative of the black rat snake) is also a good climber and often plunders bird nests. The woodpecker has a clever defense: It pecks holes on the southern side of pine trees for its nests. The sun melts out the sticky pine sap that oozes out of the hole, and it drips down the trunk making a gummy slime the snakes find distasteful and avoid.

Its funny how behavior can evolve into instinct.