Unravelling the mystery of Shining (bronze) cuckoos' dark green eggs

This week we've had a paper published in Auk which marks a personal triumph!

In 2010, I travelled to Kaikoura, New Zealand, to do some pilot work as a Phyllis & Eileen Gibbs' Travelling Research Fellow (a one-year fellowship from Newnham College, Cambridge).  With the support of Rebecca Kilner  (U. Cambridge U.K.) and Jim Briskie (U. Canterbury NZ), I investigated whether the study population of Grey warblers (a.k.a. Grey gerygones) and Shining cuckoos (Chalcites lucidus a.k.a. Chrysococcyx lucidus... phew!) could help us to unravel why Shining cuckoo eggs are dark olive-green and to learn more about their chicks.  

Most species of cuckoos in the Chalcites genus lay dark green eggs, while their hosts lay eggs that appear very different.  Back in the 1980s, Marchant and the Brookers suggested that this might be to help hosts detect cuckoo eggs, or perhaps, to avoid other cuckoos from removing their eggs preferentially.  It wasn't until 2009 however, before Naomi Langmore and colleagues produced a nice comparative study using animal vision models which started to investigate these hypotheses. 

With the help of Justin Rasmussen, who was conducting experiments for his PhD on the cuckoos, I decided to test this hypothesis with an experiment: if cuckoo eggs are visible for hosts, do they show rejection behaviour?  Justin and I added a clay egg that was a similar hue to Shining cuckoo eggs (according to calculations from an avian vision model) to as many Grey warbler nests as possible. However, these varied in brightness - same as cuckoo eggs, twice as bright, or white (the maximum brightness possible).  Despite being highly visible, none of our hosts showed any sign of rejecting the eggs, and instead incubated them along with their own eggs.

Much to our surprise, however, four of the nests were parasitised naturally by cuckoos after we'd acted as cuckoos ourselves.  And, at three of these nests, our "cuckoo eggs" were taken instead of a host egg!  

A Grey warbler (Gerygone igata, Grey gerygone) nest after predation - see the hole in the roof.

Frustratingly, many of our host nests were eaten before or during our experiments - one peril of trying to conduct experiments with New Zealand birds at unprotected sites on the mainland!  New Zealand birds just aren't adapted to coping with the hunting strategies of rats, stoats, weasels, possums, and cats that have been introduced (intentionally or otherwise) by humans.  Despite our best efforts, this kept our sample sizes smaller than we would have liked.

Luckily for us, four years later Ros Gloag, Naomi Langmore and others conducted a similar experiment with the closely related Little bronze cuckoo and its Gerygone host in Cairns, Australia.  With a great sample size, they showed that cuckoos do indeed appear to remove foreign eggs preferentially over host eggs, and at far greater rates than hosts.  The publication of this paper gave our results greater weight, because the two studies produced similar results.

So, six years later almost to the day, our efforts have finally paid off!  I also learned a valuable lesson - without pest control or physical protection, we are unlikely to learn much more about cuckoos and gerygones in New Zealand. Grey warblers inhabit scrubby edge habitats.  Unfortunately, Grey warbler habitats are rarely the types of forest chosen to be protected in conservation projects. The future might lie in finding as many nests as possible across the country. This will only be possible with citizen science projects like this one started by Michael Anderson (Massey University NZ).  

Latest research: how hosts assess parasitism risk

The outcome of two years of field experiments has been published today in Scientific Reports.  Our previous work has shown that reed warblers are much less likely to reject an odd egg from their nest than they were in the past, and that this correlates well with the local risk of parasitism.  But, how do they assess this risk?  

The decision to remove an egg is not without its own risks - hosts may remove the wrong egg, or break their own by mistake.  Close surveillance of their nests to attack a laying cuckoo is also risky because of the cuckoo's similarity to a hawk - attacking a 'cuckoo' could end up being a deadly mistake.  

Here we show that reed warblers only reject an egg when they can combine information gathered from seeing a cuckoo at their nest, with that gained by observing their neighbours mobbing a cuckoo next door.  In other words, neither social nor personal information is sufficient - instead a wide source of information is required.

See the press release here

(Image by Richard Nicoll www.richardnicollphotography.co.uk)