Hi Connie, according to Birds of North America Online the Green Heron is an “opportunistic forager with varied diet. Overall diet includes invertebrates such as leeches; earthworms; aquatic and terrestrial insects such as adult and larval dragonflies, damselflies, waterbugs, diving beetles, grasshoppers, crickets, katydids; spiders; crayfish, crabs, prawns; snails; fresh- and saltwater fish; frogs, toads, tadpoles, and newts; snakes and lizards; and rodents. Fish constitute primary food; these include topminnows, minnows, sunfish, catfish, pickerel, carp, perch, gobies, shad, silverside, eels, and in urban areas and human-made ponds, goldfish.” Kingbirds are notorious for being aggressive toward other birds, especially if they are nesting nearby, which could also have been the issue for the Swallows as well.