Pasture raised is the term to look for when buying eggs.
J Anim Sci. 2004 May;82(5):1401-9. Preslaughter stress and muscle energy largely determine pork quality at two commercial processing plants. Hambrecht E, Eissen JJ, Nooijent RI, Ducro BJ, Smits CH, den Hartog LA, Verstegen MW. The objective of the present experiment was to study physiological changes elicited in response to stress in the immediate preslaughter period [...]