Thermal Soaring by Migrating Starlings

Abstract

LARGE numbers of migrating Starlings (Sturnus vulgaris vulgaris) pass the vicinity of Yorktown, Virginia each year during September and Octo-ber. These birds usually travel in small compact groups of some 20 to 50 individuals, although some large flocks occasionally contain upwards of 300 birds. Their southward progress is punctuated by frequent rest stops in the tops of large trees along the way, and their presence is always ac-cented by a loud, incessant chattering. On the morning of 23 October 1964 the weath'er at Yorktown was par-ticularly clear, with cool air temperatures and bright sunshine, and large numbers of Starlings were observed moving southward aided by a fresh tail wind from the north. The presence of considerable thermal activity in the atmosphere was indicated by the strong and continuous soaring of numerous gulls in the area. At approximately 10:30 AM, a single hawk (tentatively identified as an Osprey, Pandion haliaetus) was observed to the north' at an estimated altitude of about 500 feet, performing the con-tinuous circling flight characteristic of thermal soaring (Cone, 1962a). In the immediate vicinity of the hawk and just above it was a flock of ap-proximately 40 Starlings, wheeling with great individual precision as a tightly compacted group around an orbit almost identical in size to that of the hawk. This intimately close circling of hawk and Starlings was ob-served continuously for some 10 minutes or longer until the birds, still cir-cling, drifted out of sight over some tree tops to the south'. I had observed and made notes on this rather unusual phenomenon on several occasions in previous years, but my observations at those times had been rather casual and of short duration, and I had tentatively surmised that the Starlings' behavior was in the nature of a harassment action, or else intended as a defensive maneuver designed to confuse the hawk and th'us discourage any predatory intentions it might have. During my most recent observations, however, the birds were visible at close range for a considerable period of time and I was able to examine their behavior in detail. I was greatly surprised to find that, contrary to my previous con-clusions as to the purpose of the Starlings' actions, these birds were making no aggressive or evasive maneuvers whatsoever, but instead were keeping a fairly constant distance above the hawk. Likewise, the hawk did not exhibit the slightest interest in the Starlings, as the group wh'eeled without flapping just above it. It appeared that neither the hawk nor the Starlings