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The ecology of Richardson’s Merlins on the Canadian prairies Hodson, Keith Alan

Abstract

It is the grassland ecosystem which supports nesting Merlins on the Canadian prairies. This study considered the effects of some factors interfering with processes in operation in the natural functioning of the grassland ecosystem that influences Merlin populations. An important part of this study was the comparison of selected habitat features near Hanna, Alberta, where a segment of the prairie Merlin population continues to nest, and those near Kindersley, Saskatchewan, where Merlins once nested but no longer are present. Additionally, data on nesting ecology of Richardson's Merlins were gathered along the South Saskatchewan River, and near Hanna, in southern Alberta, during the summers of 1968-1974. The absence of nesting merlins near Kindersley appears to be related to changing human land use patterns in that area. Since 1951, 62% of the land near Kindersley has been cultivated, while the comparable figure near Hanna, where Merlins continue to nest, is 26%. Air photo study of territories around Merlin nest sites showed that 52.3% of the area presumed to have been used by Merlins for hunting in the Kindersley area came under cultivation since the 1940's, as compared to 25.5% for the Hanna territories over the same period. Interpretation of 40 Merlin territories in use since 1971 in the Hanna area, has revealed that Merlins were hunting in areas which averaged 78% grassland. Increase in cultivation in both areas since the early 1960's has been about 7%, leaving Kindersley Merlin territories with less than 42% grassland by 1971. Assuming that at least 50%, grassland is required within a Merlin hunting territory in order to provide sufficient small bird prey, it was concluded that the Kindersley area has not been a prime Merlin nesting area since the 1940's, and that increases in cultivation since the early 1960's has probably reduced grassland below the threshold necessary to support nesting Merlins. It is felt that the heavy use of dieldrin in the early 1960's probably had the effect of administering the "coupe de grace" to the Kindersley Merlins, possibly through the reduction of grassland birds. Analysis of prey remains at nests indicated that the diet of these prairie Merlins was composed of 50% Horned Larks, 37% Chestnut-collared Longspurs, 6% sparrows, 4% blackbirds and 3%, others (passerine birds, shore-birds, rodents, etc.). Destruction of the habitats of these prey species must be viewed as destruction of the habitat for nesting Merlins. The now common practice of seeding open cattle range to crested wheatgrass and other alien monocultures is likely to lead to further regression in the prairie ecosystem and to reduce or extinguish populations of passerines and, consequently, Merlins. In Alberta, from 1971-1974, an average of 70.9% of occupied nest sites (sites with a pair of Merlins present prior to egg laying) were active, at least to the egg laying stage; 54.9% of nests with eggs hatched young, and 85.3% of these successful nests produced fledglings. Average clutch size was 4.6 eggs, of which an average of 3.5 hatched per nest with eggs hatching, producing 3.2 fledged young per nest with young reaching fledging age. A net productivity of 0.69 fledglings per occupied nest site was determined. Hatching success for all eggs was 57.8%, and 84.4%, of the young hatched survived to fledging. These figures include results from 1973 when a single storm accounted for a failure of 41.4%, of the nests in the Hanna area. Egg hatchability appears to be related (p

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