Complex sentence: Crag Martin pairs nest alone or in small colonies, usually containing fewer than ten nests, spaced on average 30 m (100 ft) apart and each pair aggressively defend the zone around their own nest against other Crag Martins and most other bird species.
Split into simpler sentences: Crag Martin pairs nest alone or in small colonies, usually containing fewer than ten nests. Nests are on average 30 m (100 ft) apart and each pair aggressively defends the zone around its own nest against other Crag Martins and most other bird species.