Two photos to see the differences between these two cowbirds: Screaming Cowbird and Shiny Cowbird. They belong to the Family Icteridae and both are parasitic 
Differences: The violaceous blue of the Shiny Cowbird and the glossy black of the Screaming Cowbird. Bill length. Shorter in the Screaming Cowbird (in Spanish it takes its name from this difference) and longer in the Shiny Cowbird. The chestnut iris in the Screaming Cowbird and the dark iris in the Shiny Cowbird.

Juvenile screaming cowbird

With shiny cowbird below

With shiny cowbird below