Little green men
The large, natural range of this small, long-tailed parrot is a band of sub-Saharan Africa and the Indian Subcontinent yet you probably know it better as the brightly-coloured alien that flocks around the parks and gardens of London and the surrounding counties.
As a popular pet, the ring-necked parakeet found its way into many urban localities where a combination of escapes and deliberate releases has lead to a number of stable feral populations in cities around the world including in South Africa, Japan, Turkey and North Africa. There are even more parakeets in Europe with populations in Belgium, the Netherlands, Germany, Italy and Spain. The biggest feral population is centred on south-west London.
The origin of the 'Kingston parakeets' is a bit of a mystery. Some suggest that they escaped from an aviary after the storm of 1987 or from the set of The African Queen whilst others believe Jimi Hendrix to have released a pair from Carnaby Street in the 1960's. In a city with many green spaces but few predators, the parakeets probably had little problem establishing a population but it would have taken some time for sufficient numbers to build up and become apparent. The population really began to expand during the 1990's, growing exponentially. Currently their numbers are difficult to estimate but there could be as many as 50,000 birds with flocks 6,000 birds strong.
It seems reasonable that such a large population of alien birds would have some impact on the wildlife of London and its environs. One study has found that typical garden birds are avoiding the large, noisy parrots who frequent the bird feeders of the masses. Parakeets also nest in tree holes which necessarily means that other species such as starlings or woodpeckers are loosing prime nesting habitat.
The ring-necked parakeet is just one of many invasive alien species to have reached our shores over the decades and centuries. Successfully invasive 'candidates' must occupy a niche which means either taking a vacant place in the ecosystem or pushing out a native equivalent. Vacant niches are hard to come by so our native flora and fauna tends to take a beating via competition for resources or apparent competition mediated by shared natural enemies such as Squirrel parapoxvirus and crayfish plague. Moreover, invasives are often released from the predation or parasitisation of their native lands allowing unchecked population growth. Grey squirrels, signal crayfish, American mink, harlequin ladybirds and Japanese knotweed are all well-known scourges of British wildlife and the list is growing.
Day 52: Hoatzin - Opisthocomus hoatzin
The crested claw
The hoatzin is a pretty odd bird to look upon. With maroon eyes set in a naked, pink face, a large, orange crest at the end of a long, slender neck and broad, rounded wings and tail the hoatzin resembles the most ancient ancestors of the birds. Furthermore, the chicks possess two clawed fingers on each wing bearing a striking similarity to the feathered arms of the small, Jurassic raptors. The uniqueness of this species meant its placement in its own order: the Opisthocomiformes. However, placement of this order on the bird tree has been very difficult with suggested relatedness to tinamous, gamebirds, rails, bustards, sandgrouse, doves and cuckoos. It took whole-genome sequencing to resolve the issue so that the hoatzin is now paired with the clade comprising Gruiformes (cranes and rails) and Charadriiformes (shore birds).
The hoatzin lives among the dense, luscious vegetation bordering the rivers of the Amazon basin. They are herbivores and their diet requires special fermenting ability akin to cows. Unlike cows (and other ruminants) they do not possess a rumen but have an enlarged crop which they use to digest the vegetation. This decomposition leads to an unpleasant smell, giving the hoatzin the alternative name of stink-bird. The hoatzin is a gregarious nester and does not need to fly far to find food. The chicks are obviously unable to fly immediately after hatching so their claws are used to scramble through the branches. This is especially important for when hawks appear as the chicks drop into the water and swim to safety before climbing out when the coast is clear. The claws of chicks lead early taxonomists to conclude that hoatzins where more closely related to Archeopteryx and its kin but the claws are actually a secondarily derived trait.
Day 53: Great spotted cuckoo - Clamator glandarius
The expert freeloader
Here we have a handsome representative of the Cuculiformes i.e. the cuckoos. You will all be familiar with our own common cuckoo (Cuculus canorus) with the famous song that gives it its onomatopoeic name. You, hopefully, will also know that the common cuckoo is a brood parasite meaning it lays its eggs in the nests of other bird species. What you may not know is that brood-parasitism is actually present in many cuckoo species in both the Old World (56 species) and the New World (three species). The common cuckoo had already been measured so I could not choose it for discussion however the great spotted cuckoo is a worthy stand-in for a discussion on cuckoos and brood parasitism.
Brood parasitism is a fascinating subject with many interesting processes. To go through these ideas it might be easy to start at the beginning. Rearing offspring is costly but many organisms do it to maximise the survival of their young. However if one could avoid that cost one could have more offspring. Many species engage in extra-pair matings so that others might rear their young. The goldeneye (Bucephala clangula) is a duck that has specialised in laying eggs in the nests of fellow females. Interspecific brood-parasitism is just one step further for a species used to pushing its eggs on others.
One might think that that when the first cuckoos began laying eggs in the nests of other species, those hosts would immediately respond. However for an unwitting host, the oblong objects in her nest have, for millions of years, always been her eggs and so she is not primed to notice if even a conspicuously different egg is added to the brood. Moreover, when the chick hatches along with the others, its loud, begging call and bright gape illicits the same instinctive feeding response as the host's actual offspring.
