One of the most unique groups of birds to occur in Iowa are the Colonial Waterbirds. Colonial Waterbirds are birds that depend on water for their food source and who nest in colonies called rookeries. These rookeries can have as many 100 or more nests and as few as two. The rookeries most often occur adjacent to or very close to water and depending on the species may be in a tree or on the ground. Great blue herons will often take over several trees in an area building somewhat flimsy looking platform nests.
The colonial waterbird species we are most interested in for Iowa are the following:
Double-crested cormorants can be found on lakes, rivers, and marshes throughout Iowa. Peak numbers are seen during spring and fall migration. Documented nesting colonies are on several islands in the Mississippi River near Sabula in Jackson County and north of Clinton in Clinton County; at Butler Lake in Allamakee County; at Coralville Reservoir in Johnson County; and at Union Slough National Wildlife Refuge in Kossuth County. Summer flocks have been reported from several other counties. They are tree nesters that build stick and twig nests lined with finer materials and greens. They lay three to four eggs that hatch asynchronously (so there may be chicks of different ages in the same nest). Parents brood chicks for the first two weeks.
Black-crowned night-herons can be found on lakes and marshes. Their colonies are located in dense stands of cattail or bulrush, over water, and in thickly vegetated marshes. Most summer sightings are in the northern two-thirds of the state. Nests are a loose arrangement of sticks, twigs, and reeds lined with finer materials. Black-crowned night-herons lay three to five eggs that hatch asynchronously. Adults forage at dawn, dusk, and at night, roosting in trees during the day.
Yellow-crowned night-herons are regular; rare nesters in the southern half of Iowa and along the Mississippi River. No confirmed nests have been reported since the early 1960's, but adults and immatures are seen regularly in late summer and fall. The preferred habitats of this bird are wooded wetlands along major rivers and large cattail marshes. They nest in trees or shrubs over land. The nests are usually substantial structures of sticks and twigs lined with rootlets and leaves. Yellow-crowned night-herons lay four to five eggs. Adults forage at night and in dim light, roosting in trees during the day.
Green herons are common and widespread nesters. They usually are solitary nesters, but occasionally form small colonies. They can be found throughout Iowa. Nests are located in woody vegetation on the borders of ponds, rivers, and marshes. They usually are concealed, near or over water, and consist of interwoven twigs lined with finer materials. Green herons lay two to four eggs that hatch asynchronously. The young are expert climbers and are fed by the adults for more than a month after they leave the nest.
Little blue herons are possible nesters in Iowa. The only recorded nesting is from 1984 at Folsom Lake in Mills County. It was associated with a colony of cattle egrets. The little blue heron's preferred habitats are lakes and marshes. Its nest is built over water in a low tree or shrub. The flimsy nest is made from sticks and twigs. Little blue herons lay two to five eggs. Chicks are brooded for three weeks.
Cattle egrets are rare summer residents found on pastures, wet meadows, and near wetlands throughout Iowa. Nesting colonies have been reported from Mills, Marshall, and Monona Counties. Nests are made of reeds, sticks, twigs, and vines. Cattle egrets lay three to four eggs that hatch asynchronously. Young are brooded for two weeks, leave the nest at 20 days, and are independent at 45 days.
Great egrets are rare summer residents found along lakes, rivers, and ponds throughout Iowa. Nesting colonies have been located along the Mississippi (Allamakee, Clayton, Jackson, Clinton, and Des Moines Counties) and Missouri (Monona and Mills Counties) Rivers and in Polk, Marion, and Emmet Counties. The frail nests are made of sticks and twigs. Great egrets may nest with great blue herons. They usually lay three eggs. The young leave the nest at three weeks.
Great blue herons are the most familiar of Iowa's herons. They are fairly common summer residents. Their nests can be found throughout Iowa near rivers and lakes. Colonies usually have fewer than 100 nests. Solitary nests also can be found. Great blue herons place their nests high in large trees. The well-made nests are large, flat structures of interwoven sticks, usually lined with greens. Great blue herons lay three to five eggs. The young fledge at 60 days.
Only conservative estimates of the number of rookeries in the state are available, because Iowa DNR staff does not have the resources to monitor many of the Iowa nests, nor to adequately search for new colonies. Great Blue heron rookeries are by far the most common and can be found statewide. Monitoring these rookery sites is important as it provides data on the reproduction and population of these important species which among other things serve as indicators of our water quality. Monitoring of nests is left mostly up to volunteers and concerned citizens. Therefore, the discovery of new nest sites, which is often by chance, is primarily by volunteers, other citizens, and agency staff.
If you or anyone you know locates a Colonial Waterbird Rookery, please report your findings to the Iowa Department of Natural Resources Wildlife Diversity Program, 1436 255th Street, Boone, IA 50036. Reporting forms with instructions can be downloaded below as well as instructions on how to get good coordinate location data on a rookery. Thank you for helping in the conservation and monitoring of a unique Iowa resource! Enjoy the outdoors!
Realize the birds are easily spooked and could abandon the nest if disturbed. Please remain 300 meters from the rookery and view them from afar. Realize that rookeries are often on private land, so please do not trespass unless permission from the landowner has been granted.
Colonial Waterbird Rookery Reporting Instructions and Form
Colonial Waterbird Rookery Reporting Instructions and Form
Questions? Contact Stephanie Shepherd at firstname.lastname@example.org for assistance.