South African Shelduck Tadorna cana Scientific name definitions

David G. Allan
Version: 2.0 — Published February 23, 2023

Behavior

Introduction

Many aspects of the basic behavior of this species are poorly known.

Locomotion

Information needed.

Self-Maintenance

Information needed.

Agonistic Behavior

Territorial Behavior

Intra-sexual fighting (chasing and pecking) has been noted for both sexes, and breeding pairs noisily drive off intruding pairs and individuals, with the female often taking the lead in such encounters (22, 1). During the incubation and chick-rearing periods, however, territorial defense is largely the preserve of the male (1). Mated females appear particularly intolerant of same-sex intruders (22, 2) and females in mated pairs within flocks can even be intolerant of other unpaired females in close proximity (1). The skewed sex ratio in favor of females may be responsible for both their active role in courtship (109) and their intense intra-sexual aggression.

Sexual Behavior

Mating System and Operational Sex Ratio

Monogamous, and pair bonds are probably long-lasting or permanent (1).

Despite being monogamous (109), information from large waterbodies shows that males are usually outnumbered substantially by females, i.e., 1.0:4.2 and 1.0:4.1, based on counts at two sites in Eastern Cape Province (22), 1.0:1.3 based on ringed birds (n = 209) and counts at Vogelvlei (102), 1:2 based on counts in the Goldfields area, Free State Province, (n = 4,224) (8), 1:1.2 in southern Free State (1), and 1.0:1.5 based on ringed birds at Barberspan (n = 375) (2). Recent counts at Barberspan (North-West Province) and Strandfontein (Western Cape Province) found a lower bias (1.0:1.1) (24). At one site (Bloemfontein sewage works, Free State) a sex ratio of 2:1 in favor of males was recorded based on counts (n = 3,019) (8).

A detailed review of sex-ratio data from counts at a large number of waterbodies from throughout the range statistically confirmed the preponderance of females (overall 1.0:1.2, n = 6,571), despite that juveniles of both sexes resemble adult males, which would tend to result in overestimates of males in some counts (109). Subsequently, however, it was found that, although sex ratio was significantly skewed in favor of females in birds ringed at Barberspan (1.0:1.1, n = 822), the sex ratio from count data was ostensibly significantly skewed in favor of males (1.1:1.0, n = 4,103) (110) . This was attributed to overestimates of males during counts due to the likely confusion of juvenile females with males.

This apparent female-biased sex ratio seems remarkable as it might be expected that the species’ unusual nesting habits would result in higher mortality among breeding females versus males, although it has also been suggested that burrow-nesting may be less hazardous than ground-nesting (109). It has also been suggested that intense competition for breeding territories and a need for vigorous territorial defense results in increased mortality among males. Information presented for ringed juveniles at Vogelvlei (1:4, n = 114) (102) and Barberspan (1.0:1.2, n = 28) (110), however, suggests that this biased sex ratio is present before either females or males attempt breeding. The shorter flightless period of females (due to their lighter body weight) may increase their survival compared to males, and may be responsible for the skewed sex ratio (2), but weight loss during wing molt apparently has little effect on survival and minimal mortality has been noted in molting flocks (9).

Courtship, Copulation, and Pair Bond

Unlike at other times of the year, when the birds spend most of the day sleeping or preening along shorelines, social activity is marked during the pre-breeding period and occurs throughout the day (1). Courtship behavior occurs on both land and water, and in the air, and is characterized by loud vocalizations by both sexes (22). It can be initiated by either sex, usually more frequently by the female. Birds pursue their mates, running at or around them in a crouched posture with their necks outstretched and their bills almost touching the ground, frequently raising their heads to utter a shrill call. The female is more likely to pursue her mate than vice versa, and similar chases and circling occur on the water. The male has been reported to bob his head in response to such attention from his mate. Copulation usually occurs on the water and a pair was observed to fly there to mate. The male initially swam rapidly around the female and then mounted her by standing on her back and held her neck in his bill, submerging her except for her head and neck. Copulation lasted just a few seconds and both birds subsequently washed and preened. A similar copulation sequence has been reported, but after mating the male was noted to again circle the female, continually and stiffly raise his head and neck up and back, then lower it down and forward (2).

Brood Parasitism by Conspecifics

Intraspecific brood parasitism is unknown in this species (111) and no parasitic egg-laying was found in 14 nests examined (112).

Social and Interspecific Behavior

Degree of Sociality

In non-breeding flocks, single birds typically stay in the center, with mated pairs at the periphery (25).

Nonpredatory Interspecific Interactions

Mated pairs are intolerant of the close proximity of Egyptian Goose (Alopochen aegyptiaca) (22). Several chicks at solar plant evaporation ponds were believed to have been killed by Cape Teal (Anas capensis) (89, 90).

Predation

Predators

Fully grown birds are taken as prey by Martial Eagle (Polemaetus bellicosus) (113) and Verreaux's Eagle (Aquila verreauxii) (114). Loss of chicks is extensive in Eastern Cape Province and is attributed to predation by freshwater marsh terrapins (Pelomedusa subrufa) (92).

Response to Predators

The South African Shelduck is notoriously shy and wary in its habits (66, 29).

Recommended Citation

Allan, D. G. (2023). South African Shelduck (Tadorna cana), version 2.0. In Birds of the World (G. D. Engelbrecht, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.soashe1.02
Birds of the World

Partnerships

A global alliance of nature organizations working to document the natural history of all bird species at an unprecedented scale.