Sentence:  Out of this background of hunting and fishing , it <WSD>was</WSD> only natural that Roy first painted subjects he knew best : hunters in the field , fishermen in the stream , ducks and geese on the wing - almost always against a vast backdrop of weather landscape . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)