THE SOCIABLE WEAVER,PART 3: BREEDING BIOLOGY AND MOULT

Maclean, G. L. 1973. The Sociable Weaver, Part 3: Breeding biology and moult. Ostrich 44: 219–240.

Rain or some associated phenomenon is the principal Zeitgeber releasing breeding in the Sociable Weaver. The species does not breed in the absence of rain. The same nest chambers are used for breeding as are used for roosting throughout the year. The Sociable Weaver is monogamous. The clutch size varies from two to six eggs, larger clutches being more common after good rains than in relatively poorer breeding periods. Food supply may therefore be the proximate factor regulating clutch size. Replacement clutches are not necessarily smaller than first clutches. The mean clutch size within a breeding period decreases with an apparent decrease in food supply. The parents share parental duties about equally. Up to four successive broods may be raised in a single breeding period; a breeding period may last up to nine months and may occur at any time of the year according to the somewhat erratic rainfall which averages about 226 mm per year in the study area.

First broods help their parents to feed later broods; fourth brood chicks may therefore be fed by as many as 11 birds (nine young and two parents). This has survival value especially toward the end of a breeding period when food is scarce. Of similar value is the habit of starting incubation with the first or second egg of the clutch; in a relatively poor season older chicks will survive while younger ones will starve, thereby effectively and quickly reducing brood size. Young birds moult into adult plumage at four months, but do not normally leave the home colony. The sexes are indistinguishable at all ages, but there is an approximate ratio of eight males to five females in the study area.

Wing moult is slow: each remex takes about a month for replacement. Body moult occurs within the space of a month, usually after rain while the birds are breeding. Primary remiges are moulted proximo-distally from 1 to 9; secondaries are moulted disto-proximally from 1 to 6. Body moult is antero-posterior with the dorsal surface slightly in advance of the ventral surface.     

THE SOCIABLE WEAVER,PART 4: PREDATORS,PARASITES AND SYMBIONTS

Maclean, G. L. 1973. The Sociable Weaver, Part 4: Predators, parasites and symbionts. Ostrich 44: 241–253.

The main nest predator of the Sociable Weaver in the Kalahari sandveld is the Cape Cobra Nala nivea. This snake causes great losses of eggs and chicks; one cobra may eat the contents of an entire nest mass at one feed. Another nest predator which causes smaller losses of eggs and chicks but great destruction to the nest masses is the Honey Badger Mellivora capensis. These are the only two nest predators on the Sociable Weaver in the study area. Predators on adult Sociable Weavers include several birds of prey and some small carnivorous mammals.

Adult Sociable Weavers have few ectoparasites and hardly any Mallophaga. A common ectoparasite on the legs of chicks is a blood-sucking Dermestes larva, which appears not to be harmful. The only endoparasite found was the nematode, Diplotriaena ozouxi, which infected the abdominal air sacs of the adults.

The nest material of the Sociable Weavers' communal nest masses was inhabited by a wealth of invertebrate animals and a few harmless reptiles such as skinks and geckos.

Some of the chambers in a Sociable Weaver nest mass may be taken over by other species of birds. Most of these, such as Redheaded Finches Amadina erythrocephala, use the chambers for breeding purposes only, but the most important avian symbiont, the Pygmy Falcon Polihierax semitorquatus, is a permanent resident in the chambers. The presence of Pygmy Falcons is resented by the weavers but the falcons may help to keep snakes away from the nest mass. Adult Sociable Weavers are not normally preyed on by Pygmy Falcons, although the falcons may occasionally take young weavers in the nest chambers.

The tops of the nest masses may be used as nest sites by the Giant Eagle Owl Bubo lacteus. Barn Owls Tyto alba may use cavities in the superstructure of nest masses for roosting in. Neither of these owls appeared to prey on the weavers.     

