Written by: Matthias Mugisha
I expected nothing spectacular in Kasangati on the morning in July. But nature had a surprise for me.
I had been photographing birds in Uganda and around my home in Kasangati for my self-sponsored bird book for a couple of years. Kasangati is about 15km north of Kampala in Uganda. The area is rapidly urbanising affecting over 100 bird species due to habitat encroachment.
On that day, I was attracted by an unusually noisy bird. It took me a while to comprehend what I saw – two different bird species sharing food. A male variable sunbird was feeding an oversized and noisy immature bird. The young bird was a Klaas’s cuckoo (Chrysococcyx Klaas). It was clear this was a classic case of brood parasitism.
Some cuckoo species like the common cuckoo and the Klaas’s cuckoo are obligate brood parasites. This means they cannot complete their lifecycle without exploiting a suitable host. Was this variable sunbird the host? It seemed so.
A female cuckoo stealthily lays eggs in nests of other species leaving the host bird to incubate the egg – thinking that the egg is its own the host bird then raises the chick. Some cuckoo species like the common cuckoo remove the host’s egg and replace it with their own.
To avoid their eggs being detected, brood parasites mostly lay mimetic eggs that resemble the host’s eggs. A female cuckoo lays about 24 eggs in one breeding season distributing the eggs in different nests.
This kind of parasitism negatively affects the reproduction of the host birds. Consequently, many host species have developed defences against brood parasites. In reaction, brood parasites have also invented techniques to counter these defences resulting in ‘an arms race.’
When the host birds learns how to recognise parasite eggs and reject them, the parasite birds, especially the cuckoos, counter this by evolving eggs that resemble the eggs of the host bird.
Other defences developed by different birds include warning calls, attacking the parasite birds, concealing the nest and shifting the breeding season to not coincide with that of the parasite.
As I stood there trying to understand what was going on, the young cuckoo started making noise and opening its mouth wide. Within seconds, the male sunbird came back. Its head almost entered the cuckoos mouth as it fed the parasite. The whole drama took a few seconds and the sunbird flew way.
Cuckoo chicks hatch after an incubation period of 11-12 days. Their survival depends on their ability to exploit the host’s parental care. In this case, the cuckoo chick destroys the host eggs or kills the host nestlings and starts loudly begging for food while mimicking the host young to avoid being deserted.
The chick remains in the nest for up to about 21 days and stays with the host bird for up to 25 days. When the chick starts flying, you cannot miss it. It loudly begs for attention from the host bird birds by flapping its wings and making loud noises.
I stood there watching, the young cuckoo again employing the attention-seeking techniques. I knew the host bird was coming. This time, it was the female sunbird. For two hours, both male and female sunbirds took turns to feed the monster chick. It was so huge that it would have swallowed its ‘foster parents’ with ease had it wanted to.
Eventually, I lost the birds as the cuckoo chick flew from one place to another after each feeding session. Nevertheless, for the next four days, it was easy to locate and photograph this natural phenomenon, thanks to the noisy chick. It was clear that the sunbirds were overworking themselves by feeding mostly nectar to a giant chick with an insatiable appetite. Cuckoos usually eat insects and butterflies.
When the parasite chicks grow up, they fly away without warning or ‘thanking’ their hosts. The female parasite chick might return to its birthplace to do what its mother did.
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