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Platte River Status: A common migrant, local breeding species, and casual winter resident. Occurrence dates at the Mormon Island Crane Meadows, Hall County, extend from 12 April to 23 October. Tout (1947) recorded great blue heron in Lincoln County during 25 March to 14 October. Although he did not find nests, Tout stated that he had reports of nests in the forest along the Platte River. Rosche (1979) stated that this species was a permanent resident, nesting in the lower North Platte River Valley.

Breeding Range: Restricted during the breeding season almost exclusively to the the Platte River Valley. Birds found occasionally in the Rainwater Basin area of the Eastern Plain are probably adults foraging.

Breeding Population: The population in 1979-1980 was estimated at 280 breeding pairs, based on the number of nesting platforms occurring at the 3 known nesting colonies. Our estimate was conservative because additional nesting colonies probably exist, especially in the western half of the study area. We are confident that we have not overlooked colonies along the river since then, but some probably exist in widely distributed tree claims away from the river valley.

Habitat: Nesting colonies are located in mature eastern cottonwood trees, generally adjacent to a large river or other permanent water source. Miller (1943) stated that the species of tree was not as important as its height and the distance from human activity. Open river channels and shorelines around lakes provide necessary foraging areas. This species forages almost entirely on small fish and other aquatic vertebrates.

Effect of Habitat Alteration: Great blue heron has benefited positively from the encroachment of wooded vegetation within the Platte River system and by establishment of tree claims in the Sandhills. This species is known to nest on the ground and in emergent aquatic vegetation, but the majority of colonies are located in tall trees (Johnsgard 1979). All of the colonies we have found were located in mature trees. The presence of chlorinated hydrocarbons and other toxic substances in heron eggs was determined from a sample of 10 eggs collected from the Lexington, Dawson County colony in 1979. Results indicated the presence of relatively high concentrations of DDE; the eggs contained more DDT than had been found in recent years (H. M. Ohlendorff, pers. comm.). Eggshell thickness in the eggs averaged 6-18% less than the pre-1947 mean reported by Anderson and Hickey (1972). Based on the egg sample we had analyzed, it appears that great blue herons in the Lexington colony are experiencing significant exposure to organochlorines either locally on migration, or during the winter. Nesting colonies are often abandoned as a result of human disturbance (Markham and Brechtel 1979). Human distrubance and loss of nesting and foraging sites have been important factors contributing to declines of some great blue heron populations (Thompson 1979, Kelsall and Simpson 1980, McCrimmon 1981). Konerman et al. (1978) believed that concentrations of DDE and dieldrin contributed to reproductive failure and colony abandonment in Iowa. We believe that the population of great blue herons nesting in the Platte River Valley will continue to thrive, and possibly increase, as long as an adequate fishery remains in the watershed, and a balance of forested and open channel areas is maintained along the river.

Nesting Data: Three active nesting colonies are known in the study area; all are located on islands in the Platte River. The largest and oldest is at Willow Island (T. 11 R. 24 W.), which supported 83 nests in April 1979. Adult great blue herons were first observed standing on nests on 13 March 1979. On 4 May 1984, there were at least 160 active nests in the colony. The Jeffrey Island colony (T. 8 N. R. 20 W.) supported 89 nests in 28 trees on 8 May 1984. Mean diameter at breast height (dbh) of the nest trees was 58.9 cm (range = 34.8 - 80.8 cm). Hatched eggshells were present on the ground beneath nest trees on 4 May. The Jeffrey Island colony was established in 1981 or 1982. The easternmost colony is located 84 km east of Jeffrey Island (T. 9 N. R. 12 W.). Discovered on 15 January 1984, this colony supported 31 nests during the 1984 nesting season. On 8 May 1984, several hatched eggshells were found on the ground beneath nest trees, and chicks were heard calling from the nests. The mean dbh of nest trees in this colony was 61.9 cm (range = 58.9 - 75.9 cm). We believe this colony was established in 1983 based on observations of adults at the Mormon Island Crane Meadows located 24 km downstream from the colony. From 17 July to 4 August 1983, up to 31 great blue herons were observed at Mormon Island; many were immatures. During the summers of 1980-1982, the maximum number of great blue herons at Mormon Island was 6 on 28 July 1980. The increase in sightings during the 1983 summer and the discovery of a nesting colony 24 km upstream in 1984 suggest that the colony was probably established in 1983. Egg dates from colonies in North Dakota extend from 27 April to 15 May (Stewart 1975) and in Kansas external from 1 March to 30 April (Johnsgard 1979).