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Home News & Events Latest Olive-headed Weaver - In search of the

Olive-headed Weaver - In search of the

Text and photographs by Dan Nuttall 
(taken from Africa Birds and Birding February 1998 Vol.3, No.1)

A first sighting is a rare pleasure, something that lingers in the minds of most birders. Today is no exception. What makes this particular sighting so indelible is my awareness that very few people have seen this species. But here, literally in hand, are 12 Olive-headed Weavers - an intriguing and rarely seen species. Courtesy of Robert Prys-Jones, I am sitting amidst the vast avifaunal collection of the Rothschild Zoological Museum in Tring, England. The collection forms a part of the larger faunal collection of the British Natural History Museum.  

It is here in Tring where I pick up the trail of some of the century's best known and pioneering birders. The specimens in front of me were collected by Jack Vincent and C.W. Benson over several expeditions spanning the period from 1932 to 1951. Each specimen tag bears a carefully written description which begins to tell the story of the Olive-headed Weaver: scientific name, sex, date of collection, location, altitude, habitat and morphological characteristics. The annals of history are their own kind of forest however, and the bird somewhat elusive within it. Geo-political boundaries have changed, names of cities and villages have changed - even the scientific name of the bird has changed several times.
    What I know for certain is that the birds in front of me were collected in Tete Province, Mozambique (then Portuguese East Africa) and Malawi (then Nyasaland); the names on their tags, Xanthophilus olivaceiceps and Symplectes olivaceiceps, are no longer in use. Currently the species is described as Ploceus olivaceiceps. The naming of the bird continues to be debated as there is the suggestion that some populations of the Olive-headed Weaver exist as a subspecies. The Atlas of Southern African Birds for example, offers the description of Ploceus olivaceiceps vicarius.
    As I have never seen the bird before, the challenge of piecing together the specimens and the written history begin to make the bird more real. I discover that the Olive-headed Weaver has previously been found in association with Brachystegia forests, is sometimes found in bird parties and is capable of existing at high elevations (to 1 560 metres above sea level).
    My research carries me into the present and once again my work is the beneficiary of those who have gone before me. It was P.A. Clancey who first discovered the bird in 1960 near Panda in the Inhambane Province of Mozambique. To my knowledge this was the first record of the species in southern Africa. The bird has also apparently been recorded in Tanzania and Malawi, although confirmation of these and more recent records in sub-Saharan Africa remains obscure. Based upon Clancey's The Birds of Southern Mozambique and Roberts' Birds of Southern Africa, it seems there is enough information to narrow my search.
    Beyond using the appearance of the bird, my search criteria will include attempting to find the nest, which is woven of Usnea, the straggly, string-like green lichen seen hanging from trees in both tropical and temperate forests. This lichen, often called a moss, is actually a composite organism made of an alga and a fungus, locked in a symbiotic relationship. The bedraggled appearance of this lichen has earned it the name 'old man's beard'.
    This glance backwards to the history of the Olive-headed Weaver is also the compass to the future and points my research in one direction - to the Brachystegia forests near Panda, Mozambique, to search for both birds and nests. One last question remains: has the Olive-headed Weaver been seen since the 1960s?
    As recent as Clancey's work seems to be, sub-Saharan Africa has changed drastically in the past 30 years. Since Clancey's work, Mozambique has achieved independence (in 1975), endured a 17-year civil war and now finds itself listed by the World Bank as one of the world's 10 poorest countries. The process of rehabilitation has been slow but steady. The civil war was particularly devastating to wildlife resources as both the incumbent party, Frelimo, and opposing Renamo forces used national parks as headquarters. During these periods camps were destroyed, landmines were laid and local wildlife resources were used to feed troops. It is difficult to assess the losses of wildlife, but an inventory of Marromeu park, for example, indicates an 89 per cent reduction in animal numbers.
    But the Olive-headed Weaver survived. In 1996, under the auspices of the Endangered Wildlife Trust, Vincent Parker confirmed the presence of the bird, again near Panda. Now there was no doubt in my mind - my first base camp would be set up in Panda. With the assistance of Vincent Parker I packed my tent, binoculars and field equipment, and left.
    Those first few weeks in the field were unforgettable. I had heard Paul Simon sing about 'African skies' but had always pictured the skies I had seen from my home in Canada. Africa changed forever my memory of starry nights. Here in Panda all I had to do was tilt my head back to be immersed in our galaxy - not just a diorama or proscenium painted with spots of lights, but milky whorls and sparkling eddys of the afterbirth of our universe.
    And for every star there was an insect or a bird. My academic exposure to biodiversity seemed but a single note when compared to the chorus of life around me. At night, while reading by flashlight in my tent, there was a constant staccato barrage of insects hitting the canvas roof and sides. I opened and closed the flap of my tent quickly to avoid this rain of chitinous bod- ies, gossamer wings and jointed legs.
