Why would the woodpecker delay the space shuttle?

Woodpeckers are a problem for much the same reason as anything that would puncture a hole into the shuttle or its fuel tanks. In 1995, the shuttle 'Discovery' was rolled back to the Vehicle Assembly Building only a week before launch because woodpeckers had drilled dozens of holes in the insulating foam of the external fuel tank. A hole in that tank would be explosive, and the effect would be similar to what happened in the Challenger explosion, where a chain of other events led to the puncture of the tank.

When the shuttle Columbia was torn apart during reentry, the tiles on a crucial part of the orbiter had been damaged, allowing heat to enter and damage the frame of the shuttle's wing. So, a single bird could completely destroy a shuttle and its crew by drilling into the shuttle or its tanks just as a single piece of falling foam has already done.