Zigzag Heron

Zebrilus undulatus (Gmelin)

Ardea undulata Gmelin, 1789, Syst. Nat. 1 (2), p. 637: Cayenne.

DESCRIPTION
The Zigzag Heron is a small, short tailed, black crested, cryptically colored dark heron, generally occurring in dense South American forests.

ADULT: The Zigzag Heron’s head and neck are mottled grey with a black crest forming a prominent mane giving the bird a thickheaded appearance (Davis et al. 1980). The bill is relatively short and dark. It may be all black, but they may also have a black or grey upper bill and horn or yellow lower bill. The irises are yellow and the lores are grey to black. The brown and black back and upper wings are barred and speckled and with buff white vermiculation. Under parts are buff cream, heavily barred, speckled and streaked with black. The legs are grey horn with yellow toes. The upper part of the leg is nearly fully feathered.

VARIATION: The sexes are alike. There appears to be considerable individual variation that requires further study.

JUVENILE: The juvenile plumage differs from the adult. The immature bird has a rufous forehead, sides of head and neck. The bill is yellow green. It is paler on the back with more rufous zigzag barring.  The under sides are tawny, with sparse indistinct black stripes. The legs are yellow green.  The immature plumage was once thought to be a rufous phase.

CHICK: The chick has yellow cream or white down.

VOICE: The “Kreeik” call is the alarm call. It is a “kreeik” becoming a “krik”  (Matthews and Brooke (1998). The “Oooop” call is a hollow hooting used by adults, to which birds responded  aggressively on playback (English and Bodenhorst 1991).The “Ahnnn” call also is a distance communication call given most frequently by juveniles. It too is a call also responded to aggressively by both adults and juveniles and used in chorusing between individuals (Ascanio-Echeverria and Davis 2000). The “Oooop” and the “Ahnnn” calls are also combined and given in a series. These are loud long carrying calls, such as would be expected in a forest heron, and appear to be recognition, distancing and very likely territorial calls.

WEIGHTS AND MEASUREMENTS: Length: 28-30 cm.  Weight: 123 g.

FIELD CHARACTERS 
The Zigzag Heron is identified by its small size, dark vermiculated color scheme, and cryptic habits. It flies in an indirect weaving manner (Lyles pers. comm.). It sits uncharacteristically for a heron in a perched posture, rather horizontal with the neck pulled in (Davis et al. 1980). It is distinguished from an immature Green Backed Heron and small bitterns by the white barring on its black back.

SYSTEMATICS 
The systematic position of this species has been a matter of great debate, being moved about taxonomically to reflect closer affinities to the tiger herons or to the bitterns. Recent molecular studies revealed the Zigzag heron to be a bittern (Sheldon et al. 1995). In addition to the biochemical evidence, it similar to bitterns in having 10 tail feathers (unlike the 12 of typical herons), scutellate tarsus, a long inner toe, and white eggs (not splotched like tiger herons). The Zigzag Heron’s habit of using thorns in the nest is similar to the habits of two forest bitterns, the African Dwarf Bittern and Black Bittern. But it also is similar to some of the tiger herons in having three powder down patches, rather than two in bitterns. It is encouraging to have the lineage of this long time problem heron resolved.

RANGE AND STATUS 
The Zigzag Heron occurs in northern South America.

BREEDING RANGE: The breeding range has been poorly known, with reliable sight records and museum records providing limited information. However, the known range has increased substantially since Hancock and Kushlan (1984), such that it is now clear that it encompasses the Amazon and Orinoco river drainages.  It occurs in east Colombia  (Arauca, Vaupés), Venezuela (Montagas, Cojedos, Apure, Delta Amacuro, north Bolivar, south Amazonas), the Guianas, Brazil (Amazona, Mato Grosso), north Paraguay, Bolivia (Beni, Santa Cruz), east Peru (Loreto, Madre de Dios), east Ecuador ( Napo, Pastaza) (Davis et al. 1980, Scott and Carbonell 1986, Mathews and Brooke 1988, English and Bodenhorst 1991, Robbins et al. 1991, Biazis pers. comm., Lyles pers. comm., Ascanio-Echeverria and Davis 2000,  Morales 2000).

MIGRATION; As far as is known, the Zigzag Heron is sedentary.

STATUS: Nothing is known about population sizes of this species but it is now understood to be widely distributed in the forests and wetlands of north South America and it is increasingly observed as it is now being expected by ornithologists and ecotourist guides.

Zigzag Heron

HABITATS 
Its primary habitat is tropical forest near and along the edges of rivers and oxbow lakes (Davis et al. 1980, Mathews and Brooke 1988, English and Bodenhorst 1991). It specifically occurs along slow moving rivers and streams lined with thick overhanging trees, exposed roots, and fallen logs, from which it can fish. It uses this habitat within undisturbed tropical forest and also the narrow gallery forests of the llanos (Ascanio-Echeverria and Davis 2000). A second habitat it uses is the edge of depression ponds, river pools and oxbow lakes, surrounded by either trees or grass. Although observations are few, it appears that a third principal habitat is the forest floor. It occurs from the flat lowlands to about 200-500 m.

FORAGING
It has a diversity of feeding methods (Davis et al. 1980, Mathews and Brooke 1988). Most typical is feeding by Standing on a perch above the water on branches, logs, or roots. It stands with its neck retracted. When it sees a fish, it often wags its tail back and forth, bends more horizontally, extends its neck and then lunges, while holding onto the branch. When it lunges vertically it depresses its tail and catches the branch to provide additional support.  It also uses Standing while wading in the water. It Walks slowly in the water, along the shore or on the forest floor. While Walking or wading, its capture is Pecking at the surface rather than Stabbing. It has also been described as hopping along the ground on the forest floor (Scott in Sick 1993). It uses Standing Flycatching from a Bittern Posture to catch flying insects.

It feeds during the day, particularly at dawn and dusk when it also calls. When not feeding, particularly midday, it perches 5 – 10 m high in trees, where it is most often seen standing, preening and tail flicking. When perched its has been observed to jump into the air and turn 180o on its perch (Davis et al. 1980).

It eats small fish and insects. Probably its diet is varied, depending on what is available.

BREEDING
The Zigzag Heron breeds April- July in the west of its range. It nests solitarily in trees near and overhanging water. There may be other herons nesting nearby, particularly the Rufescent Tiger Heron. The nests are shallow round platforms, averaging 17 X 22 cm (English and Bodenhorst 1991). They are made of twigs and often incorporate thorny branches along the periphery. Nests are placed low in the trees or bushes next to the water, 1.5 – 3 m high. They may be low enough to be washed away in unexpected flood events.

Nothing has been reported on courtship. The egg is white and unspotted. Evidence so far suggests that the species lays only one egg, which would be very intriguing if it turns out to be so.  There is evidence of multiple clutches in a year (English and Bodenhorst 1991). 

Incubation time has not been recorded.

Both adults attend young. One adult is usually at the nest until the young are larger than they are usually not in attendance, probably because of the need to feed. When disturbed at the nest the adult assumes the Bittern Posture.  The plumage of the chick changes with age (English and Bodenhorst 1991). Chicks darken to rufous on face and back; bill and legs are yellow green and irises yellow. Older juveniles become uniformly yellow cream with grey wings and no rufous. The head had neither white of the hatchling nor black of the adult.  There is no information on breeding success other than on individual nests.

POPULATION DYNAMICS
Post nesting, nothing is known of the population biology or demography of this species.