Hutson, G. D. 1992. A comparison of operant responding by farrowing sows for food and nest-building materials. Applied Animal Behaviour Science 34, 221-230.

Operant responding for food completely overshadows operant responding for other potential reinforcers both before and after farrowing [because, obviously, food-restricted sows have an extremely high motivation to obtain food]. .. Any potential welfare problems caused by frustration of nest-building behaviour seem insignificant in comparison with the prolonged frustration presumably experienced by food-restricted pigs. Food-restricted sows are so hungry that they have little interest in straw. ... It now seems clear that the nest-building behaviour of sows, as exemplified by rooting and pawing behaviour, is under tight endogenous control. The performance of nest-building behaviour is in itself reinforcing independent of functional results.