A robin is a type of bird. Father is another name for priest. "Birds of a father flock together" sounds like "Birds of a feather flock together" which is a common saying meaning that people having the same interests or beliefs will be found spending time together. This joke is playing on "birds of a feather flock together" except "feather" is replaced with "father". 
A bunch of robins followed the priest wherever he went . Birds of a father flock together .