Why does the biggest communal nest of any bird occur only in one southern African semi-desert? part 1

@tonyrebelo Hi Tony, does this rationale make sense to you? With thanks from Antoni

The communal nest of Philetairus socius (https://en.wikipedia.org/wiki/Sociable_weaver and http://www.oiseaux-birds.com/card-sociable-weaver.html and https://www.nwf.org/Magazines/National-Wildlife/2002/Oh-What-a-Nest and https://phys.org/news/2019-07-massive-sociable-weavers-house-species.html) of southern Africa is the largest on Earth.

This species has no counterparts on other continents or even in ecologically similar parts of the same continent such as the Sahel or northeastern Africa.

Which combination of ecological factors has produced this adaptation?

Why does such a large construction (https://www.inaturalist.org/observations/47986970 and https://www.inaturalist.org/observations/37159904 and https://www.inaturalist.org/observations/11064462 and https://www.youtube.com/watch?v=HPdLqL_Tzso and https://thefunambulistdotnet.wordpress.com/2010/12/18/architectures-without-architects-sociable-weavers-nests/) occur in Namibia and Northern Cape province of South Africa (https://en.wikipedia.org/wiki/Sociable_weaver#/media/File:Philetairus_socius_distribution_map.png), as opposed to any other semi-desert with scattered trees?

In the summer heat and winter frost of the dry interiors of all vegetated continents, there are obvious economies of scale in large communal nests, for shelter and thermodynamic homeostasis (https://www.researchgate.net/publication/229635804_The_thermal_significance_of_the_nest_of_the_Sociable_Weaver_Philetairus_socius_Winter_observations and https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-048X.2012.05797.x).

However, it is only in one situation on Earth, and one species of bird, that communal nesting on this scale has arisen. In East Africa, related and comparable birds do occur in the form of genus Pseudonigrita (https://en.wikipedia.org/wiki/Pseudonigrita), but their nests are far less communal and thus far smaller (https://www.inaturalist.org/observations/28021578 and https://www.inaturalist.org/observations/108449462 and https://www.inaturalist.org/observations/47881640 and https://www.inaturalist.org/observations/47120758 and https://www.inaturalist.org/observations/109837557).

One obvious environmental prerequisite is freedom from wildfire. This helps to explain why no comparable nest occurs in, for example, Australia, where wildfire is remarkably widespread under semi-arid climates except on saline/sodic substrates (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x).

However, this is an insufficient explanation because extensive semi-arid areas of the Americas, as well as northeastern Africa, are also free of wildfires.

The availability of suitable forms of grass, such as Stipagrostis (https://en.wikipedia.org/wiki/Stipagrostis), is also a prerequisite. However, such grasses are far more widespread (e.g. https://en.wikipedia.org/wiki/Stipagrostis_uniplumis) than is the sociable weaver.

I suggest that the crucial factor is dietary: although the sociable weaver belongs to a mainly granivorous family, and although it eats grass seeds as one of its staples, it is odd in its dependence on a peculiar kind of termite.

This explanation invokes the perennial availability of a large-bodied, surface-foraging, diurnal hodotermitid: Hodotermes mossambicus (https://en.wikipedia.org/wiki/Hodotermes and https://termites.myspecies.info/content/hodotermes). This insect is arguably crucial to the sociable weaver - which seldom drinks because it lives remotely from surface water - as a source of not only food but also hydration.

The sociable weaver is not specialised enough on social insects to qualify as myrmecophagous (https://en.wikipedia.org/wiki/Myrmecophagy and https://www.tandfonline.com/doi/pdf/10.1080/02541858.1988.11448112).

However, it does rely on eating a species of termite (https://www.tandfonline.com/doi/abs/10.1080/00306525.1973.9639162?journalCode=tost20 and https://www.tandfonline.com/doi/pdf/10.1080/02541858.1988.11448112 and https://ielc.libguides.com/sdzg/factsheets/sociableweaver/diet) that is itself odd in being less a detritivore than a grazer. This insect forms part of a guild of grazers including ruminants and hares, and continues to be active on the ground surface even in the frosty mid-winter, at the driest time of year.

However, the availability of Hodotermes is also insufficient to explain the extreme adaptation of the sociable weaver. This is because this genus of grass-eating termites is abundant also in semi-arid East Africa, where it does not seem to be important in the diets of the closest relatives of the sociable weaver (https://academic.oup.com/auk/article/97/2/213/5188549?login=false).

The difference is that southern Africa has at most one rainy season per year, whereas northeastern Africa reliably has two (https://journals.ametsoc.org/view/journals/clim/28/6/jcli-d-14-00484.1.xml). This makes desiccation a greater risk in the habitat of the sociable weaver than in the habitats of Pseudonigrita spp.

As far as I know, no other species of bird, beyond Africa, relies on a combination of seeds and termites to the degree shown by the sociable weaver.

Although seeds are available in all semi-deserts, Australia and the Americas lack any counterpart for Hodotermes. Because termites contain sufficient water to allow independence from drinking in the dry season, a potential niche has arisen in southern Africa for a sedentary bird such as the sociable weaver. And the means whereby this niche has been made viable is gregariousness in foraging, roosting, and breeding, facilitated by an extremely economical nest-complex, plus a degree of cooperative breeding (https://en.wikipedia.org/wiki/Cooperative_breeding and https://citeseerx.ist.psu.edu/viewdoc/download?doi= unusual in Ploceidae.

A closely related and partly sympatric ploceid, namely the white-browed sparrow-weaver (Plocepasser mahali, https://www.inaturalist.org/taxa/14115-Plocepasser-mahali), has a diet remarkably similar to that of the sociable weaver (https://www.tandfonline.com/doi/pdf/10.1080/02541858.1988.11448112). So, why are the nests non-communal in genus Plocepasser?

A possible answer is that the white-browed sparrow-weaver

Furthermore, of the other three species in this genus,

to be continued...

Posted on April 08, 2022 04:38 PM by milewski milewski



Posted by milewski over 2 years ago

@alexanderr Hi Alex, has Dasypeltis been recorded raiding the communal nests of the sociable weaver? If not, why do you think that specialised egg-eating snakes are excluded from the nests of Philetairus socius? With many thanks from Antoni

Posted by milewski over 2 years ago

I'm not sure about Dasypeltis, but Cape Cobras are often seen raiding their nests (for chicks I think). Being bright yellow they are quite noticeable.

I'm sure Bryan can help: @bryanmaritz

Posted by alexanderr over 2 years ago

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