Prey Size Selection By Lapwings in Lapwing/Gull Associations

Abstract

1. Flock size in lapwings correlated positively with the mean length of worm taken and birds in large flocks tended to take a more profitable mixture of worm sizes than those in small flocks. They also did better than expected on the basis of worm size mixtures in the ground. 2. Worm size taken was influenced by whether or not birds crouched before pecking. Crouching was associated with birds finding larger worms and taking predominantly the most profitable worm size classes. Crouching may have helped birds avoid broken fragments of worm common in feeding areas and small worms near the surface of the ground. 3. The proportion of pecks preceded by crouching increased significantly with flock size but decreased with the numbers of kleptoparasitic gulls in the flock. Perhaps as a consequence, lapwings tended to take smaller and, overall, less profitable mixtures of, worms in the presence of gulls. 4. The positive relationship between crouching and flock size appeared to be due to the relationship between flock size and worm density, birds feeding more selectively when searching costs were low. The negative relationship between crouching and the number of gulls, however, may have been due to lapwings reducing their risk of attack.