Stripe-backed bittern

The stripe-backed bittern (Ixobrychus involucris) is a bird species belonging to the family Ardeidae, which includes Herons, Egrets and Bitterns. Commonly found near freshwater swamps, marshes, lake shores and streams, stripe-backed bitterns span over a wide distribution that divides into the northern and southern populations, spanning on both sides of the Andes. Although increasingly recognized and researched worldwide, much life history details are lacking concerning this species, which remains understudied compared to its similar looking sister clade, including the least bittern (Ixobrychus exilis).

Taxonomy
The stripe-backed bittern is a member of the genus Ixobrychus in the Ardeidae family. This genus comprises nine extant small wading bird species distributed globally. This species of bittern was formally described in drawing by Vieillot in 1823, whereby a binomial name was attributed to it; Ixobrychus involucris.

Description
Stripe-backed bitterns are very small, averaging from 28 to 35 cm in length. Their pale brown back is ornamented with both black and yellowish-beige streaks, the latter also running along their broad wings. The colour patterns on their back complements the ones on their wings as to suppress obvious contrast between the two. Black stripes continue their way on the darker brown crown, while the underbelly is lighter brown and striped with white. As for the beak, the colour can differ from pale green to orange. This bittern species exhibits no sexual dimorphism, making it difficult to distinguish males from females. Furthermore, juveniles are not phenotypically described in the literature. Discriminating gender and age for this species has proved to be a challenge, so has the simple quest to even observe them in the wild. Stripe-backed bitterns tend to adopt an upright stance in which they “freeze” in place to observe their surroundings, making it more efficient to hunt their preys. When threatened, they respond by pointing their neck and bill skyward. This augments the difficulty to spot them easily. Following a solitary lifestyle, chances of finding them in groups are scarce but sights in pairs have been reported in the past.

Similar species
The stripe-backed bittern is easily confused with the least bittern. The most striking difference consist of the contrast between the dark coloured, completely streak less back of the least bittern, compared to its patterned wings. Least bitterns also differ by the rufous coloration on the sides of their head, also present on the neck of females.

Distribution
Displaying a disjunct, yet wide distribution, stripe-backed bitterns are found on each side of the Andes. A northern population is distributed in large patches across South America, located in Colombia, Venezuela, Guyana, Suriname, French Guiana and the island of Trinidad, while a southern population is found in central and southern South America; in Paraguay, Uruguay, Argentina, Chile, southern Brazil and Bolivia. In 2018, the presence of stripe-backed bitterns have also been confirmed in northeastern Ecuador, as well as in southeastern Peru.

Habitat
Freshwater sources are favoured as habitats for stripe-backed bitterns. Ideal ecological niches consist of highly vegetated reed beds, swamps, rushes, lakes and mountain streams, marshes, and rice fields. Such aquatic environments are essential in food acquisition, and in requirements for nest building.

Diet
The most prominent food source for stripe-backed bitterns, comprising half of their overall diet, are insects. More specifically, they preferably consume insects from the orders Odonata, Coleoptera, Hemiptera and Orthoptera. The remaining half of their food intake is sourced from small fishes, crustaceans, and arachnids. Stripe-backed bitterns are diurnal birds. They tend to intensify their activities such as active hunting either early in the morning or in the afternoon, prioritizing rest at noon and at sunset. They have been recorded to feed either alone, or in pairs. Night feeding has also been reported, but only scarcely.

Reproduction
The reproduction cycle and breeding details of stripe-backed bitterns have not been extensively studied, and much of it is still unknown. What we do know is that this species is not a strong flier, and only does fly across short distances. Their nests are consequently not found high up in trees. Stripe-backed bitterns makes small nests of tightly bound reeds and stems, which are found above water level, among reeds. Incubation period for eggs is unknown, and breeding seasons appear to vary widely based on location.

Eggs
Stripe-backed bitterns have not been represented widely in the literature, especially due to the rarity of sight, as well as the scarcity of information concerning many of their life history traits. However, one topic has often resurged throughout history; the evolutionary explanation for stripe-backed bitterns’ distinct egg colour.

Ardeidae is a family of birds known to yield eggs of either elliptical or spherical shape, which are often pale blue in colour. The translucent layer of their eggs, combining the commonly called cone and palisade layers, allows a glimpse into the blue or white inner shell. The eggs of stripe-backed bitterns, uniquely, exhibit pale blue cone and palisade layers, covered by an outer translucent green layer. Although recurrent observations/descriptions have been published, the evolutionary explanation for such a unique egg coloration within the Ardeidae bird family, remains unsolved to this day.

Vocalization
Sounds have been elaborately described and are sometimes the best way to determine the presence of stripe-backed bitterns, emitting calls through distinct, low-pitched ooks, or through gargling. Territorial or warning vocalizations consist of four low-pitched short “huu” or “ook” notes, followed by a second type of vocalization; a faster and higher-pitched “g’u’u’u’a’a” gargle.

Status
This bittern is considered to be of least concern, due to its wide range, and its large, stable population. As per information gathered by the IUCN Red List of threatened species, stripe-backed bitterns do not receive much conservation efforts, nor do they constitute a priority for outreach education programs, as their status is well anchored in the Least concerned section and is not foreseen to switch to a Vulnerable status anytime soon.