THE SOCIABLE WEAVER,PART 2: NEST ARCHITECTURE AND SOCIAL ORGANIZATION

Maclean, G. L. 1973. The Sociable Weaver, Part 2: Nest architecture and social organization. Ostrich 44:191-218.

Sociable Weavers build nest masses in a number of indigenous tree species (especially on Acacia giraffae branches) and on artificial nest sites like telephone poles. They never build in exotic trees. Nest masses are built of grass straws and roofed over with a superstructure of coarser material such as thorn twigs. The grass substructure contains the nest chambers which do not interconnect. The substructure may be divisible into two or more levels, each forming a social unit comprising the birds inhabiting it.

Each social level of birds is confined to its own structural level at all times, but a bird may roost in any chamber within its own level. The superstructure is not divisible into social units and any bird may build or perch on any part of the superstructure. Movements of birds from one colony to another are rare. The colony at one nest mass leaves the nest at about sunrise in summer, a little later in winter, and flies to the feeding grounds; the birds return to the nest mass for a siesta lasting from about 10:00 hours to 14:00 hours in hot weather, less than this in cool weather. They depart again for their feeding grounds until about sunset.

The internal temperature and RH of the nest chambers were not found to be significantly different from ambient temperature and RH when ambient temperatures were > 21,7°C. At ambient temperatures < 26,7°C the RH of the nest chambers was significantly lower than ambient Rh, but temperatures were not significantly different during the day.     

NEST BUILDING AND NESTING BEHAVIOUR OF THE SOCIABLE WEAVER PHILETAIRUS SOCIUS

The behaviour of the Sociable Weaver Philetairus socius was studied briefly in South Africa in 1969, and more extensively in aviaries in southern California for over three years (1972-75). The species was bred successfully in captivity, apparently for the first time. Building of the compound nest in nature is reviewed in the light of preceding accounts, both by ourselves and by others. The optimal preference for nest materials was experimentally determined. Male and female appear identical and there seems to have been no previous detailed account of the behaviour of Sociable Weavers based on distinctively colour-ringed individuals of known sex. Among the sex differences in behaviour are the amount and nature of building, dominance, malespecific song, and sex differences in relative share of incubation and in the care of nestlings and fledglings. In addition, we describe in detail for the first time certain rarely observed behavioural acts including precopulatory courtship and copulation. Allopreening and water bathing are very rare in this species, and the birds do not take dust baths.     

THE THERMAL SIGNIFICANCE OF THE NEST OF THE SOCIABLE WEAVER PHILETAIRUS SOCIUS: WINTER OBSERVATIONS

The Sociable Weaver of southern Africa builds the largest of all birds' nests. Individual nests contain many chambers and may be occupied by a colony of a hundred or more pairs. Most aspects of the biology of this species are linked, either directly or indirectly, to the nest and its construction. One readily demonstrable function served by this enormous nest is the amelioration of the impact of low environmental temperatures.     

