I have had the same issue in the past. I too have an upstairs dryer, and a long dryer vent run, and have had birds find a nice dry home in the vent, however I did fix mine. If you go to your local 'major home center' they typically have brushes that are extendable to clean dust and cobwebs from high walls. This is what i used to clean mine out by shoving it down the pipe after I too the pipe apart at each section. If you can't do that (take it apart) due to access, believe it or not, a leaf blower might just do the trick. I got a ladder and pulled a lot of the crud out of mine from the outside and blew the rest of it out with a leaf blower once. Another thing that you must do is to get a screen to cover the dryer vent outlet. My house had the vent outlet built into the vinly siding to where I couldn't get it out easy, so I took some quarter inch screen and trilled it down, then shoved it into the hole behind the flap, and watched the bird come back and fight with it. The vent is still working fine now, but if you don't screen the entrance to keep the birds out, they will rebuild within a week. Good luck!