Cuckoo

 The turn of the seasons is marked by the arrival of the summer migrants. One of these is the cuckoo, Cuculus canorus, which travels north from Africa to breed in Europe. 

Cuckoos have grey upper parts and chest, and barred underparts.
(James West, creative commons license)

This species is well known for its brood parasitism - laying its eggs in the nests of other birds. In Britain, the most commonly parasitized species varies depending on the habitat type. In moorland and healthland meadow pipits are the primary targets, reed warblers in marshland, and dunnocks in woodland and farmland. In the absence of these species cuckoos will also parasitize the nests of robins and pied wagtails. The slight size difference between male and female cuckoos is likely to have come about due to selection pressure on females to be smaller, therefore laying smaller eggs to match the small host eggs. This is different to many bird species where size dimorphism evolves due to sexual selection on males to be larger. 

Host parent birds will reject an egg if they detect that it's not their own. This leads to selection pressure on cuckoo eggs to effectively mimic host eggs. Individual females tend to have a preference for one host species, with her eggs resembling the eggs of that host. Cuckoo eggs mimic host eggs in multiple ways, and is more accurate when aiming to deceive species with stronger rejection. Different factors determine how easily an imposter egg can be detected - background colour, luminance and pattern are important, whereas pattern colour is generally less important. To decrease the chance of detection, the female cuckoo removes one of the host's eggs before laying her own. The cuckoo chick hatches relatively quickly - after 12 days - and pushes out the host eggs or chicks. After 19 days the chick fledges but is fed by the host parents for another 2 weeks before becoming independent. 

The distinctive song of the cuckoo, possibly the most easily identified birdsong. Recorded in woodland on the UEA campus. 

In the last 20 years the number of cuckoos breeding in the UK has declined by approximately half. This is in part due to high mortality rates on their migration. As cuckoos don't have to remain at their breeding site to rear young, they are some of the first summer visitors to leave, migrating back to Africa as early as June, though some stay until September. There are two routes taken by cuckoos from the British Isles. Birds taking the east route fly over Italy or the Balkans, then south over the Sahara, whereas birds on the west route travel through Spain and Morocco, then southeast to converge with the east route in the Congo Basin, central Africa. Satellite tracking by the BTO revealed that mortality rates are significantly higher for cuckoos on the shorter, western route. This is thought to be due to conditions in stopover locations in Europe, as most deaths occur before the desert crossing. Possible causes are increased drought conditions and wildfires in Spain, or large scale habitat change. Surviving such an energy intensive activity requires cuckoos to build up fat reserves at their breeding location, so decreased food sources in the UK may also increase mortality. 

There are currently believed to be around 15,000 breeding pairs of Cuculus canorus in the UK. Due to their rapid decline cuckoos are listed as a UK Red List species, and are protected by the Wildlife and Countryside Act 1981. More research is needed to comprehensively understand what is causing the decline of cuckoos, so effective conservation actions can be taken.