Travelling through Southern Africa in October, I was excited to photograph these characters...
The weaver birds (Ploceidae) are famous for their elaborate nest construction. They mainly inhabit sub-Saharan Africa, although a few species occur in Asia too. For many of the Ploceus weavers (e.g Village and Masked weavers), the complex nest is crafted by the handsome male, who weaves strips of grass and leaves together using a similar technique to human weavers. Somehow he can complete this exquisite construction within a day or two!
Once the nest is complete, the males try to attract females by hanging upside-down underneath it and fluttering their wings whilst producing a metallic 'churring' song. If a female approves of a male's nest, she lines it before she mates with the male and lays her eggs. Afterwards, the male's attention rapidly shifts towards building more nests and attracting as many females as he can.
Female weaver birds have some battles of their own to fight. For starters, other female weavers in the colony might try to sneak their eggs into her nest! But there is a far more sinister invader about which she should be concerned. A number of these colonial breeding weaver bird species (such as Village, Masked and Cape weavers) are hosts for the brood parasitic diederik cuckoo (Chrysococcyx caprius).
Although the diederik cuckoo males are gorgeous metallic green-backed birds with vivid red eyes, their females are somewhat duller and more difficult to spot. The females have to be sneaky, so that they can slip in and deposit their own egg in a weaver bird nest the moment the opportunity arises.
Weaver birds and diederik cuckoos have provided an interesting natural experiment because weaver birds were introduced from mainland Africa to the islands of Mauritius and Hispaniola, where they live life diederik-free. Studies conducted by David Lahti suggest that the absence of diederik cuckoos on the two islands has resulted in the weavers eggs becoming less variable in appearance across females and less consistent within a given female's clutch. So without the pressure of brood-parasitism, weaver birds appear to have reduced the pigmentation that they add to their eggshells. However, the weaver birds are no less likely to reject foreign eggs than mainland birds, when the harder task of distinguishing between eggs due to the greater within-clutch variation is accounted for.
So it seems as though weaver egg appearance on mainland Africa has evolved in response to parasitism by diederik cuckoos. This has arisen via an evolutionary arms-race between cuckoos producing mimetic eggs to fool the weavers, and weavers producing eggs that minimise the risk of accepting a cuckoo egg or rejecting their own eggs by accident.
I enjoyed reflecting on these fascinating interactions between diederik cuckoos and weaver birds as I watched this male diederik relentlessly calling out to attract a female... "dee-dee-dee-dee-derik". Meanwhile, in a tree not more than 50 meters away from their foe, the weaverbird males hung upside down flapping their wings and churring to impress their own ladies, blissfully unaware of the imminent dramas that would unfold in their carefully woven nursery baskets.
behaviour and evolution