Great Blue Heron

Ardea herodias

These large wading birds are commonly seen foraging for fish and other aquatic animals in shallow waters during the spring, summer and fall. They breed colonially, typically in floodplain forests. Colony sites are used annually, some for over 35 years (Butler 1992). In Missouri their stick nests are placed in the crowns of large trees, especially sycamores (Platanus occidentalis).

Great Blue Heron numbers declined during the '50s, '60s and '70s throughout much of their range, including Illinois (Graber et al. 1978) and the Upper Mississippi River (Thompson 1978). Beginning in the early 1980s, growth of the breeding population was documented in many areas including Illinois (Kleen 1987) and Michigan (Butler 1992). The Missouri Department of Conservation 1978 survey of Great Blue Heron colonies recorded only 123 colonies averaging 29.3 active nests per colony. In 1992, 192 colonies were located, averaging 29.2 active nests.

Code Frequency

While Great Blue Herons are easily sighted, their presence does not necessarily indicate a breeding colony. Some of the individuals found in Missouri during the breeding season may have already concluded breeding at colonies farther south (Heitmeyer 1986). Also, although Great Blue Herons most often forage within 2.3 to 6.5 kilometers of breeding colonies, they may range up to 30 kilometers (Butler 1992).

Considering these factors, the possible and probable breeding blocks on the accompanying map provide a misleading view of breeding distribution. Fortunately, their breeding colonies are relatively conspicuous, so confirmed breeding locations suggest the true distribution of breeding Great Blue Herons in Missouri.

Distribution

Great Blue Herons bred most densely in southwestern Missouri and were more scattered elsewhere in the state. Although breeding colonies were found on a variety of streams and lakes, none were located on the Missouri River nor on the Missouri side of the Mississippi River during the Atlas Project.

Average Number of Birds / 100 stops

Abundance

Great Blue Herons appeared to be most abundant in the Osage Plains and Ozark natural divisions and least abundant in the Ozark Border, Mississippi Lowlands and Big Rivers natural divisions.

Phenology

Great Blue Herons return to Missouri in late February and early March (Robbins and Easterla 1992) and apparently initiate breeding activities immediately. Late dates for nesting activities may have resulted from re-nesting subsequent to nest failure or been due to late nest initiation. It is unclear how many late broods are first or second nesting attempts (Butler 1992).

Breeding Evidence

Reported in 320 (26.5%) of 1,207 blocks

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