A bouquet of Reed Warblers, the origin of the Cuckoo and the strange case of the Black Robins of the Chatham Islands

 

I had a potter around Grimley last Friday and was amazed by the number of Reed Warblers now present. I though Reed Warblers were territorial, indeed I have recently seen two males fighting over real estate – click here. Yet the sheer concentration of Reed Warblers in some of the patches of reeds left me wondering how territorial they really are. I tried to count how many adults were in one patch of some 10 x 10 meters. Any number is only a rough estimate as they rather obviously do not all show at the same time but I guesstimate 6 adult birds in this small patch of reed, probably more. Two singing males were right next to each other seemingly quite comfortable with close proximity. I noticed that where the concentration of Reed Warblers was at its highest the males were quite showy often signing from the tops of the reeds. In contrast the birds at Otmoor RSPB close to where I used to live seem to mainly sing from deep within the reeds.  I guess that the high concentration of birds at Grimley may lead the males to be more showy and vocal in their search for a mate.

 

I have a book of collective nouns for birds but there was no entry for Reed Warbler. The best google could come up with was the collective noun for a group of warblers, a bouquet which I rather like.

 

There were at least two male Cuckoos flying around and I wondered how many of the Reed Warblers nests our migrant brood parasite had found? It will be interesting to see how many juvenile Cuckoos are around later in the summer.

 

I’ve often wondered how brood parasitism evolved. By definition it can’t the result of gradual continuous evolution as it’s a pretty digital activity, i.e., you either do it or you don’t. Nature does have another form of evolution, or perhaps it should more rightly called revolution, when a DNA copying error leads to a major discrete change that in some instances can be highly advantageous.

 

A bit more googling didn’t give me a clear answer as to how brood parasitism evolved but there were some clues.  There are 127 species of cuckoo worldwide but only some of them are brood parasites. Some others use old nests of other birds rather than build their own but still incubate their own eggs which seems to be an intermediate step. 

 

For whatever reason it would seem that the eve of all brood parasitic cuckoos at some point started laying her eggs in other bird’s nests. Perhaps she had a genetic change which meant she no longer recognised her own nest? Most times the owners of the nest would recognise the egg as sufficient different to their own to be foreign and reject it. But I guess by chance some matched and brood parasitism was born conveying an enormous evolutionary advantage over previous generations who had to expend enormous energy building their own nest and raising their young. 

Colin the brood parasite, May 2018

 

Measures of genetic difference indicate that brood parasitism evolved some tens of millions of years ago. Since that time there has been a co-evolutionary race as the host evolved better ways of detecting the alien presence in its nest and the Cuckoo developed better egg mimicry and other techniques to outwit the host. I read that some Cuckoo species specialise on one or a few similar hosts and have evolved such good host egg mimicry that the only way for humans to distinguish the cuckoo egg is by its thicker shell! It’s been recently discovered that a Cuckoo has the truly remarkable ability to hold its egg inside it for an extra 24 hours before releasing it into a host bird's nest to make sure that its embryo is more advanced, and will hence hatch faster than, the hosts own clutch of eggs.

 

This reminds me of the story of the Black Robins on Chatham Islands that I learnt about on our big tour of New Zealand a few years ago.  By 1980 only five birds remained comprising four males and one female called old blue. I guess you can’t get any closer to extinction than that. The eggs were removed from the last nest and placed in a surrogate nest and old blue re-laid resulting in a slow recovery of numbers. Researchers noticed that one of the new females was laying her eggs on the rim of her nest so the researchers put the eggs back in the nest to save them. Unfortunately, this intervention propagated a characteristic that would normally have been natural selected against and at one point 50% of the females were edge layers. So the researchers had to stop putting the eggs back in the nest and natural selection quickly evolved the trait out.  As a result, today only nine percent of females lay their egg on the rim of their nests and the population has recovered to some 300 birds.

 

By reconstructing the bird’s lineage, the researchers found that the original reproductive male—the Adam of Chatham’s Black Robin population—was a silent carrier of the dominant genetic trait for the rim-laying behaviour. That means that half of his female offspring were likely to inherit the trait by laying eggs on the rim of the nest, and so on through the generations, and the other half inherited the recessive, or normal egg-laying gene.

 

The robin’s situation demonstrates how conservation interventions can have unforeseen, and potentially dangerous effects on the recovery of a species!

 

 

 Footnote – my blogs are posted with sometimes rather imaginative spelling and grammar due to my extreme dyslexia!