    My first few days in the forest are a feast for the eyes. All these avian forms, colours and calls are new to me and it seems at times that the forest is conspiring to distract me with a kaleidoscope of movement and sound. The recipe for locating birds seems as simple as pointing my binoculars and establishing focus on some form of vegetation. At other times the forest seems strangely silent. But in no time my pencil is ticking off a record number of firsts and the list expands rapidly: Green Tinker Barbet, Racket-tailed and Broad-billed rollers, Bateleur Eagle, Red-billed Woodhoopoe, Trumpeter and Crowned hornbills, Rudd's Apalis, Mashona Hyliota, Mozambique Batis, Scarlet-chested Sunbird, Grey Waxbill.
    My encounters with the birds are by no means merely 'sightings', the birds seem interactive and unaware of my presence which allows for more intimate encounters. I sit within two metres of a Mozambique Nightjar which is also sitting on the ground and, while my eyes are wide open, the bird's are closed, completing her camouflage. A single nestling presses closely against her side. Later that afternoon for half an hour I follow a Giant Eagle Owl through the forest, awed at the bird's size. A pair of Red-winged Pratin- coles hover above me in flight, suggesting that their nest must be nearby. Eyes trained to the ground, I step carefully and am rewarded by finding a pair of beautifully camouflaged eggs. My base camp is a theatre of avian activity. Above me, in my 'shade tree', a Grey-headed Bush Shrike brings food to its young. In the adjacent tree a Grey Penduline Tit is nesting. Once, and only once, a Red-billed Helmetshrike presents itself while its cousin, the White Helmetshrike, passes by me frequently in noisy, chattering and seemingly oblivious groups. The forest is training my eyes and ears and slowly I am becoming attuned to my surroundings. But so far not one Olive-headed Weaver.
    The forest in this area is becoming a patchwork. I am traversing both mature Brachystegia forests and burned areas which are being prepared for planting of cassava, maize, pineapple and squash. The earth is warm and smoking beneath my feet and my face registers vertical strands of radiant heat as I pass each scorched tree trunk. Passage is easier now - no thorny scrub, no grasses, no hidden vines or branches to tangle my feet. The forest is more open, pared down and revealing. A pair of stark white eggs sit newly exposed in the now sooty ground. They are too hot to touch and I leave them in their ashen depression knowing no Phoenix will rise. At night red-orange eyes stare with Cyclopean intent as distant embers signal the boundaries of a transforming forest.
    And on the seventh day it finally happens. A single female alights in the canopy of a Brachystegia tree and begins foraging. It appears to me that she is examining me as much as I am examining her. And while she is nonchalant about my presence, I am in fervour over hers. Our introduction lasts less than a minute and she undertakes a long flight, disappearing into the lattice of tree canopies.
    Over the next few days my eyes become more adept at locating Olive-headed Weavers and my ears begin to discern their call from the chorus around me. Even with my newfound ability to see and hear these birds there are still many days when I see nothing. A month later, with the aid of my field assistant Nelson Manave, we have located a total of 18 nests of which four are active. These are the first nests to be recorded in southern Africa. Given the unique characteristics of these nests I have chosen to focus on the nest as an introduction to the Olive-headed Weaver.
    The nest is similar in form and size to that of the Spotted-backed Weaver Ploceus cucullatus, although the spout appears to be somewhat longer. All of the nests are located in areas which have not been burned in preparation for subsistence agriculture. All are composed of a single material, the lichen Usnea. The Olive-headed Weaver begins the nest by integrating it with existing long vertical strands of the lichen which are attached to a branch. This allows the nest to sway in the wind. Within the tree canopy the point of attachment of the nest is never the end of a branch but deeper within the canopy, along branches with a diameter that appears to be greater than a few centimetres. This is different from the nesting behaviour of some other weaver family members who attach their nests to reeds or to the ends of branches. Why would the Olive-headed Weaver evolve such a strategy?
    There are many possible answers. A first suggestion might be that if the nest is not at the end of a branch it is not as conspicuous, which thus decreases the risk of predation. However, all of my initial observations were based upon trees which were not in leaf. The nests were completely 'exposed'. At the same time I did not directly observe any examples of nest predation. It appears that even when it is situated in a tree without leaves, the nest is well camouflaged.
    A second answer might be that the placement of the nest deeper in the canopy may be an adaptation which allows the weaver to ensure a securely attached nest. This is because the ends of branches are areas of growth with a very small diameter - both factors which create a paucity of lichen. As branches mature and thicken they begin to provide a substrate for colonization by both flora and fauna. Ridges occur within thickening bark and an increase in branch diameter means an increase in the area of a relatively 'flat' surface on the upper side of the branch. These changes associated with increasing branch thickness allow for the collection of debris and attachment of epiphytes, lichens and associated fauna. It thus follows that larger branches possess more dense clumps of Usnea which, coincidentally, have longer strands and more points of attachment for a nest. Thus a nest placed deeper within the canopy is a more secure nest.
