Complex sentence: The early re-colonisation was very slow, because of contamination of the food chain by organochlorine pesticides, and the activities of egg collectors, and had reached only 14 pairs by 1976.
Split into simpler sentences: The early re-colonisation was very slow because of contamination of the food chain by organochlorine pesticides and the activities of egg collectors. As a result the breeding population had reached only 14 pairs by 1976.