Seasonal Timing, Geographic Distribution, and Flight Behavior of Broad-Winged Hawks During Spring Migration in South Texas: A Radar and Visual Study

-Spring migration of Broad-winged Hawks (Buteo platypterus) through south Texas was studied using vertical, fixed-beam, and surveillance radars with simultaneous visual observations. Five years of records from Santa Ana National Wildlife Refuge and 33 years of reports from American Birds also were employed. The peak of migration occurred during the last week in March through the second week in April, with the migration of adults commencing two weeks earlier than immatures. Almost all migrants (>99%) passed east of 99?10'W. Winds during the migration season in south Texas were from the southeast at the surface, and from south-southeast to south at the altitude of migration, minimizing the potential for lateral drift. Only 4% of the 85,000 migrants counted in 1982 flew on days with opposing winds. Net tracks of hawks soaring in thermals were nearly downwind toward 3120. Tracks of glides were toward 6? and did not vary with wind direction, probably because of the extreme predictability of wind direction. Daily flight time averaged 8.7 h and was greater on days with good thermal lift (low percent cloud cover) and favorable winds. Altitude of migration increased from early morning to midday, averaging 652 m at midday, and varied inversely with the amount of cloud cover. Hawks often flew too high to be seen without binoculars, so direct visual counts were strongly dependent on radar detection. Ninety-two percent of the hawks flew in flocks of >100 birds after 1100, and approximately 90% landed in flocks of >40 individuals. Flocking was hypothesized to facilitate the random encounter of thermals during interthermal glides, saving time and energy. Received 30 October 1984, accepted 10 April 1985. MANY species of soaring birds regularly undertake long-distance migrations (Brown and Amadon 1968, Pennycuick 1972, Smith 1980). During these flights they depend on atmospheric lift, usually in the form of thermal convection or orographically deflected wind, and infrequently resort to more costly flapping flight. These migrants also must fly through wind fields that vary with altitude and geographic location. Therefore, soaring migrants should adopt flight behaviors appropriate for the lift and wind conditions encountered at different times and locations. Little is known, however, about the flight behavior of soaring migrants in different lift and wind conditions (Kerlinger and Gauthreaux 1984, Kerlinger et al. 1985). In this paper we examine several aspects of spring migration of the Broad-winged Hawk (Buteo platypterus), a long-distance, soaring migrant. Specifically, we report daily and 1 To whom reprint requests should be addressed. seasonal timing of migration in south Texas, geographic distribution, flocking, altitude, and direction of flight in relation to wind. STUDY AREA AND METHODS Study area.-Santa Ana National Wildlife Refuge (26?07'N, 98?18'W), Alamo, Hidalgo Co., Texas (Fig. 1) was selected as our study site after consulting past spring migration issues of American Birds. Observations were conducted from the parking lot in front of refuge headquarters, where there is a nearly unobstructed view of the horizon. The refuge (elevation 20 m) encompasses slightly more than 2,000 ha of woodland on the north bank of the Rio Grande River and is one of the few remaining roosting sites for migrant Broad-winged Hawks in the Rio Grande Valley. Surrounding the refuge is a vast expanse of flat farmland that is almost devoid of wooded tracts. Observations at the refuge were conducted from 28 March to 16 April 1982, from the Avian Migration Mobile Research Laboratory, a 7-m motor home equipped with two radars and other electro-optical equipment. Observations of migrants landing in the r fuge were conducted on foot from an elevated (3735 The Auk 102: 735-743. October 1985 This content downloaded from 207.46.13.149 on Wed, 19 Oct 2016 04:04:37 UTC All use subject to http://about.jstor.org/terms 736 KERLINGER AND GAUTHREAUX [Auk, Vol. 102 5 m) dirt dike that extends approximately 1.9 km along the north boundary of the refuge. We determined the distribution of migrant Broadwinged Hawks in the southern United States from southeastern Texas at the Gulf of Mexico westward to the Pacific Ocean by summarizing observations published in spring migration issues of American Birds (1949-1981). Seasonal timing of migration was determined using 5 years of unpublished daily counts made by the refuge personnel (1963, 1964, 1965, 1980, 1981). Standardized methods were not employed, although several refuge personnel counted hawks daily during their normal work. To explain day-to-day variance in numbers of hawks counted in 1982, we examined the influence of surface wind upon the numbers of migrants counted. Wind direction and speed at 1200 were used to compare following winds (south-southeast >2 m/s) with opposing winds (northwest-northeast >2 m/s). Two days when winds were variable or 1 yr old) of low-flying migrants also was noted. Net tracks of birds soaring in thermals and gliding tracks between thermals were taken from the radar screen or by hand-held compass. Altitude could not be measured with the surveillance radar because of the wide vertical beam. We employed an inclinometer to measure the elevation of the target above the horizon and then calculated altitude from the elevation angle and range of the target. The surveillance radar was most useful early in the day when migration was below approximately 300 m. As the altitude of hawk migration increased, usually after 1000-1100, the vertical, fixed-beam radar (a modified Marconi LN66 marine radar, 3-cm wavelength, 10 kW peak power, 40 conical beam, range ?2%) was used. Altitudes of migrants passing through the radar's narrow beam were read directly from the radar screen (a modified Plan Position Indicator), while simultaneous visual observations were made. The vertical radar was always used on the 4.8-km range setting. For each radar echo, we noted flock size and flight behavior (soaring or gliding). Flight direction was recorded for gliding tracks either as a disappearing azimuth or until a bird began climbing in a thermal. Soaring flight direction (net track) also was recorded as a disappearing azimuth or until birds began to glide. Radar operations ceased at about 1600 (1500 on two days) or when hawks were noticed descending. At that time, one observer (PK) walked along the dike and observed descent and landing of hawks in the refuge. Operations were halted at 1800 on all but two days (1830), although casual observations were continued until sunset. In this study takeoff time was operationally defined as the time when the first two hawks were seen aloft simultaneously. Because departures of Broadwinged Hawks lasted for at least 1 h, the number of birds taking off in that hour was the datum used in all analyses involving takeoff. Only mornings when >25 hawks were seen were included in the analyses. When flocks of migrating hawks began descending toward the forest in the afternoon, landing times were recorded to the nearest minute for each flock containing more than 40 birds. The average daily landing time was the midpoint time for the flocks ob-