THE THERMAL SIGNIFICANCE OF THE NEST OF THE SOCIABLE WEAVER PHILETAIRUS SOCIUS: SUMMER OBSERVATIONS

Extending a previous study of the thermal significance of the nest of the Sociable Weaver carried out during the winter, we measured temperature and humidity in the matrix and chambers of a large nest of this species in the Kalahari Gemsbok National Park, South Africa, during the austral summer of December 1973. Air temperatures outside the nest ranged from 16 to 33.5°C but temperatures in occupied chambers varied over a range of only 7 or 8°C and remained well within the zone of thermal neutrality for a passerine bird of this size. Compared to outside air temperatures, those within the nest matrix were lower during the day and higher at night. Thus, the nest ameliorates the effects of external temperatures and allows maintenance inside the chambers of a range of temperature favourable to the birds. In winter we found up to five roosting adults per chamber, with some chambers left empty. In the same nest in summer we found no more than two adults per chamber but virtually all chambers were occupied. The principal mechanism for maintaining chambers within the zone of minimal energetic cost is changes in the number of birds in the nest chambers at night. Humidity inside the occupied and unoccupied chambers was somewhat higher in the former but always less than that of outside air in both cases. Air movement through the desiccated nest materials causes uptake by these materials of most of the water vapour introduced by the birds, and this moisture is dissipated to the outside during the day so that the nest remains dry. The highly social and colonial habits of the birds and their year-round occupancy and maintenance of the nest favour a system of opportunistic breeding that may be initiated by rainfall at any season. Larger nests provide the most favourable environment for energy conservation and successful reproduction. Even the largest nests, however, do not prevent predation during the warm season by snakes such as the Cape Cobra, which may consume all the eggs and young in all the chambers of a large nest. The effects of such heavy predation may be offset by the birds' capability for breeding during times too cold for reptile activity. It seems likely that in smaller nests such as those on telephone poles, lack of predation would favour summer breeding while thermal problems would limit breeding success in winter. In larger nests, breeding success may be lower in summer because of predation and higher in winter when reptile predation is lacking and thermal problems are minimized by the nest structure. The large nest not only makes possible the success of the Sociable Weaver in desert areas, but the nest could only exist in such areas and the species' range is thereby restricted. Higher humidity and heavier rainfall would cause fermentation within the nest mass, loss of its thermoregulatory advantages, and ultimately its decomposition and destruction. Therefore, the unique nesting system of the Sociable Weaver appears to be initially self-reinforcing and ultimately self-limiting.     

THE FOOD AND FEEDING APPARATUS OF THE BLUE DUCK HYMENOLAIMUS

This paper describes a preliminary study of the feeding behaviour of the territorial Blue Duck. The bird is unusual in possessing lateral bill flaps and, in a number of head and skull characters, differs from a typical dabbling duck. Herbst's corpuscles are absent from the outer 60% of the width of the flaps and this is thought to indicate that any tactile function is subsidiary. It is tentatively suggested that an additional function is protective, and that the flaps cushion the jaw edges during vigorous food searching beneath stones on rocky stream bottoms. Food taken in summer was analysed from remains in droppings and from gut contents and was found to consist of aquatic insects, principally case-living and free-living caddis larvae, and algae. Some short-term variation in the diet was noted. It is suggested, also tentatively, that the decline in Blue Duck numbers since the 19th century has been associated with the introduction of trout which are potentially direct food competitors. Further ecological research on this unique species is required.     

STUDIES OF THE PURPLE HERON,PART 2: BEHAVIOUR PATTERNS

Tomlinson, D. N. S. 1974. Studies of the Purple Heron, Part 2: Behaviour patterns. Ostrich 45: 209–223.

Some aspects of Purple Heron Ardea purpurea behaviour have been covered although the study is not complete. Further observations are needed, especially on pair formation, nest building and copulation. Many of the displays described for the Ardeidae are clearly homologous and only slight differences exist. The throat puffing component in the Aggressive Upright Display appears unique to A. purpurea as does the clappering and sudden retraction of the neck in the Stretch Display. Birds raised in captivity provided valuable information on agonistic behaviour patterns and the behaviour of young.     

BIOLOGY OF THE BANK CORMORANT,PART 3: FORAGING BEHAVIOUR

Cooper, J. 1985. Biology of the Bank Cormorant, Part 3: Foraging behaviour. Ostrich 56: 86–95.

The Bank Cormorant Phalacrocorax neglectus, endemic to southern Africa, is primarily a solitary inshore forager. Bank Cormorants forage Primarily on the bottom among kelp beds but also may forage over shingle or coarse sand substrates or in midwater. Breeding birds forage up to 9 km from their colony. Little is known of foraging depth but birds may dive as deep as 28 m. Mean dive duration was 44,9 s and ratio of dives to surface rests was 2,18. In most cases prey is swallowed under water, presumably to avoid kleptoparasitism. Bank Cormorants foraged during daylight hours from before sunrise to after sunset. Birds did not forage in exceptionally rough seas. Mean female foraging bout duration (84,3 min) was significantly longer than that of males (68,4 min) in breeding individuals. Breeding males undertook significantly more foraging bouts (3,47 boutdday) than did females (3,02 bouts/day). No significant differences were found between the sexes when total time spent foraging/day by breeding birds was compared. It is not clear why males foraged more often, but for shorter periods, than did females, but the differences may be related to sexual dimorphism, males being larger than females.     