Clearly it is costly for the host species to expend so much energy feeding a chick that isn't its own so their is an evolutionary pressure on the host to act. The host can avoid parasitism by nesting in less accessible places and earlier in the year or it could learn to recognise the cuckoo and harass it until it flees. If this fails, hosts can learn to asses the number of eggs in the nest or to distinguish the cuckoos egg from her own. The host might respond to parasitism by evicting or piercing the new egg or abandoning the nest entirely. However, once host parents begin to evict parasitic eggs, there is a pressure for the cuckoo's eggs to resemble the host. Thus, an evolutionary arms race ensues as the hosts' powers of discrimination increase hand-in-hand with the mimicry of the cuckoo.
Mimicry requires specialisation on the part of the cuckoo and some species do not bother, preferring to have many, infrequent hosts which never cotton on to the cuckoos' ploys. Other species are able to focus on one, sufficiently abundant host species. However there are a few species, including the common cuckoo, which are able to mimic multiple hosts. This was perplexing for biologists as specialisation has a genetic basis yet members of the same cuckoo species could interbreed without messing up the different specialisations. For this to be possible, egg mimicry should be coded by genes which are only passed down the female line such as mitochondrial genes or genes on the female-determining W chromosome. This would allow females to form specialised gentes (singular gens) whilst males can mate indiscriminately as their genes will not affect the mimicry ability of their offspring.
In addition to egg mimicry, cuckoos have developed many other tactics to maximise the chances of their eggs and chicks. Some cuckoo males resemble hawks and lure aggressive host parents away allowing female cuckoos to stealthily lay an egg (the common cuckoo somewhat resembles the sparrowhawk, Accipiter nisus).Some cuckoo species have short incubation times allowing the chick to hatch before the host's. As soon as the cuckoo hatches, it instinctively rolls the host's eggs, one at a time, into a cleft in its back before thrusting them out of the nest. Whether or not to kill one's step-siblings is an interesting question. Research into brood-parasitic cowbirds suggests that when one's host has large chicks its better to evict them early to avoid later competition whereas, when the host's chicks are small, the cuckoo chick can high-jack the collective begging power of the nestmates before stealing the lions share of food.
Who wins this evolutionary arms race? Well there are many reasons why cuckoos should always be one step ahead. Firstly, cuckoos have much more at stake. Every cuckoo necessarily must outwit a host in order to succeed in reproducing. The hosts, however, are far more numerous and therefore parasitism is unlikely for any given couple, reducing the strength of the pressure. Secondly, as cuckoos' eggs become near exact replicas of the hosts' eggs, the hosts must have incredible powers of discrimination or else suffer the risk of accidentally rejecting their own eggs. Thirdly, it is suggested that some cuckoos have strategies which force hosts to comply. The brilliantly named Mafia hypothesis suggests that if a host rejects a cuckoo egg or chick, the cuckoo parents trash the host nest meaning that the risk of defection is greater then the cost of rearing the cuckoo. Evidence for this startling strategy has been put forward for two species, one of which is the great spotted cuckoo which parasitises magpies and is less likely to predate the nests of compliant hosts. #thuglife
Day 54: Little bronze cuckoo - Chrysococcyx minutillus
Cuckoo jewel
This gorgeous little cuckoo was a regular and welcome presence during my sampling in the riparian rainforests of Cape Tribulation, Queensland. I liked this species because it had a simple yet pleasant descending call and catching a sight of its metallic green back and red eye was always nice, especially when the bird in question was bashing away at a thick, juicy caterpillar. The little bronze cuckoo was a regular attendant of mixed species flocks. These loose groups of insectivores move through the understory, sallying and rummaging for bugs amongst the foliage. When any given species is locally rare, safety in numbers is provided my members of different species including spectacled monarchs(Symposiachrus trivirgatus), rufous fantails (Rhipidura rufifrons), fairy gerygones (Gerygone palpebrosa) and little shrikethrushes (Colluricincla megarhyncha).
Hearing little bronze cuckoos and the other species of the rainforest was not only a pleasantry but a necessity for the purposes of fieldwork. From day one of sampling I needed to be able to identify the species I heard to get an accurate representation on of the presence and absence of the jungle denizens. This meant learning the calls of up to 100 species in advance with only a week or so in the field to actually hear and see them in real life. Thus, much time was devoted to sitting in front of an Australian bird app, playing calls until my partner and I could ID successfully. By the end of sampling, some species were so familiar I could ID them in my sleep but rarer individuals such as the congeneric shining bronze cuckoo (Chrysococcyx lucidus), still required the playback of our digital recordings.
This process of learning bird song in advance gave a real appreciation for its necessity in fieldwork and in birding in general. In the rainforest, most identification is based on song as small, skulking birds at some distance through the dense foliage can be difficult to observe. I'm pretty good at identifying British birds by song or call but that is from years of passive (and sometimes active) learning, one species at a time. However I had never attempted the songster crash course before on other international trips (which would have been especially useful in Honduras). From now on I will make an effort to get to know the birds of a new country in advance, by sight and by sound.
The little bronze cuckoo is another brood-parasitic cuckoo but there are many members of the Cuciliformes which are neither brood-parastic nor especially cuckoo-like including the charismatic roadrunners (Geococcyx spp.).
