The arrival of the cuckoos

Last week I heard my first cuckoo! We had three (brief!) sightings whilst setting up a camera trap on Slimbridge reserve, and ever since I've heard them on nearly every visit out along the river and canal!

Cuckoos are fascinating and there is such an amazing diversity of behaviours and tactics that can be seen in cuckoo species around the world. They are brood parasites; they sneakily lay their eggs in the nests of other birds to gain the benefits of offspring without the costs of parental care. The impacts of this parasitism on the hosts own reproductive abilities vary. Some species rear chicks alongside the cuckoo whereas some fail to do so if the cuckoo out-competes or ejects the hosts offspring.

The cuckoo and its host is a fine example of co-evolution, with each adapting and fine tuning to the others tactics. Host specific races can be found, with specific egg variants being produced to beautifully match the egg type of the host, with these variants being genetically maintained by the female lineage. The host on the other hand tends to exhibit behaviours such as host egg rejection and mobbing, often stimulated when parasites are present in the area, with the aim of limiting the likelihood of successful parasitism.

The diversity of adaptations in cuckoos across the world is really cool - the Horsefields cuckoo for example evicts the hosts own offspring and has gape coloured patches on its wings that it will shake to trick the host into thinking it has many more mouths to feed.

Hawksfield cuckoo with gape coloured patch on its wings zoo2.zool.kyoto-u.ac.jp

The european cuckoo is a obligate breeder, laying an egg in the nest of an unsuspecting host, with successful parasitism resulting in complete failure of the host clutch, with just the parasite being reared as one of its own. This leaves the cuckoo with cheap offspring, and only a fleating visit to the UK, before heading back to Sub-saharan Africa. European cuckoos select host species that have open nests and short nestling periods, with over 100 host species being found in Europe. European cuckoos are in decline, and the BTO have been tracking 20 indvidiuals to try and understand why. Check out this link for more detailed info.

So far the project has identified timings of arrival and departure of the cuckoos from the UK to their wintering grounds, how long they are spending in different areas, and identified important stop-over sites for their migration.

One of the factors likely to be influencing cuckoo populations is climate change. Global warming can result in the change of a range of biological events; the first bud burst, the development of caterpillars, and many reproductive events such as the timing of clutches to match food supplies. A mismatch in timing between the arrival of cuckoos and the stage of breeding of their chick rearing hosts could have large impacts on cuckoo success. For example, an early Spring in the UK could result in earlier breeding times for hosts, resulting in incubation being too far along by the time cuckoos arrive from Africa. Of the over 100 cuckoo host species, some are short-distance migrators, wintering either in the UK or Europe, and some, like the cuckoo themselves, are long-distance migrators. Research shows that short-distance migrating hosts are advancing their migration times to match temperature changes, whereas long-distance migratory birds and cuckoos are not. This shows that cuckoos are not tracking phenological changes in all of their hosts, which could be resulting in loss of successful egg laying opportunities, affecting host-parasite dynamics and potentially contributing to the decline of common cuckoos.

Remember to record any cuckoo sightings (or if you hear one!), to help scientists track their movements and arrival times! Check out Bird Track, or find your county bird recorder, for more information.