Confused Enemy Recognition
Confused Enemy Recognition.-Regarding the phenomenon of enemy recognition by birds (Cross, Auk, 67, 1950:512; Davis, Auk, 67, 195O:SlS; Rand, Auk! 68, 1951:524-525), certain field observations suggest to me that a bird’s reaction may be directed not only against a recognized enemy but against any form which resembles an enemy.
On May 26, 1951, Yellow-headed Blackbirds (Xanthocephalus zanthocephalus) nesting in an extensive cattail marsh southeast of Fort Collins, Colorado, commenced an excited harangue when a high-flying B-29 airplane passed over the marsh. A week later these same birds in a three-hour period harangued a Great Blue Heron (Ardea herodias), American Crow (Corvus brachyrhynchos), and Common Nighthawk (ChordeiJes minor) which crossed above the marsh.
Although none of the objects which excited the blackbirds can typically be considered in an enemy status in this locality, there is a decided resemblance between each of the objects and various hawks, especially the Marsh Hawk, which harass the blackbirds. Since it is not uncommon for ornithologists mdmentarily to mistake some of the aforementioned objects for birds of prey, it is not surprising that the blackbirds appear to err likewise, particularly when under the stress of defending a nest.-RICHARD G. BEIDLEMAN, Zoology Department, Colorado Agricultural and Mechanical College, Fort Collins, Co!orado, March 31, 1952.
