A guest blog by Dave Leech, BTO who reports back on our Springwatch 2014 star, 'Cuckoozilla'. 

 


As a volunteer nest recorder for the British Trust for Ornithology (BTO), where I also work, I’m used to failure. I’m not talking about my own inability to locate nests, though I’m only too aware that many elude me, but rather the outcome of the breeding attempts themselves. Data collected by the Nest Record Scheme over the past 75 years show just how perilous life as an egg or chick can be, particularly for open-nesting species like Blackbird and Chaffinch, where the probability of young fledging successfully is around 50%.


While parent birds are able to counter the pressures of weather and predation by replacing lost clutches quickly and laying lots of eggs per season, the resulting rollercoaster ride places a severe emotional strain on the recorder. Will they succeed this time? What if the snake comes back? It’s been raining for hours – are the chicks still OK?

As someone who monitors around 500 nests a year, I learnt my lesson long ago; my role is to record, not intervene, and I should never get too attached. For the most part, this works, but then along came Cuckoozilla. Our Reed Warbler population typically hosts 10 or so Cuckoo eggs a year, so I’m used to seeing chicks, but thanks to the Springwatch footage, we could all follow his/her progress in minute detail. Who could resist? The fact that Cuckoozilla reached the grand old age of 21 days, when even the laziest chick is supposed to fly the nest, was celebrated enthusiastically in our household. So, when I visited the site again several days later hoping to hear begging calls in a nearby willow, I was completely mortified to find a fully-feathered body floating just a few metres from the nest. 

So, what happened? My best guess is that it met the same fate as at least four of its siblings this year and many others before - the strong gusts of wind that battered the reedbeds over the following weekend toppled the chick into the water and it was unable to pull itself out. The large bulk and relatively weak grip of a Cuckoo is hardly suited to life in the reeds (in strong contrast to the warblers they parasitise) and I do sometimes wonder how well the species actually fares in these managed beds, where the willow and sedges that may provide structure and shelter in more natural situations are largely absent. And could such fatalities become more commonplace as the climate warms, with increasingly erratic weather forecast for UK summers?

One thing this unfortunate event does highlight is the importance of recording how well young fare once they’ve left the nest as well as the number that leave, achieved through a number of techniques, including bird ringing. A study on Great and Coal Tits in Switzerland found that juvenile mortality rates were as high as 5- 10% per day in the four days after fledging and that only 53% were alive after 20 days. All the nestling reed warblers and cuckoos we find are ringed at our site to enable us to monitor how long birds stay before heading south for the winter and what proportion return to breed.  


A sad end for Cuckoozilla then, but we should not lose sight of the fact that the data that he has generated lives on and will feed into conservation efforts combatting the species’ decline. And it’s not bad news for all concerned – rather than spending the rest of the season tending a chick that wasn’t theirs, the unwitting foster parents have built a new nest several metres away and are currently incubating a clutch of four, all of them warbler eggs!

Warbler nest by Dave Leech, BTO

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