|Great spotted cucko (Clamator glandarius)|
The fascinating results of research into the relationship between a species of parasitic cuckoo, the great spotted cuckoo, Clamator glandarius, and one of it's host species, the carrion crow, Corvus corone corone, was published in Science this week and illustrates an interesting aspect of evolution - how competing evolutionary pressures can produce a dynamic equilibrium.
The study was carried out in northern Spain where 67.7% of carrion crow nests are parasitized by the great spotted cuckoo, which also parasitizes another member of the crow family, the magpie (Pica pica), which has a broadly similar geographical range to the carrion crow. Unlike the magpie, which will often evict alien eggs from it's nest and will mob great spotted cuckoos in the vicinity, carrion crows exhibit neither of these defence mechanisms. It is normally assumed, when this is found in other cases of avian parasitism that the species has only recently begun to be parasitized, so has not yet had time to evolve these defensive behaviors. However, it is also possible that there has been no evolutionary pressure to evolve defences because there may be a degree of mutualism in the relationship. In other words, the parasitized species might be deriving some benefit.
Avian brood parasites lay eggs in the nests of other birds, which raise the unrelated chicks and typically suffer partial or complete loss of their own brood. However, carrion crows Corvus corone corone can benefit from parasitism by the great spotted cuckoo Clamator glandarius. Parasitized nests have lower rates of predation-induced failure due to production of a repellent secretion by cuckoo chicks, but among nests that are successful, those with cuckoo chicks fledge fewer crows. The outcome of these counterbalancing effects fluctuates between parasitism and mutualism each season, depending on the intensity of predation pressure.
Canestrari et al., From Parasitism to Mutualism: Unexpected Interactions Between a Cuckoo and Its Host;
Science, 343 (6177): 1350-1352; 21 March 2014:DOI: 10.1126/science.1249008
|Carrion crow (Corvus corone corone)|
The difference in nest location and construction between carrion crows and magpies is also significant in explaining the different responses of these two species to cuckoo parasites. Carrion crows nest in the tops of tall trees and construct an open nest, making the brood prone to predation by predators such as falcons and buzzards. Magpies, however, build a nest in a thicker cover and construct a roof over it, making it harder for flying predators to take the chicks.
This difference in nest construction has created a different dynamic because it means the cuckoo chicks in carrion crow nests are also more liable to predation. This has produced evolutionary pressure for them to evolve a defence mechanism - they secrete a substance which repels predators. This in turn protects the host chicks in the same nest.
|Magpie (Pica pica)|
Carrion crows, on the other hand, which can, in situations where predation is high, actually benefit from having a parasite chick in the nest, have had little evolutionary pressure to evolve avoidance strategies and might even be expected to evolve strategies to encourage it. However, given that predator numbers can fluctuate and there are years when predator numbers are low and so any potential benefit is not realised and loss due to competition can be the more significant effect, there may also be pressure acting in the reverse direction.
So, there is a dynamic of competing forces at work here which has probably reached an equilibrium at which the carrion crow does little or nothing to avoid parasitism because of the benefits it can get from it, but it has not evolved behaviours to encourage it because it also often suffers from it. Being dynamic of course means that it is relatively easy for a small change in one of the forces to push the dynamic in one direction or another, maybe over just a small part of the range, especially if the species involved is relatively sedentary.
And the evolution of the different strategies between magpies and carrion crows has its origins in the different nesting strategies adopted by the two species, which was itself probably produced by different responses to evolutionary pressures at some point in their evolutionary histories.
The historic forces which shaped evolution in the past may not even be present today so it can be difficult to see why something like different nesting behaviours in birds developed as related species diverged until we understand how natural selection operates. I'll be looking at this in my next blog.
As always, I'd love a creationist to come here to explain how this finding fits in with their currently fashionable 'intelligent design' notion. None have yet managed it with any other blogs on similar topics, but there is always hope that one will one day find the moral courage to defend their idea or to have the integrity to say they can't.
NB: Corvus corone corone is a three-part name to distinguish it from the Eastern subspecies, C. corone orientalis. The species formerly included the hooded crow, previously Corvus corone cornix. However, since 2002, these have been recognised as distinct species, C. corone and C. cornix respectively, and there is disagreement about the exact taxonomic status of C. corone orientalis. The reasons for this classification debate is interesting in itself and illustrates the problem of precise definition of the terms 'species' and 'subspecies' as an original species diverges and speciates, and may well be the subject of another blog here soon.
'via Blog this'