Bio-musings

Nesting a tangle of cause and effect

Grand constructions by non-human organisms seem to defy our self-concept as the engineering animal. Yet some of the greatest feats of engineering can be found in Nature itself: beaver dams and lodges; mima mounds on a landscape scale; termitaria five metres high; and the breakwater of coral reefs – a rampart of natural concrete built by the most passive looking of brainless algae[1].

Bird nests are another intriguing example of engineering by modest minds. The constructions of most birds are, almost by definition, lightweight. However, one woven abode is the avian equivalent of a multi-storey building: the communal nest of the sociable weaver (Ploceidae: Philetairus socius). This thatch-like conglomeration knits together the nest tunnels of many pairs of the species, forming a single structure of exceptional size.

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Dear readers, we present the sociable weaver, a rather ordinary-looking[2], sparrow-like bird which lives in the southwest Kalahari, extending into semi-arid parts of southeastern Namibia and the Northern Cape of South Africa. Because its habitat is so sparsely vegetated, the nests of the sociable weaver are the most prominent features in some vistas of southern Africa.

Biology, like other branches of science, is ultimately about cause and effect. In this blog from the Bio-edge, we begin our exploration into the ecological significance of an avian feat. What is the web of cause and effect that has led to the unlikely fabrication of the largest bird nest on Earth by an unassuming passerine in a semi-desert environment? 

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It seems fairly obvious that the{njaccess 2, 3, 4, 5, 6, 7, 8} communal nest of the sociable weaver provides valuable shade in summer, blankets the birds against cold in winter, and allows the conservation of body water during drought. But all of these advantages simply beg a question. Why do the other semi-arid parts of the subtropical and temperate latitudes on four continents – as well as extensive semi-deserts north of the equator in Africa itself – lack any rival for this record-breaking nest? After all, large nests bring cons as well as pros. For example, predators such as the Cape cobra[3] can potentially exploit the sociable weaver with concentrated destruction.

Some lateral thinking is needed and our answer – to appear in a forthcoming e-book – is subtle enough to involve another lineage of ecosystem engineers: termites. The link between the sociable weaver and termites might escape most visitors to this habitat, because the termite concerned is usually unapparent.

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The sociable weaver, like other sparrow-like birds worldwide, eats seeds as a preferred staple. Where it is odd is in its routine consumption of not just any termite, but a remarkable species: the northern harvester termite (Hodotermes mossambicus)[4]. The significance of this possible co-staple seems to have been overlooked in potential explanation of the scale on which the bird builds its nests.

Although the sociable weaver is less specialised on a diet of termites than several other animals in its habitat[5], it may nonetheless turn out to be the most termitivorous of all sedentary[6] birds found under any dry climate on Earth. If so, could this be a crucial factor explaining its exceptional nesting habits?

The termite species eaten by the sociable weaver is unusual in being large, fast-moving, and capable of activity on the ground surface during the day – even in the cold winter months. Surprisingly for such an abundant and voracious insect, the northern harvester termite builds no visible termitaria or mounds, each enormous colony instead maintaining several large hives deep underground, each larger than the typical beehive. Its activity on the surface is frenetic but so sporadic on any given site that it makes observation of this insect a matter of sheer chance.

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Instead of eating wood as might be expected for a termite, here is an insect that eats mainly grass. And unlike most other termites, it harvests not only dry, dead straw but also grass green with life. Perhaps because it operates as a grazer rather than a detritivore, the northern harvester termite is extraordinarily productive for a termite.

So we can bring you some original hypotheses. The largest bird-nest on Earth is associated with what is possibly the most fecund and fast-growing termite on Earth. The northern harvester termite is unique among termites in its capacity to support termite-eaters. Not only is it extreme among termites in the degree to which it competes with large herbivores for the consumption of grass, but the fact that this colonial insect remains active on the surface during winter means that it can sustain several species of consumers through the leanest time of the year.

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Even the most productive of termites are fairly poor food in the scheme of things, because much of their body consists of a plastic-like chitin and their guts are full of fibre. As a result, specialised termite-eating animals tend to have relatively slow metabolism, reproduction, and growth. Although the sociable weaver retains a fairly broad diet and the particular termite that it eats is in many ways more rewarding fare than other termites, we can expect this bird to be economical in its use of energy and nutrients. On the other hand, in a semi-arid habitat with little drinking water, the water content of the termite body is a vital asset to consumers such as the sociable weaver. Although the emergence of this termite species may seem unreliable to the human observer, the bird has the advantage of winged mobility to seek out the current point of activity over a large home range.

We have been pondering the possible ways in which termites could be a crucial factor in explaining the exceptionally large nest of the sociable weaver. An availability of the insects may allow a termite-eating bird to be sedentary, provided it has limited metabolic and reproductive demands. The water content of the termites (in contrast to the dryness of seeds) potentially emancipates the bird from drinking water, provided it can economise on bodily water in other ways. And an exceptionally large nest can buffer a termite-eating bird from aridity, while also allowing cooperative breeding on a colonial scale.

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However, the thread of cause and effect is complicated by fire and herbivorous mammals. Such large nests made of extremely flammable material can only last for any length of time in a habitat free from fire. Although the habitat of the sociable weaver is semi-arid, it potentially produces enough grass, particularly in relatively rainy years, to fuel grass fires. Freedom from fire in this environment thus depends on consumption of the green and dry grass, which is achieved by both termites and large grazing herbivores. The habitat of the sociable weaver is exceptional in having a rich fauna of large grazers. Among these, particularly noteworthy is one[7] of the species of oryxes, the only semi-specialised grazers[8] on Earth that do not need to drink free water.

So these are the elements we plan to weave into a forthcoming e-book: an exceptionally large nest in a habitat with enough straw to build the nest but not enough straw to burn; large herbivores powerful enough to minimise the incidence of fire but too scattered to eat the straw before it can be used to build the nest; and termites powerful enough to emulate large herbivores while at the same time allowing year-round residence by providing the sociable weaver with a dietary source of water.

One strand of logic nests in another, and our explanation is, in the end, mere theory. But we are attempting for the first time to provide a whole explanation. And in this spirit, dear readers, we look forward to seeing you in our e-book...{!njaccess}… See the hidden half of this blog-post by subscribing here{/njaccess}

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1 e.g. Porolithon

2 14 centimetres long and 26-32 grams body mass

3 Naja nivea

4 The northern harvester termite, although restricted to seasonally dry grassland and savanna in southern and eastern Africa, is far more widespread than the sociable weaver. Only in its driest habitats does this bird species occur.

5 The habitat of the sociable weaver supports many other partly or mainly termite-eating animals: aardvark (Orycteropus afer), aardwolf (Proteles cristatus), bat-eared fox (Otocyon megalotis), yellow mongoose (Cynictis penicillata), crowned plover (Vanellus coronatus), ant-eating chat (Myrmecocichla formicivora), blind snakes (Typhlops), thread snakes (Leptotyphlops), Kalahari round-headed worm lizard, Cape spade-snouted worm lizard, striped blind legless skink, bushveld lizard, ground agama, Bradfield's dwarf gecko, common barking gecko, and driver ant (Dorylus). Many of these termite-eating animals lack counterparts under similar climates in Africa north of the equator, as well as in Arabia and Asia.

6 The double-banded courser (Rhinoptilus africanus), sympatric with the sociable weaver and extending also to northeastern Africa, appears to be the most specialised bird on Earth for termite-eating, but differs from the sociable weaver in being nomadic.

7 Oryx gazella

8 as opposed to browser of the dicotyledonous foliage of woody and herbaceous plants

 
 
 

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