    A third mitigating factor may be that nest placement is a trade-off which minimizes risk of predation. Predation at the nest is a major cause of nest failure among passerines. In this scenario the bird must place its nest as far away from predators as possible, which would suggest the smallest and finest branches of the tree. At the same time the bird must move in towards the trunk to find suitable support from Usnea. Moving in towards the trunk of the tree, at any height, places the nest closer to potential predators. An optimal strategy might be to locate the nest in those clumps of Usnea which offer the greatest structural support (large thick clumps, with multiple points of attachment) and are farthest from the ground (higher up, as far as possible out along branches).
    A final factor may be microclimate. As mentioned previously, during initial data collection none of the Brachystegia trees were in leaf. This means each and every nest is situated in the upper reaches of the tree where it is directly exposed to the sun. It is possible that such a position allows the warming rays of the sun to assist in creating a temperature suitable for incubation. In the early morning, it is the upper branches of the tree which receive sunlight first. Assisted by this boost in temperature, adult birds may be able to leave the nest earlier than would be possible if the nest were much lower in the canopy. This means that the sun assumes some of the energetic contribution of the parents. A bird which can leave the nest earlier in the day may be able to provide more food over the entire day. Such a strategy would be adaptive over the long run, increasing the amount of food provided to nestlings. However, the converse must also be considered: if the nests warm easily, do they 'overheat' in the midday sun and how has the species adapted to counter this possibility? The insulative qualities of the Usnea, its tensile strength and the movement of air through the sides of the nest (cooling) would merit investigation.
    Beyond the strategies for nest placement, the nest itself possesses some special characteristics. Because the initial strands which are attached to the branch are alive, and additional strands are also living, the entire nest continues to live. This makes the 'living nests' of the Olive-headed Weaver unique within this family. How long the nests remain living is unknown at this point.
    The appearance of the nest is also unique. Given the inherent structural qualities of the Usnea, the nest's spout is less rigid than that created by the grass-weaving weavers. The spout has a somewhat collapsed appearance, looking like the open end of a dangling sock. In addition, the slightly flattened and loose nature of the siphon provides the adults with manoeuvring space as they fly directly into the nest. The central chamber has a compact, woven appearance while the trailing edge of the spout has loose strands of lichen which move in the wind and predispose the adults to timing their entrance to coincide with a lull in air movement. The thickness of the nest walls varies but is around two centimetres. Given the softness of the nest, it comes as no surprise that it is not lined with feathers or downy material. The nests are found consistently within the upper reaches of mature Brachystegia trees which have an average diameter of approximately 385 millimetres at a height of 1.5 metres above ground level. Trees containing nests are usually between 15 and 18 metres in height.
    Overall, this suggests that the Olive-headed Weaver requires mature stands of Brachystegia forest which has implications for conservation and resource utilization, particularly in forestry management practices. While these mature trees are ideal habitat for the bird, they also pose logistical problems in terms of research methodology: how may a researcher peer into a nest or observe fledglings? How would a researcher observe the nests when the trees are in leaf? Scaling the nest tree or ones adjacent to it to record observations may seem logical, but the abundance of trees blown over by the wind and the fact that large sections of trees periodically crash to the ground during the day suggest that such an approach may be difficult.
    In addition to nests being found in mature Brachystegia trees, the predominant vegetation in the nesting areas is also Brachystegia, while smaller quantities of Balanitacea spp. and Acacia spp. occur. The predominance of Brachystegia is consistent with the observations made by Vincent, Benson, Clancey and Parker. There is limited understorey of immature Brachystegia and Julbernardia, while the groundcover is predominantly grasses interspersed in a loose sand substrate.
    This initial glimpse into the life history of the Olive-headed Weaver reveals a remarkable bird whose known geographic distribution appears to be limited. In addition, both the weaver and local human communities are competing for the same resources - the forests of Brachystegia.
    A possible solution is to undertake a collaborative and participatory approach with local communities and governmental and non-governmental organizations which allows for sustainable use of resources by both human and animal communities. Given the current economic situation of Mozambique, a means for generating much-needed currency, increasing the standard of living and protecting resources for use by future generations is required. Ecotourism presents a possible alternative given these requirements and it is an alternative which will be pursued through this research.

