Overland Nighttime Flights

We know of only two accounts of Ospreys migrating overland at night. At 2155 H on the night of 26 September 2004, one of us (R. DeCandido) watched an Osprey migrating over Manhattan Island, New York City. The bird, which was flying at 550–600 m, was seen from a 320-m high Observation Deck on the 86th floor of the Empire State Building (ESB) in downtown New York. The sky was clear at the time, the winds were northeast at 25–40 kph, and the barometer was rising. At 2246 H on the night of 26 October 2004, R. DeCandido observed another Osprey migrating over the ESB. The bird was estimated to be flying at 600 m. At the time, the sky was clear, winds were north at 25–40 kph, and the barometer was rising.

Overwater Nighttime Flights

Reports of Ospreys making landfall at dawn on the central Mediterranean archipelago of Malta indicate that individuals sometimes attempt to cross the Mediterranean Sea at night (Beaman and Galea 1974). In addition, R. Dennis (pers. comm.) reported that one satellite-tracked individual migrating between Scotland and Morocco flew over the Atlantic Ocean at night. On the other hand, three studies of satellite-tagged Ospreys migrating south from Europe presented no evidence of migration at night (Hake et al. 2001, Kjellen et al. 2001, Triay 2002).

Migration data from North America for southbound Ospreys indicate that nighttime flights are not unusual during overwater travel. From 2000–04, two of us (R. Bierregaard and M. Martell) tagged 10 Ospreys (seven adults and three juveniles), and collected data on 17 north or southbound migrations made by these birds between their breeding grounds in the eastern United States and wintering areas in South America. Five individuals left the North Carolina coast and flew south across the Atlantic to Florida, covering between 400 and 700 km of open water. All birds crossed 180–290 km of open water between Florida and Cuba. From Cuba, all but two crossed 560–960 km of the Caribbean Sea while traveling between Hispaniola or Puerto Rico and South America. Two birds flew 550–650 km over water between Cuba and Central America or Mexico.

In spring, two Ospreys on their northward migration flew directly from South America to Cuba, a nonstop flight of approximately 1000 km. Another northbound bird leaving Mexico flew 790 km across the Gulf of Mexico toward the Florida Panhandle, but disappeared approximately 100 km west of the Florida coast.

Of five flights from North Carolina to Florida, two definitely included nighttime travel, two others probably did, and one did not. Of the eight trips between Florida and Cuba for which we could estimate flight timing, only one included nighttime flight. All three crossings between Central America and Cuba or Florida, and all 11 Caribbean crossings for which departure and arrival could be estimated, included nighttime flight.

We estimated ground speed for five birds traveling across open water, based on the most accurate locations (Location Classes 2 and 3) from the Service Argos, Inc., Largo, MD telemetry data. Calculated speeds ranged from 27–64 km/hr, with three flights between 40–56 km/hr. At 45 km/hr (the median speed), the Florida to Cuba crossing would take between 4 and 6 hr. The Cuba to Central America crossing, or the Caribbean crossing to South America would take between 12 and 21 hr. The South America to Cuba direct flight would take approximately 22 hr.

Any trip of more than 12 hr requires some nighttime flight unless the birds leave at dawn. We estimated departure time for 17 overwater crossings: no birds departed before daybreak, seven set out in the morning, and nine in the afternoon. Based on these data, there was no tendency by Ospreys to start overwater trips early in the day to reduce the need for nighttime flight.

Our observations lead us to conclude that Ospreys migrate at night more frequently than has been suspected. The paucity of earlier observations is most likely due to two factors. First, few raptor biologists regularly watch for nighttime migrants. Second, at high latitudes, Ospreys tend to be relatively uncommon broad-frontal migrants whose low numbers at any one place reduce the chances of observing nighttime movements. Currently, at least ten other species of diurnal birds of prey are known to migrate at night (Table 1). As greater numbers of raptors are tracked by satellite, we expect that the numbers of known nighttime migrants will increase substantially. In the words of Beaman and Galea (1974), “Nocturnal migration by raptors may thus be on a larger scale than is generally realized, if only as an involuntary consequence of adverse conditions encountered during a sea-crossing.”

Table 1