The application of the telescopic method held promise of supplying definite data on the numbers of trans-Gulf migrants, however high their flight levels. The roll and vibration of the ship had prevented me in 1945 from making telescopic observations at sea. Since no immediate solution to the technical difficulties involved presented itself, I undertook to reach one of the small cays in Alacrán Reef, lying seventy-five miles north of Yucatán and in line with the coast of southern Louisiana. Because of transportation difficulties, my plans to place a telescopic station in this strategic location failed. Consequently, I returned in 1948 by freighter to Progreso, Yucatán, where telescopic counts were made for three nights, one of which was rendered almost valueless by the cloud cover. [Illustration: FIG. 35. Positions of the cone of observation at Progreso, Yucatán, on the night of April 23-24, 1948, from 8:53 P. M. to 3:53 A. M. Essential features of this map are drawn to scale. The telescope was set up on the end of a one-mile long wharf that extends northward from the shore over the waters of the Gulf of Mexico. The triangular (white) lines represent the projections of the cone of visibility on the earth at the mid-point of each hour of observation. Only briefly, in the first two hours, did the cone lie even in part over the adjacent mainland. Hence, nearly all of the birds seen in the course of the night had actually left the land behind.] The observation station at Progreso was situated on the northern end of the new wharf which projects northward from the beach to a point one mile over the Gulf. As will be seen from Figure 35, the entire cone of observation lay at nearly all times over the intervening water between the telescope on the end of the wharf and the beach. Therefore, nearly all of the birds seen were actually observed leaving the coast and passing out over the open waters of the Gulf. The hourly station densities are shown in Table 7 and Figures 24 and 36. In the seventeen hours of observation on the nights of April 23-24 and April 24-25, a total computed density of 59,200 birds passed within one-half mile of each side of Progreso. This is the third highest density recorded in the course of this study. The maximum for one hour was a computed density of 11,900 birds. This is the fourth highest hourly density recorded in 1948. [Illustration: FIG. 36. Hourly station density curve for night of April 23-24, 1948, at Progreso, Yucatán.] TABLE 7.--Computed Hourly Densities at Progreso, Yuc., in Spring of 1948