S1, E6: The animal building trade has craftsmen and 'cowboys' too. Eager to cut corners on building costs, cliff swallows renovate and re-use last year's nests, unneighbourly shags steal materials from next door, and hermit crabs aggressively play the housing market. Expertly engineered, the most desirable animal dwellings boast air-conditioning, solar heating, and defensive moats.