Male Passer domesticus
The female is generally lighter-colored
and absent the black bib and eye shadow.

House Sparrow/English Sparrow:
is possibly the worst of the bunch. It belongs to the Finch family. It is very adaptive, aggressive, and reproductive. It is known to throw other nestlings out of bird houses. It will nest in any protected area from lighted mall marquees to large building supply warehouses, where it appreciates the water fountains and damaged bird feed bags.

It is the number one enemy of the Eastern Bluebird. 
Many traps and devices have been invented to exclude them from Bluebird houses, but the similar body size makes the chore a difficult one.

Sturnus vulgaris
Male and Female are very similar

European Starling:
is a tossup for Songbird enemy number one. It is every bit as aggressive and adaptive as the English Sparrow. And it must surely be Purple Martin enemy number one.

Most songbirds have a particular diet, but these two imports will thrive on burgers and fries in fast food dumpsters, seed from your feeders, bugs from your car bumper, or undigested grain in livestock manure. Neither requires a bird house, but will nest in most protected areas. They are both on the most wanted list of the Pennsylvania Game Commission, which has a program underway to bring back the insect-eating Eastern bluebird.

I have seen the Starling evict a much larger Flicker from its tree cavity, and drop the Flicker eggs to the ground.  In early spring, they often travel in 3's. Its sound is very varied and sometimes appears imitative. It is often seen in large flocks with other birds that are black.

Cyanocitta cristata

Blue Jay:
is less well known as  the villain it is. It will rob bird nests of eggs and nestlings. It will also ruin the fruit and berries in your orchard. And it's a notorious bird bath and feeder bully.

Corvus brachyrhyncos

Crows:
are much the same as Blue Jays, but on a larger scale. They will even pilfer the nest of the Blue Jay.

Molothrus ater (male)

Cow Birds:
are not aggressive or adaptable. They are just clever. They watch a nest from a nearby lookout, and when the incubating parent leaves the nest for a snack, the cowbird deposits her egg in the nest. The Cowbird chick hatches first, grows fastest, and shoves its nest mates from the nest. All the while, the tenants think this is their own nestling, and feed it to adulthood.

Grackles:
eat insects, invertebrates, eggs, nestlings, lizards, and minnows. And have earned a bad reputation as a significant pest in corn fields, and also stealing food from other ground-foraging birds such as Robins.