Audubon Southwest's Bird of the Month: May 2021

The Least Bittern (Ixobrychus exilis); Avetoro menor (Spanish)

A denizen of wetland marshes, this secretive marsh bird is a treat to hear or see in the arid southwest. I have spent many years hearing and not seeing a Least Bittern. It took me a while to detect the soft “coo” of the courting male. The Kek-Kek-Kek voice of this tiny bird heard year around is loud and easy to remember. Earning the title of World’s Smallest Heron, they are about 13 inches tall. The heron closest in size and coloration is the larger Green Heron (about 18 inches). The streaked whitish underparts and bright buff wing patches are diagnostic identification marks for Least Bittern. The thin body, about 2 ¼ inches across, is ideally suited to slipping through the reeds in search of food or to hide.

The largest breeding United States populations are in estuaries in the east, along the southern gulf coast, the Mississippi River, and the prairie potholes of the Midwest. These birds migrate to Central and South America for the winter. Southern Florida and the Lower Colorado River and delta, including the Salton Sea, have year around populations. Breeding populations are also in central Canada, Mexico, Central, and South America. Because of the extremely large distribution, the Least Bittern is globally of least concern. Current population trend is considered stable.

Destruction of wetland habitat is the greatest threat to the species, so it was my delight to discover Least Bitterns at municipal water treatment wetlands. So, after years of hearing and not seeing this diminutive bird I now often spot a Least Bittern clambering in the cattail and bulrush reeds at a local water treatment pond. My work on the Salt and Gila Rivers west of Phoenix has highlighted the value of the City of Phoenix Tres Rios Wetlands to a community of secretive marsh birds that in addition to Least Bittern include Virginia Rail, Sora, and the endangered Yuma Ridgway’s Rail.

The photograph of the Least Bittern for this field note is a bird that has a mate and nest in bulrush reeds adjacent to a heavily traveled walkway at the Gilbert Riparian Preserve, Arizona’s smallest Important Bird Area. The wetlands are part the city water treatment facility.

A distinctive trait of Least Bittern is the habit of perching sideways on a reed stalk, often at eye level. I am amazed at how well the bird melts into the greenery. Like its larger relative, the American Bittern, this bird will “freeze” in place, head and bill pointing to the sky, making detection that much harder. They will even sway to resemble reeds blowing in the wind.

The nest is an elevated platform of crimped and bent over reeds with a canopy built by the male, often while the female watches. A pair will lay 1-6 blue/green eggs, with one egg laid each day. Both parents share incubation and raising the young. The young are semi-altricial, meaning they open their eyes within minutes of hatching and able to grasp vegetation and assume a “bittern stance” on a reed stem away from the nest by the 5th day. Young are able to fly after a month. An interesting nest-keeping trait is the adults poke holes in the nest bottom to drain fecal matter and to aerate the nest. The diet is small fish and insects, also small crayfish, tadpoles, and frogs. The adults feed the young by regurgitating partially digested food, a common feeding technique of fish eating birds.

I am so happy that this tiny heron is tolerant of humans and has accepted our constructed wetland marshes for places to raise their young gives me hope. A secretive marsh bird that maybe is not so secretive!

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