Ground-feeding birds must peck for food and scan for predators such as hawks - but they cannot do both at the same time. Because birds can barely move their eyes they must turn their heads to look for predators, making them temporarily unable to peck for food. Their feeding is a sequence of "pecks" (strikes at the ground) and "scans" (cocking their heads to look for predators). In a field study of Yellow-eyed Juncos (Junco phaeonotus), biologists observed that a scan is always followed by a peck, and a peck is followed by either another peck or a scan. For example, after a scan, a sequence might be 3 pecks followed by a scan: PPPS. The biologists theorize that the length of scan sequences is affected by the flock size -- the number of Juncos feeding in the same place. The basis of this theory is the belief that individual birds in larger flocks do not need to scan as much, since others will be "helping" to look for predators. The biologist's observed counts of scan sequence lengths for Junco flocks of size 2 and flocks of size 5 are shown in the table below.

Observed counts of scan sequence lengths for flock sizes of 2 and 5 Sequence Length 2 3 4 5 6 (Sequence) PS PPS PPPS PPPPS PPPPPS Flock size = 2 186 108 62 18 7 Flock size = 5 45 40 32 20 11