Summary1. Population viability models usually consider only the female segment of the population. However, random variation in sex ratio as well as the mating system may also affect variation in fitness among females. Here we develop population models incorporating demographic stochasticity in both sexes. Furthermore, we consider the effects on the estimated time to extinction and whether density regulation acts only on females or on total population size. 2. We applied these models to two populations of polygynous great reed warblers Acrocephalus arundinaceus L. with differences in population trends to investigate the importance of considering sex in population viability models. 3. Demographic stochasticity was larger in a polygynous than in a monogamous mating system. 4. The estimated time to extinction was considerably shorter for a monogamous than for a polygynous mating system, particularly if density regulation acted only on females than rather on the total population. 5. This study demonstrates that structure of mating system must be included when making population viability analysis based on counts of total population sizes. It is especially important to model the specific effects of density regulation on the two sexes.

Comparative analyses of interspecific differences in response to climate change can provide important insights into the factors initiating seasonal onset of reproduction in various species and subsequent fitness consequences. We present a comparative analysis based on a 30-year breeding survey of two related migratory songbird species [Reed Warbler Acrocephalus scirpaceus (RW) and Great Reed Warbler A. arundinaceus (GRW)], which coexist in reedbeds by means of various interspecific interactions. The RW advanced breeding by 15 days and shortened its breeding time window, which is a combined effect of higher temperatures, alleviated competition as a consequence of population declines in the dominant GRW, and reduced RW population. Although the breeding period of GRW changed only slightly, its clutch initiation was likewise related to temperature. Most probably, advanced breeding in RW is favoured by changes in food supply and accelerated reed growth, which provides the necessary nest concealment earlier, whereas this does not affect the GRW, a species less vulnerable to nest predation. Clutch size decreased later in the season, so that earlier breeding produced a net increase in both species. An additional increase of clutch size in GRW can be explained by the use of higher-quality territories in today's smaller population. The main causes of nest losses were predation in RW and adverse weather in GRW, but reproductive success increased over the study period in both species, which was a consequence of larger clutches in RW, but of favourable weather during rearing and fewer total losses in GRW. Our results document that different causal mechanisms are involved in the reproductive changes of the two congeneric species living in the same habitat: RW breeding earlier by making use of competitive release and other ecological improvements, and GRW by benefiting from better rearing conditions. As species respond differentially to climate change depending on ecosystem and biotic interactions, predictions of population dynamics will remain vague until the specific response mechanisms have been elucidated.

In some hosts of avian brood parasites, several populations apparently escape parasitism, while others are parasitized. Many migratory specialist brood parasites like common cuckoos, Cuculus canorus , experience a short breeding season, and in order to maintain local parasite populations host densities should be sufficiently high to allow efficient nest search. However, no studies have investigated the possible effect of host density on presence of cuckoo parasitism among populations of a single host species. Here, we investigated possible predictors of common cuckoo parasitism in 16 populations of reed warblers, Acrocephalus scirpaceus , across Europe. In more detail, we quantified the effect of host density, number of host breeding pairs, habitat type, mean distance to nearest cuckoo vantage point, predation rate and latitude on the presence of cuckoo parasitism while controlling for geographical distance among study populations. Host density was a powerful predictor of parasitism. We also found a less pronounced effect of habitat type on occurrence of parasitism, while the other variables did not explain why cuckoos utilize some reed warbler populations and not others. This is the first study focusing on patterns of common cuckoo-host interactions within a specific host species on a large geographic scale. The results indicate that if host density is below a specific threshold, cuckoo parasitism is absent regardless of the state of other potentially confounding variables.Most studies on avian brood parasitism have focused on single populations (Rothstein and Robinson 1998, Davies 2000). However, spatially heterogeneous environments are likely to influence both distribution of populations and their dynamics (Hanski 1999). Therefore, spatial differentiation of various populations of hosts and parasites (metapopulation scenario) may lead to local adaptations and spatial variation in coevolution of parasites and their hosts (Thompson 1994(Thompson , 2005. The investigation of host Áparasite dynamics in a metapopulation context has so far mainly been applied to plants and their pathogens (reviewed by Kaltz and

In some hosts of avian brood parasites, several populations apparently escape parasitism, while others are parasitized. Many migratory specialist brood parasites like common cuckoos, Cuculus canorus , experience a short breeding season, and in order to maintain local parasite populations host densities should be sufficiently high to allow efficient nest search. However, no studies have investigated the possible effect of host density on presence of cuckoo parasitism among populations of a single host species. Here, we investigated possible predictors of common cuckoo parasitism in 16 populations of reed warblers, Acrocephalus scirpaceus , across Europe. In more detail, we quantified the effect of host density, number of host breeding pairs, habitat type, mean distance to nearest cuckoo vantage point, predation rate and latitude on the presence of cuckoo parasitism while controlling for geographical distance among study populations. Host density was a powerful predictor of parasitism. We also found a less pronounced effect of habitat type on occurrence of parasitism, while the other variables did not explain why cuckoos utilize some reed warbler populations and not others. This is the first study focusing on patterns of common cuckoo-host interactions within a specific host species on a large geographic scale. The results indicate that if host density is below a specific threshold, cuckoo parasitism is absent regardless of the state of other potentially confounding variables.Most studies on avian brood parasitism have focused on single populations (Rothstein and Robinson 1998, Davies 2000). However, spatially heterogeneous environments are likely to influence both distribution of populations and their dynamics (Hanski 1999). Therefore, spatial differentiation of various populations of hosts and parasites (metapopulation scenario) may lead to local adaptations and spatial variation in coevolution of parasites and their hosts (Thompson 1994(Thompson , 2005. The investigation of host Áparasite dynamics in a metapopulation context has so far mainly been applied to plants and their pathogens (reviewed by Kaltz and

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