Some birds have development an interesting reproductive strategy to deceive other birds and put the eggs in their nests, so foster parents are forced to feed other chicks. But what is behind this strange behaviour?
WHAT IS THE BROOD PARASITISM AND HOW MANY DIFFERENT TYPES ARE THERE?
The brood parasitism is a type of biological interaction between two organisms, in which one of them (the parasite) obtains resources from the other one (the host). In birds, the parasite obtains some benefits of parental cares from the host, developing a breeding strategy cold brood parasitism. The brood parasitism, although has been studied mostly in birds, also happens in other groups of vertebrates: for example in fish (Sato 1986, Baba et al. 1990) and some insects such as Himenoptera, Coleoptera and Heteropterous.
According the characteristics of each relation, there are different types of brood parasitism:
- Optional brood parasitism: the parasite species is capable of breeding a part of its own offspring and also, to parasite other individuals. A example in birds is in genus Coccyzus (Cuculidae).
- Forced brood parasitism: hosts breed all the offspring of the parasite bird, as happens in common cucko (Cuculus canorus).
- Intraspecific brood parasitism: host and parasite are of the same species. This is a common strategy in colonial species and in other species with nidifugous chicks.
- Interspecific brood parasitism: host and parasite are the different species.
In addition parasites are classified, by their specialisation on one or several host species, in general parasites (parasite large number of species) or specialist (only parasite one or a few species).
WHAT IS THE ORIGIN OF THIS BEHAVIOUR?
Everything suggests that the main focus of this behavior was decreased the parental investment (less cost) increasing the chance of success (major benefits), although this is not always the case.
There are several hyphotesis to explain the origin of the brood parasitism in birds:
- Firstly, it is probably that parasites were displaced individuals that did not have any territory or lost their laying, and they try to lay their eggs in other nests to achieve greater breeding success (Sorenson 1998, Sandell y Diemer 1999).
- Other hypothesis suggests that this parasitism could be a stable strategy for evolution of the population, which has similar benefits to breed its own offspring (Eadie y Fryxell 1992).
- Finally, the third hyphotesis considers this parasitism like an additional strategy to the parent care and some individuals could be used it to reduce the sibling competition in their nest, or to reduce the number of chicks to feed without decreasing the breeding success (Moller 1987, Jackson 1993).
HOW CAN THEY PROTECT THEMSELVES?
Hosts have learned to protect their offspring about the threat posed by brood parasitism.
Common cuckoo (Cuculus canorus) and rufous bush robin (Cercotrichas galactotes) have this relation, the first one lay a egg in the rufuos bush robin nest that it will be born before other chicks and kill them, capturing all the parental care.
One of the main host defences against brood parasites is the recognition and rejection of parasitic eggs. Because obligated brood parasites need appropriated individuals hosts for reproduction, such host defence-mechanisms simultaneously select for counter-defences in brood parasites, causing a coevolutionary arms race between hosts and parasites.
A FIGHT CONSTANTLY EVOLVING
There is a particular case, the strategy of the great spotted cuckoo (Clamator glandarius) when is a parasite of the common magpie (Pica pica). The great spotted cuckoo lays an egg in the nest of common magpie and this chick possess adaptations to exploit the host parental care, it does not kill directly their siblings but has advantages in relation to begging behaviour and in the order of birth (cuckoo chick is born four or six days before magpie chicks.
Magpies have development a selective advantage to recognize and ejection of parasitic egg. However, it has been observed that cuckoos react to this behaviour returning to the parasitized nest and destroying it. This situation conditions the behaviour of the magpies in the future and they are forced to accept the parasite egg.
The result of this evolutionary fight is the mafia-type behaviour of cuckoo that leads to a co-evolutionary arms race between species to avoid parasitism, in one hand, and maintain it, in the other one.
- Parasitism and nest predation in parasitic cuckoo (American Naturalist, 1995)
- Mafia Behaviour and the Evolution of Facultative Virulence (Journal of Theoretical Biology, 1995.)
- Magpie Host Manipulation by Great Spotted Cuckoos: Evidence for an Avian Mafia? (Evolution, 1997.)
- Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs (PNAS, 2006)
- Cover photo: Cuckoo chick in parasitic nest reciving food of host – http://www.guaso.com/bestiario_el_cuco.htm