Aerial radio-tracking of Whooping Cranes migrating between Wood Buffalo National Park and Aransas National Wildlife Refuge, 1981-84 /

From 1981 to 1984, Whooping Cranes Grus americana migrating between their summer range in and near Wood Buffalo National Park in Canada and their winter range on and near the Aransas National Wildlife Refuge in the United States were studied by means of radiotelemetry. Objectives of the aerial study included a precise determination of migration routes, duration of occupancy and location of stopover sites, and documentation of migrating crane behaviour and mortality. In 1981, 1982, and 1983, 15 juvenile Whooping Cranes were captured on the breeding range and equipped with single radio transmitters weighing between 62 and 77 g each. Nine families, pairs, singles, or subadult groups in fall and five such aggregations in spring were monitored through all or part of their migration by teams of airborne and ground observers. The present report summarizes observations by the airborne radio-monitoring crews. Radio contact with migrating cranes was maintained by means of leg-band radio transmitters, antennas attached to aircraft struts, and radio receivers carried in the aircraft. Radio signals could be picked up from distances up to 155 km, with shorter receiving ranges (up to 56 km) when cranes were on the ground. The radio system allowed operators to "home in" on cranes, and visual contact was maintained for up to 50% of the migration, enabling air crews to obtain data on flight behaviour. Whooping Cranes do not all migrate simultaneously. Fall migration of yearlings and subadults began after the middle of September, with family groups and some paired adults following in early October. In spring, migration was under way by the middle of March, with family groups and paired adults leading, followed in April by subadults. Whooping Cranes usually migrated as pairs, family groups, singles, or small aggregates of up to four or five subadults. Sometimes pairs joined family groups for all or part of the migration. Even though age-related differences in onset of migration and resumption of interrupted flights were evident, weather conditions were also important stimuli for the initiation or termination of migration flights. Southward fall departures from the nesting range coincided with northerly winds, rising barometric pressure, and good visibility. Weather conditions symptomatic of the passage of a low-pressure system (decreasing pressure, low ceiling, precipitation) forestalled departure of migration or forced migrating cranes to land. If fall departures were delayed, the birds might eventually begin their flight under less than ideal conditions (freezing, snowfall). Northward spring migration from the winter range was facilitated by the eastward passage of a high-pressure system producing warm southerly winds, relatively high barometric pressure, and good visibility. Weather conditions such as those enabling birds to begin fall and spring migrations also contributed to resumption of migration after unfavourable weather had forced down flying birds. Unfavourable weather conditions notwithstanding, onset or resumption of migration among experienced breeding pairs (with or without their juveniles) often occurred under less than ideal conditions, as if the birds were in a hurry to reach breeding territories. Yearlings and other subadults appeared to have a more "casual" approach to migration. The period required to complete the 4000-km fall migration occupies a large part of the crane year. The migration, taking up to 50 days, consists of three parts: a two-day flight from the breeding range to the staging area in Saskatchewan; a oneto five-week staging period on prairie grainfields and wetlands; and a rapid week-long journey through the United States. Spring migration, particularly for established breeding pairs, may be completed in only 10-11 days, because no spring staging occurs. The migration corridor followed by migrating Whooping Cranes measured 80 km at its narrowest and 300 km at its widest point. This corridor generally paralleled but did not cross the continuous 1000-m contour line that extends north from Mexico to the Northwest Territories. Migrating Whooping Cranes display various forms of flight, such as relatively slow, low-level flapping flight and a rapid, high-altitude flight with little wing movement. The most common and undoubtedly most energy-efficient flight was a combination of spiralling and downwind gliding. This method of flight, resembling that of sailplanes, allowed cranes to utilize thermal updrafts and favourable winds during fair weather and resulted in rapid progress in the form of nonstop journeys of 9-10 hours and 700-800 km daily. During normal flying days, generally unaffected by weather disturbances, mean daily flight distance (and duration) and ground speed were about 400 km (7.5 hours) and 53 km/h. Spring migration flights were generally longer in distance and time than fall migration flights. Ground speeds of wind-assisted cranes reached over 100 km/h. Radio-tracked Whooping Cranes were observed as high as 1950 m above ground level, and, although migration at altitudes of 1500-1800 m was not uncommon, most flights occurred below 600 m. The most common configuration of flying Whooping Cranes was the V-formation or modifications thereof. These formations were generally assumed by spiralling or gliding cranes. The echelon formation was also commonly observed, during wind-assisted high-altitude flights and low-level flapping flight. Although Whooping Cranes sometimes formed single flocks with other Whooping Cranes or Sandhill Cranes Grus canadensis, rarely did they appear to seek them out. In some instances crane families on spring migration remained intact until they reached the breeding range, whereas in other circumstances juveniles separated from their parents in Saskatchewan or at Aransas National Wildlife Refuge. Whooping cranes are diurnal migrants: nocturnal flight was recorded rarely. When night flying did occur, it was done most often by nonbreeding birds and usually at the termination of long daytime migration flights. There was some evidence that birds recognized landscape features during daylight flights. Six radio-equipped juvenile Whooping Cranes were found dead on the breeding range. Most likely these birds, not quite capable of sustained flight, died as a result of wolf Canis lupus prédation. The remaining nine cranes were radio-tracked during the present study, and six of them died after contributing varying amounts of data. Two birds died as a result of power line collisions, and four were found dead or disappeared at the Aransas National Wildlife Refuge. Collision with power lines was the most serious threat to migrating Whooping Cranes.