CUCKOOS and CLIMATE CHANGE
When I was teaching at primary school I remember that one of the the objectives in the Science curriculum was to try and instil in children a sense of awe and wonder at the natural world. This was often easy to do by playing extracts from David Attenborough documentaries, though sometimes the awe was a little overwhelming. We once played our classes the famous scene of the killer whale snatching a seal pup off the beach once; the shock amongst the ten year olds was palpable.
The complexity of the relationships that have evolved between some animals is not quite as obvious as between orca and sealion. One of our parishioners, Professor of Behavioural Ecology, Nick Davies, has documented the strategies used by the cuckoo to gain the upper hand in its battle with the species it parasitises. These species vary but two of the most common are the dunnock and, especially at Wicken Fen, the reed warbler.
At a time when we have been reminded the 2019 State of Nature report of the catastrophic fall in the populations of many of our most well known birds such as the cuckoo, Professor Davies's observations in his book, published in 2015, after 30 years of research at Wicken Fen, are not only fascinating but profoundly worrying.
He shows how the relationship between cuckoo and target victim has evolved over thousands of years in an evolutionary arms race. Cuckoos have to get the timing of their egg laying exactly right to prevent their egg being rejected; but they have also developed mimicry in the coloration of the egg. The victim species has often developed the ability to detect these rogue eggs and eject them, in some cases so successfully that particular species are no longer vulnerable and cuckoos races have evolved to target other birds.
The cuckoo chick has become capable of ejecting not only other eggs, but other chicks from the nest, with immature wings and a body shape that has evolved especially for the purpose. There are many other extraordinary behaviours that cuckoo and victim species have evolved in the race to be one step ahead of the other. All of these qualities had for thousands of years enabled an equilibrium to be achieved. Cuckoos thrived and the birds whose nests were parasitized gradually adapted and created new obstacles for the cuckoo.
But the cuckoo is now on the verge of extinction in the UK. They are rarely seen and in locations such as Wicken Fen, successful rearing of chicks has virtually ceased.
And yet there are plenty of reed warblers, dunnocks and meadow pipits, the three most popular host species for the cuckoo. What is going wrong?
Professor Davies describes how it has been noticed that many birds are nesting earlier and earlier as the climate warms and insects are available for food. The cuckoos are arriving too late to be able to place their eggs in the host nests. Why is this?
Cuckoos actually spend most of the year in Africa, many in the skies above the Congo. It seems that their return to Europe is triggered by the changing length of the day. Unlike the global temperature, this pattern hasn't changed so cuckoos have become desynchronized from their age old rhythms.
Cuckoos may evolve to overcome this mismatch in the timing of their arrival. But this may not happen in time to prevent their disappearance from UK skies at least. Some species may be coping with rapid climate change, but many others such as the cuckoo, are not.
The evolution of the cuckoo's behaviour matched by that of its target species is an extraordinary example of how evolution has produced remarkable complexity in creation. That such ancient complexity is now so vulnerable to the results of recent global warming should raise questions on our competence as stewards of this planet.
Nick Davies, Cuckoo: Cheating by Nature, Bloomsbury 2015
State of Nature, National Biodiversity Network, 2019