Environment – Deadbeat parents

Raising babies is hard work. The challenges associated with parenting require investments of time, energy and resources. Among bird species, there is an astonishing diversity in how parents solve these challenges. 

In most species, like the toroa (royal albatross), both Mum and Dad contribute equally to incubating the eggs and raising the chicks. In some species, like the tūī, the female does most of the parenting. In others, like the kākāpō, Dad has absolutely nothing to do with the youngsters and Mum does all the work. And there are some species that even recruit parent helpers, typically grown offspring from previous broods, that chip in with chick feeding. This happens in pōpokotea (whitehead) for instance.

Remarkably, there are a few species where neither parent contributes anything to raising young. Their youngsters still need lots of care, but the parents have pawned off all that hard work onto others. There are two species that do this in New Zealand: the pīpīwharauroa (shining cuckoo) and the much rarer koekoeā (long-tailed cuckoo).

Shining cuckoos are relatively abundant on the Hibiscus Coast, and they are gorgeous birds if you are lucky enough to see one. Cuckoos are far more often heard than seen however, and the loud plaintive songs of this migratory species are a welcome sign of spring. So how did shining cuckoos manage to evolve out of their parenting responsibilities? They are brood parasites that trick our endemic riroriro (grey warblers) into raising cuckoo young as their own.

An amazing series of adaptations allow shining cuckoo to get away with this ruse. First the female spies out potential grey warbler nests during patrols of suitable forest habitat. When a promising nest is found, the cuckoo will simply swoop into it, quickly lay an egg and depart with one of the host’s eggs in its bill. All of this takes less than 20 seconds! Then, after the cuckoo egg hatches and the nestling is just a few days old, the baby cuckoo will toss out all the grey warbler eggs and chicks. Finally, when the chick gets a bit older, the youngster mimics the sounds of baby grey warblers, ensuring the host parents are fooled into feeding it. 

When the cuckoo chick leaves the nest, it is a remarkable sight to see a pair of devoted grey warbler parents furiously collecting food for the considerably larger and ravenous young cuckoo. Such trickery is very costly to the grey warblers. Indeed, in many other species of cuckoos, their hosts have evolved counter-adaptations to defend against parasitism, such as being able to recognize cuckoo eggs and ejecting them. However, grey warblers have not evolved any of these adaptations (yet), and so in this evolutionary arms race, pīpīwharauroa has the current advantage.