Heronry

Herons, members of the Ardeidae family, have been returning to area nest sites for the past couple of  weeks.  Herons are mainly tropical birds, but they have spread out during warmer months to every continent except Antarctica.  They can be found in all types of habitats except where high and low temperature extremes are the norm, such as high altitudes and dry deserts.  Herons build nests in a heronry, a descriptive term for a rookery or colony of herons.

A heronry can include only a few nests or several hundred in one location.  Nest sites are often located on islands, especially those created from dredging.  These do not typically support other wildlife populations.  Predators searching for eggs and young chicks, especially raccoons, find it difficult to access nest sites.  Nests can be located near the water’s surface, in reed-beds, or high up in the tops of tall trees, from 40′ to 100′ off the ground.  It is not unusual for a single tree to support 8-10 nests.  Individual birds tend to colonize with their own species, although a good nest site may support several colonies of different species adjacent to each other.

Herons eat a wide variety of foods, but are strictly carnivorous.  Their main food comes from aquatic habitats and includes fish, crustaceans, and amphibians.  They occasionally hunt in non-wetland habitats, including agricultural fields and grasslands, eating voles, field mice, and other live prey.    A primary reason for colony style nesting is the scarcity of nest sites that are safe from predators and still close enough to food sources, typically from one to three miles away.  One advantage is that the food source can be shared among many individuals.

Once a suitable location for a heronry is established, nests will be reused from year to year.  Returning males arrive first, choose a nest, and then find a mate.  Birds may choose different mates each spring, but will remain monogamous for that year.  Nests are constructed by weaving together twigs of various lengths and start out about 20″ in diameter.  As nests are reused by new pairs of birds, they are continually rebuilt and improved each year.  Older nests may reach massive proportions of up to five feet across.

Adult herons are at the top of their food chain, their only predator is humans.  As such, they are considered an excellent indicator of the health of our wetlands.  Draining wetlands for urban or agricultural development removes them as a food source, and a heronry may be abandoned if there is not enough food available for raising young.  Human disturbance can also have negative effects on reproduction.  Repeated intrusions into nesting sites results in nest failure and abandonment of eggs or chicks.  It is against the law to disturb any rookery or heronry. 

To learn more and safely view all of the activity of a heronry, consider a visit to one of Illinois’ premier sites, the Lake Renwick Heron Rookery in Plainfield, Illinois.  They have limited days and hours, so before you go, check their website at:  https://www.reconnectwithnature.org/preserves-trails/visitor-centers/lake-renwick-heron-rookery-visitor-center