S51:Summary: New models in avian host-brood parasitism systems

Manuel Soler1 & Eivin Røskaft2

1Departamento de Biología Animal y Ecología, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain, e-mail jsolerc@goliat.ugr.es; 2Norwegian Institute for Nature Research, Tungasletta 2, N-7005 Trondheim, Norway

Soler, M. & Røskaft, E. 1999. New Models in avian host-brood parasitism systems. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 3095-3096. Johannesburg: BirdLife South Africa.

Interspecific brood parasitism is a breeding strategy where parasitic females lay their eggs in the nests of other different species which incubate and rear the brood parasitic offspring. Therefore, this strategy greatly reduces the cost of reproduction of brood parasites but inflicts important fitness costs on hosts mainly due to three causes. Firstly parasitic females usually eat or destroy one or more of the host eggs. Secondly, the parasitic chicks ejects all host offspring or out competes host chicks during the nestling period, and thirdly, the host frequently provides extensive parental care during a period which is considerably longer than when the host provide care for the conspecific young.

Thus, there is a strong selection pressure on hosts to evolve defence strategies against brood parasites. Such host defence strategies are mainly aggressive behaviour towards the parasite and recognition and rejection of parasitic eggs. However, brood parasites have developed counter-defences that counteract such host defences increasing survival probabilities of parasitic nestlings.

This means that brood parasites and their avian hosts are involved in a coevolutionary arms race. In fact, avian brood parasitism has been considered as a model system for the study of coevolution, mainly because most hosts are usually parasitised by only one brood parasite species.

In this symposium we dealt with some of the most interesting questions of the relationships between avian brood parasites and their hosts, always in the context of the coevolutionary process described above.

The first paper (which is not published in the present journal) deals with the most fascinating example of a quick development of defensive mechanisms by one host species. In Japan, the common cuckoo started to parasitise Azure-winged Magpies Cyanopica cyana about 30 years ago and, over a very short time, only ten years, the magpie, which in the beginning was lacking defensive mechanisms against the cuckoo, developed effective counter-adaptations. Such counter-adaptations were aggressiveness towards the cuckoo and rejection of cuckoo eggs. Interestingly, these parasitic relations between the cuckoo and its magpie host have provoked dramatic changes in the population dynamics of both the brood parasite and the host.

In the second paper, an original new point of view is developed. The idea is based on the obvious fact that brood parasites and their hosts are different species, and thus, with a different evolution behind their life-history traits. It has usually been concluded that the main advantage of the brood parasite reproductive strategy is the reduction in the cost of reproduction. However, in this second paper it is emphasised that the brood parasite enjoys three important advantages only as a consequence of the fact that it is from a different species than the host. Advantages that in the past usually have been neglected in the study of interactions between brood parasites and their hosts.

The third and fourth papers deal with the same subject: the rejection response, and also the lack of it has usually been described as a fixed response to parasitism. However, recently, several studies have proved a different response to the parasitic egg according to the variation in the circumstances of parasitism; and, furthermore, behavioural differences between close populations are unlikely to reflect genetic differences. An alternative hypothesis is that the behavioural differences reflect individual flexibility in egg rejection according to the risk of parasitism. The process of rejection of an odd egg includes two different steps; first, to recognise the foreign egg, which should be genetically determined, and second, to decide whether reject it or not, which could be determined by individual flexibility.

However, during the general discussion it was emphasised that phenotypic plasticity can not be infered from results showing differences in ejection rate between different populations of the same host species, or between years in the same population. It is true that these differences in responses to the parasitic eggs are unlikely to reflect genetic differences. However, phenotypic plasticity has to be critically tested by performing several recognition experiments on the same individuals in a colour ringed population. Phenotypic plasticity will thus only be demonstrated if the same individual responds to the same model egg differently in subsequent tests and under different contexts.

Finally, the last paper discusses the role of theoretical models in explaining the relationships between brood parasites and their hosts. One mathematical model is constructed to describe the relationships between brood parasites and their hosts, and analyse how the host defence spreads under selection pressure imposed by the parasite.

The symposium has come up with several new models in avian host-brood parasite systems. In the future, more research will probably be concentrated at understanding the variation between individuals and populations in responses towards brood parasites. In addition models considering other benefits than direct reduction in reproductive costs of brood parasites will probably also be further developed.