Family THRESKIORNITHIDAE Poche, 1904

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Introduction

Worldwide within the Threskiornithidae there are 35 species in 13 genera, five of which occur in Australia. Traditionally the family has been divided into two subfamilies (Threskiornithinae and Plataleinae) although this arrangement has been debatable. All are large, long-necked and long legged; in Australia, ibis (Threskiornis and Plegadis) are mainly white, or partly black and white or glossy brown, the face or neck having bare skin. All ibis species have a long, rounded and decurved bill. Spoonbills (Platalea) are white, with feathered heads and soft parts black or yellow; the bill is long, flattened and spatulate. Threskiornithids have long, broad wings and short tails, making them capable of strong flight andable to soar through the use of thermals; such flights are often interspersed by gliding.

In flight, threskiornithid necks and legs are extended and flocks often form V-shapes or long lines in which their flapping actions are in unison. In all species the feet are anisodactyl while the hallux is reduced. The family is widespread in tropical to temperate zones throughout both the New and Old Worlds. All members exhibit strong movement patterns which are influenced by rainfall, nesting sites or insect irruptions.

The two Australian Threskiornis species feed in both wet and dry habitats (freshwater or estuarine, swamps, marshes, flooded agriculture, rice paddies, river and lake edges, dry pasture, grassland); whereas Plegadis and Platalea prefer shallow flooded pastures or calm to slow moving water and pondage where aquatic and emergent vegetation is either sparse or absent. ll feed on crustaceans, molluscs, aquatic insects, insect larvae, small fish and frogs in wetlands while ibis take swarming insects such as grasshoppers, locusts, insects.

There are two distinctive feeding methods: ibis use their decurved bill to probe silt or soil; spoonbills differ by wading through shallow water sweeping the bill through water and silt and filtering food through their spatulate bill. They are generally silent although in flight or at the nesting colonies their vocalisations consist of a series of harsh, guttural or grunting calls.

Nesting may occur annually although it is generally associated with rainfall. All species nest colonially, often including mixed species, the monogamous pairs forming temporary bonds at the nest site. Such pairing may include a display at the nest; this nest is strongly defended by either or both birds. The nest is constructed by both adults using sticks placed in low trees or reeds usually over temporarily flooded areas or permanent wetlands. This nest is a flat or platform nest where a clutch of between two and five oval to elongate-oval eggs is laid. The base egg colour varies from a lightly chalky white (Platalea and Threskiornis) to a pale blue-green (Plegadis). Surface patterns are lacking apart from occasional nest staining.

Both adults share the responsibility of incubation, feeding and care of their young. Incubation lasts approximately 21–29 days and the newly hatched young are nidicolous and semi-altricial. After caring for their young at the nest for the first week both adults leave them in order to procure food. The young often leave the nest and wander locally but always return to the nest for feeding by regurgitation. They are capable of flight between 30 and 50 days. Nest failure is often encountered due to a failure of food availability or the loss of the surrounding wetlands.

Database Notes

Threskiornithidae Poche, 1904 given precedence over Plataleidae Bonaparte, 1838 and Eudocnimidae Bonaparte, 1854 as the name for the family of ibises and spoonbills: ICmZN Opinion 1674

General References

Christidis, L. & Boles, W.E. 2008. Systematics and Taxonomy of Australian Birds. Melbourne : CSIRO Publishing 288 pp.

History of changes