Environment – The secret lives of tūi

Despite tūī being a common and iconic endemic species of Aotearoa New Zealand, we still have much to learn about the biology of this remarkable bird. Indeed, only recently have biologists resolved the details of the tūi mating system. 

The tūi breeding season has been underway for some time, and like most species of birds, they form pair bonds during nesting. It is a great time to watch tūi right now as they court, chase each other, do energetic display flights, sing loudly (and early!) and generally just make spectacles of themselves. 

Ultimately though, a male and female will form an ongoing association as they cooperate to raise a brood of youngsters.

Dr Sarah Wells of Unitec Institute of Technology did her PhD research at Massey University on tūi mating behaviour. Females build the nests on their own and once the eggs are laid, do all the incubation. When the eggs hatch, females are also the primary caregivers. They feed their young insects, and you may have noticed tūi hunting them in bushy trees like mānuka and kānuka. They also like to get into the eaves of houses and scout about for spiders. While mum does most of this work, dad will also bring food to the chicks. Some dads are pretty good food providers and some not so much. Why are tūī dads such slackers?

To address this question, Dr. Wells caught mated pairs of tūi and took tiny blood samples from them. She also took tiny blood samples from the chicks after they had grown. She then analysed the DNA in the blood to resolve the parentage of the chicks. The results of this research were quite a surprise.

Overall, fully 50 percent of the chicks were not related to the males that were taking care of them. This is known as extra-pair mating and is a common phenomenon in bird species that form monogamous pair bonds. 

However, tūi have an exceptionally high rate of extra-pair young. This probably explains why tūī dads do not contribute that much parental care. If, on average, half the young in the nest are not actually theirs, then natural selection would favour those males that pull back on investing in these young, and instead channel their energy into other pursuits.