Least Bitterns are the smallest member of the heron family. Their diminutive size allows them to move about in dense emergent vegetation, making them hard to see. A marsh bird through and through, they may be found in freshwater or brackish marshes provided there are sufficient cattails, sedges, rushes, or arrowheads for nesting.
Where to look for Least Bittern in the final year of the Atlas
Below is a map highlighting atlas priority blocks missing Probable or Confirmed codes for Least Bittern, but where the NY Natural Heritage Program has records of Least Bittern breeding in the past. Keep an eye (and ear!) out in these blocks for Least Bittern breeding behaviors! (Tip: Click on the icon in the upper right of the map to make the map full size and zoom in to your area.)
Top Tips
- Plan a dawn or dusk outing in May or June to a freshwater or brackish marsh, one with dense, tall vegetation like cattail or other reeds. Look for open-water areas and scan the edges for foraging birds while you listen for the soft coo-coo-coo song of males.
- Watch over the top of the marsh vegetation for birds flying from one location to another.
- If you are visiting a large wetland, try hitting the water with a canoe or kayak to access more remote portions of the marsh.
Where to Find Them
Least Bitterns are most frequently associated with large, freshwater cattail marshes interspersed with open water. They also use freshwater and brackish tidal marshes in the Hudson Valley and on Long Island.
They are locally common, especially in marshes greater than 5 ha (12.3 acres), such as at the Montezuma and Iroquois wetland complexes. They can be found scattered throughout much of the state, most frequently along the Hudson River and Great Lakes Plain.
It’s worth checking out any large freshwater or brackish marsh for these birds because they are often overlooked. It’s quite possible you could discover a new nesting location! See the current Atlas map.
Distribution of Least Bittern from the 2000-2005 Atlas.
When to Find Them
Least Bitterns usually arrive in April and May, breed from mid-May to mid-Aug, and leave by October. They time their breeding cycle around the emergence of aquatic insects, usually feeding young in June and July to coincide with peak insect season.
Breeding phenology plot showing the time of year each breeding behavior was recorded for Least Bittern based on Atlas data from 2020-2021 (266 observations). Standard box plots with median and first and third quartiles plotted. Based on raw, unvetted data.
Breeding Calendar
- Late-Apr to mid-May: Arrival, pair formation, and courtship
- Mid-May to mid-June: Nest building, egg laying, and incubation
- Jul-Aug: Fledglings, feeding young
- Sep-Oct: Departure
How To Find Them
Of course, getting to the appropriate habitat is only part of the puzzle; you also need to find the secretive bitterns. The best times of day to listen for them are early in the morning and again in the evening. They are most vocal during the nest initiation phase, which spans from mid-May to late July in NYS.
Look for open-water areas and scan the edges for foraging birds while you listen for the soft coo-coo-coo-coo song of males. Watch over the top of the marsh vegetation for birds flying from one location to another.
To maximize your chances of encountering Least Bitterns, consider taking to the water. Going out in a kayak or canoe can greatly increase the amount of area you can cover and get you closer to dense vegetation. Note that you are unlikely to find them or their nests, though it can happen.
Keep in mind that a Probable status is perfectly acceptable and likely the highest level you will get for this species. There is no need to disturb the birds unnecessarily to try and Confirm them.
Behaviors To Look For
The cryptic nature of Least Bitterns means many of their breeding behaviors are difficult to observe. Raising a breeding status beyond Possible requires intention and a bit of luck. If you do hear one, return a week later to bump the code up to S7, which is Probable breeding.
- Singing. Like other secretive marsh birds, it is important to be familiar with their vocalizations. Listen for the coo-coo-coo song given by males to advertise their presence. The cooing is reminiscent of Black-billed Cuckoo, but lower and less musical, and should be recorded as S (singing bird). Females may respond to males with a ticking call, and it’s thought that a gack-gack contact call might be given from the nest. Juveniles can be quite vocal and by one week old give shrieks. Female and juvenile calls should not be coded as song.
- Pairs. Code P (pair) should only be used if a male and female are together; two bitterns calling are not a pair. Note that courtship is undescribed for this species.
- Nest building. The nest is built using vegetation at the site, so you won’t see them carrying nesting material. Instead, use code NB (nest building).
- Feeding young. Adults feed their chicks through regurgitation, so you won’t see them carrying food, and code FY (feeding young) should only be used after the chicks have left the nest.
- Nest with young. Chicks should always receive code NY (nest with young) when they are in the nest, regardless of other behaviors.
- Recently fledged. Finding a Least Bittern in juvenile plumage is insufficient evidence for code FL (recently fledged young). There must be additional characteristics indicating that it is indeed recently fledged, such as down still on the feathers, begging behavior, or agitated parents.
Life History
The bright, buffy tones of Least Bitterns are visually distinctive, particularly in flight on the outside of the wings. Females appear browner and less contrasty than males do; the male’s black crown and back is largely responsible for this dimorphism. Juveniles resemble females but have a paler, browner crown and heavier streaking on the throat. An extremely narrow body permits this bittern to pass easily through tangled vegetation.
When encountered, Least Bitterns typically burrow like rodents through dense vegetation, fly away weakly over marsh vegetation with legs dangling, or “freeze” with their bill pointed upward, feathers compressed, and eyes directed forward.
Their nest is an elevated platform with an overhead canopy built of emergent aquatic vegetation and sticks. Nests are built high enough above the water to avoid flooding during storms, but low enough to be well hidden by vegetation from above. Males do most of the nest construction and will continue adding material to the structure throughout incubation. As the nest’s brood grows, the parents will use their bills to create holes in the nest for fecal matter to drip through. They also build small foraging platforms at rich feeding sites, catching fast-moving prey (mainly small fish and insects) with their small, thin bills.
Aside from nest building, males and females appear to help equally in rearing the young. Four to five pale blue or green eggs are laid at a rate of one per day; incubation begins after laying the first or the second egg. Both adults participate in incubating the clutch for 17–20 days. Since incubation begins when the first egg is laid, hatching is asynchronous and takes three or four days.
Chicks usually leave the nest when they are around thirteen to fifteen days old. They will stick close to the nest site, not traveling much more than 50–100 feet from where they hatched. Parents feed their offspring by regurgitation, and will continue to care for their chicks until the chicks are able to fly at about a month old.
Most Least Bitterns leave New York in September but some remain into October and November on Long Island.