Odd egg out

This is a great time of year to hear the distinctive call of the Cuckoo (Cuculus canorus) as it spends the summer in the UK. Collections Manager Eileen Westwig recently shared Cuckoo specimens with the public in one of our Spotlight Specimens sessions. You missed it?! No problem, here she is with the fascinating story of this threatened bird…

Cuckoos could be described as absent mothers, laying their eggs into the nest of a ‘host bird’, such as Dunnocks, Meadow Pipits, Garden Warblers, Whitethroats or Flycatchers. When she finds a suitable nest, the female Cuckoo will remove one of the host’s eggs and lay hers in its place. She lays between 12 and 22 eggs in a season, all in different nests. No worries befall her about building a nest, brooding out any eggs or raising her young as she leaves it all to strangers. One challenge for the Cuckoo is to make sure her trickery is not discovered.

When the female host returns to her nest, she will inspect it for any changes and if she discovers the intruder’s egg, she will simply toss it out. So the female Cuckoo has to be pretty good at forgery and mimic the host bird’s egg ‘signature’, copying the colour, pattern and shape of the original eggs. This is the only way to get away with her ‘brood parasitism’. Around 20% of Cuckoo eggs never make it. In the top picture, you can see the nest of a Garden Warbler with three Warbler eggs and one larger Cuckoo egg, on the top left.

An adult Garden Warbler (Sylvia borin borin) can reach a weight of 16-22g with a wingspan of 20-24.5cm

After twelve days, the Cuckoo hatches and pushes the other nestlings out. As the single remaining occupant of the nest, it has the full attention of the host parents, which try to feed a nestling soon outweighing. An adult Cuckoo is more than 6 times the weight of an adult Garden Warbler. The Cuckoo young will leave the nest after 19 days, but gets fed by the parents for a further two weeks. That is one busy summer.

OUMNH.ZC.11868_Cuculus_canorus_canorus_Eileen_Westwig

An adult Cuckoo (Cuculus canorus) can reach a weight of 105-130g with a wingspan of 55-65cm.

According to the RSPB, there are about 15,000 breeding pairs in the UK and Cuckoos are now included on the Red List, giving them the highest conservation priority. Ten years ago, numbers of this migrant bird fell by 21% and more than half of the population has disappeared in the past 25 years. Threats include damage to the bird’s winter habitats and a decline in large insect species that are its major food source.

Cuckoos migrate to West Africa over the winter months and can be seen in the UK from late March or April through July or August. Young birds leave a month or so later to give them time to grow and prepare for the long journey ahead. Wintering grounds are not exactly known but include Cameroon, Gabon and other African nations.

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