Peregrine Falcon biology and management in Colorado 1973-2001

Craig, Gerald; Enderson, James

In the early 1970s nesting peregrine falcon (Falco peregrinus anatum) population in Colorado had declined due to the adverse effects of DDT in prey birds. With the cooperation of federal agencies and the Peregrine Fund, about 400 young were successfully released 1974-90. We report here on the expansion and performance of nesting peregrines in Colorado in the 35 years ending in 2001 at a time when more than 100 pairs were found on territory. In the 1970's Colorado peregrines persisted on tall cliffs exceeding 60 m, and since population expansion in the 1990's they also have accepted cliffs as low as 30 m. About 39% of recent nests were on cliffs in the lower half of the range of cliff height and prominence. Nest ledges were found at all compass orientations at elevations up to 2300 m, but most faced southerly to easterly in the 2300-3200 m range. In about 60% of the cases at 34 long-studied sites, pairs selected ledges each year not known to have been used previously. The same ledge was used more than three years in 12% of the cases. Five radio-tagged adults hunted over areas as large as 1,440 km2 and ranged up to 43 km from the eyrie. Twenty species of prey occurred more than 12 times in remains of 1,106 prey individuals found in nests. Most frequent were white-throated swift (Acronautes sextalis), mourning dove (Zenaida macroura), common nighthawk (Chordeiles minor), and rock dove (Columbia livia); collectively these remains comprised 45% of the top 20 species. Egg-laying centered in mid-April. Clutches averaged about 3.5 eggs, and brood size was 2.9 chicks. Sex ratio of nestlings were 1.22 females to males (n = 577). We found a daily chick mortality rate of 0.01 using the Mayfield method. Natural productivity (young fledged per pair on territory) was about 1.2 (1973-80) and about 1.7 in 1995-2001. The overall rate was 1.67 young/attempt (n = 381). We estimated the productivity necessary for an equilibrium population based on mortality of age groups from band recoveries and re-sighted peregrines; if 50% of two year-olds breed, about 1.2 young per pair would be required. We estimated that adult annual survivorship averaged about 80%. Adults identified through photographs or colored markers occupied territories an average of 2.7 years and 25% were present at least 4 years. We found no change in productivity of males or females present up to 9 years on territory. Productivity was normal even when both adults were new on the territory. Recent productivity was similar at newly used sites compared to sites we studied before 1985; newly used sites were not poorer in terms of nest success. In 65 cases, causes of mortality were known; golden eagles (Aquila chrysaetos) killed 11 peregrines (8 nestlings), and great horned owls (Bubo virginianus) killed 8 (7 nestlings). Collisions killed 13 peregrines (1 with a jet fighter). DDE declined in the contents of eggs from 82 sets, 1973-90. The thickness of eggshells from whole eggs (n = 213) increased from about 0.300 ppm in 1973 to 0.330 ppm in 1998, the latter still below 0.359 ppm we took as normal for the species. We found wide within-clutch variation in the thickness of intact shells and shell fragments, and no direct correlation between shell thickness and nest productivity. In fall, Colorado peregrines (n = 14) generally moved south to northern Mexico, but one juvenile was found in Panama. Birds fledged in Colorado (n = 15) were later found at nest sites; 10 were in Colorado, 3 in Utah, 1 in New Mexico, and 1 in Arizona. We estimated that the known Colorado peregrine population has grown about l0-fold in the 35 years of this study. Before 1985, searches for new sites were mainly fruitless, then success improved. After 1996, searches were markedly productive. Occupancy and survey efforts were evaluated and protocols were recommended for future monitoring. (From report Abstract)

Peregrine Falcon biology and management in Colorado 1973-2001

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