On Tue, 11 Sep 2012 12:48:21 -0700 (PDT), Larry Fishel
wrote:

Maybe the bird didn't get around to building a nest on/in your vent pipe until recently...

I'd expect that to cause a slow flush, especially on a septic system.
Tree frogs seek refuge in vent pipes in South Florida, avoiding heat.
Seeking a cool location.

The option is to flush them out with a water hose.