THE FOOD AND FEEDING HABITS OF GOLDENEYE BUCEPHALA CLANGULA IN GREAT BRITAIN

The food and feeding habits of Goldeneye Bucephala clangula are described, based on the analyses of 51 stomach contents and a brief review of the literature. The type of food taken depends on the type of habitat being used, the availability and size of the food items and on the methods of feeding—though animal material largely predominates.
In estuarine and coastal areas, small crustaceans and in particular Carcinus maenas figured highly in the diet, with some molluscs and small fish. In two brackish-water feeding birds, seeds formed the bulk of the food, some caddis-fly larvae also being taken. In freshwater feeding birds insects predominated, particularly Trichoptera larvae, chironomid larvae and the adults of Corixa spp. Some crustaceans ( Asellus, Gammarus ) and small molluscs were taken and occasionally small fish. A small amount of plant material was taken in the form of seeds, mainly Potamogeton spp.
The relation between Goldeneye and fishing interests is briefly discussed and it is concluded that fish normally form a small part of the diet and it is unlikely that any serious depredation of fish stocks is common.     

THE BREEDING OF HEUGLIN'S MASKED WEAVER AND ITS NESTING ASSOCIATION WITH THE RED WEAVER ANT

Grimes, L. G. 1973. The breeding of Heuglin's Masked Weaver and its nesting association with the Red Weaver Ant. Ostrich 44: 170–175.

The breeding season of Heuglin “s Masked Weaver Ploceus heuglini at Legon began in the latter half of the major dry season in January and February; continued through the main wet season and ceased in late August and for the rest of the year. The Accra colony had a similar breeding activity but in addition was active for a short period in November which had ended by mid-December. Although the majority of males formed colonies in which the number of males ranged from two to twenty, solitary breeding occurred equally frequently. Most males within the Legon colony, and possibly this is true for all colonies, were polygynous while most solitary males were monogamous. More nests were built by each male than the number of females involved in the polygyny.

A close nesting association with the Red Weaver Ant Oecophylla longinoda existed and the data suggested that the weaver sought the ant. Two cases were found of nesting association with the wasp Belanogaster grisens.     

FOOD, FEEDING RATES AND BODY TEMPERATURE IN THE NESTLING HOUSE SPARROW PASSER DOMESTICUS AT OXFORD

Diet, feeding frequency and body temperatures were studied in nestling House Sparrows at Oxford. In the diet, studied principally from crop contents, dipterous flies, caterpillars and aphids were most prominent in early May, early June and July respectively, while bread and dry grain, apparently less suitable foods, were more evenly distributed throughout the breeding season but apparently acquired an importance when insect foods were scarce.
Adult House Sparrows increased the frequency of their feeding visits to the nestlings proportionately with brood-size for broods of 1–3, but for larger broods the feeding frequency did not increase. This abruptly achieved upper limit presumably gave rise to inadequate feeding of the larger broods (including the principal brood-size). The male's share of the visits decreased markedly in the later stages of the nestling period, corresponding to his initiation of a display directed at the female. His displaying at this stage probably allows the female to begin a further clutch soon after the previous nesting has finished.
Nestlings were unable to maintain a steady body temperature above the ambient air temperature outside the nest before nestling day 10½. Maintenance of a steady body temperature depends on the nestling's feather covering, which is effectively complete at 9½ days. Before this, temperature control is possible within a limited range of ambient temperatures.