Product review (5 stars): Excellent!  Get it!
To put bird guards on my vents, I was quoted $480 by a company.  I purchased 2 packs of these vent guards for less than $10 each and installed them myself.
If you do not need to get on a ladder to install them in high vents, then you can literally install one of these in seconds and completely solve your problem of birds nesting in your vents!