Individuals wishing to contribute to Dan Nuttall's Olive-headed Weaver research should contact the Endangered Wildlife Trust; telephone Jose Alves in Johannesburg on (011) 486-1102, or Antonio Reina in Maputo, Mozambique on (092581) 42-4832. Alternatively, a trip to see these birds may be co-ordinated through either of these offices or Penny McKibbin in Nelspruit (013) 752-3797.

Author's acknowledgements
The author is grateful for the financial support received from the Environmental Capacity Enhancement Project (ECEP). Additional support was provided by Mountain Equipment Co-op (Canada), The Body Shop (Canada), Clark Beenen, Bruce Henderson, Frank Infante and Susan Selbin. Logistical and administrative support has also been provided by the Endangered Wildlife Trust (EWT) in Johannesburg. Support in Mozambique was provided by the EWT Field Office in Maputo, the Ministry of Agriculture and Fisheries, the Department of Forestry and Wildlife, the Natural History Museum of Maputo, and Eduardo Mondlane University.
    A special thanks must be extended to those individuals who have taken it upon themselves to become actively involved in conservation efforts by helping this research project achieve the seemingly small day-to-day goals. These are the backbone of any field research project. In particular I am indebted to Jose Alves, Penny McKibbin, Vincent Parker and Mike Rees for transportation to and from my base camp, accommodation in their homes and warm hospitality.

Last Updated ( Sunday, 24 October 2010 11:44 )  


Newsflash

While leading a birding trip to Southern Mozambique, Maans Booysen located and photographed no less than 3 Great Knots (Calidris tenuirostris) at the Barra Peninsula, near Inhambane, Southern Mozambique. There is one previous record of Great Knot for the Barra Peninsula by Faansie Peacock in December 2004, but given these birds propensity to return to the same area, it is conceivable that one might be the same bird that Faansie recorded in 2004.