I had to chime in on the cowbird topic. There's a guy in Virginia who is a big purple martin landlord (over 70 pairs of birds), and cowbirds are their arch enemy. I've been to his place for the lessons he gives on the topic...purple martins only exist east of the Mississippi because of folks who set up and maintain houses for them. Out west, native Americans made (make?) houses out of gourds for them (which is why the houses out this way are designed like that.)
Anyway...cowbirds will go into the nest of other birds (like purple martins) and lay their eggs for the resident adults to brood, hatch and feed. Cowbirds do not take over the nest...they let the resident birds do all the work. The hatched cowbird chicks crowd out the other chicks, who starve. (Audubon refers to this specific bird behaviour as "parasitic.") Cowbirds come back to the nests every once in a while to see if their eggs are still there and are unmolested. If the cowbirds' eggs have been removed or are damaged in any way, they destroy the resident bird's eggs. Nasty little buggers...
Those PVC gourds you see in the video that are used for purple martin houses usually have an access port in the back for the landlord to check on them and to do any required maintenance. If cowbird eggs are present, you take them out and addle them...then put them back. They'll never hatch, but they will be there when the adult cowbird comes back to inspect things. Of course, if the cowbird meets an untimely end, then none of this matters. ETA: They're protected by the Migratory Bird Act.
Regarding bluebirds and sparrows...I've heard of bluebirds coming to the windows of the people who feed them, freaking out because of sparrows taking over their nest and [apparently] asking for help.
ps: I want one of those FX airguns.