Between 1970 and 2006 reed warblers Acrocephalus scirpaceus started breeding progressively earlier; both the initiation of breeding (the earliest first egg dates) and peak of breeding (median first egg dates) advanced. Median first egg dates correlated significantly with increasing MayÁJuly mean temperatures. However, in contrast to other studies showing the advancement in laying dates, the end of the season did not shift. As a result, the breeding season is now longer increasing re-nesting opportunities. Individuals are able to re-nest 4Á5 times, which might have important implications for the species. It was also found that in warmer seasons the population suffered fewer nest losses. Both factors, higher re-nesting potential and a trend toward fewer losses, should lead to increased fitness of individuals in the studied population.

Many bird species are advancing the timing of their egg-laying in response to a warming climate. Little is known, however, of whether this advancement affects the respective length of the breeding seasons. A meta-analysis of 65 long-term studies of 54 species from the Northern Hemisphere has revealed that within the last 45 years an average population has lengthened the season by 1.4 days per decade, which was independent from changes in mean laying dates. Multi-brooded birds have prolonged their seasons by 4 days per decade, while single-brooded have shortened by 2 days. Changes in season lengths covaried with local climate changes: warming was correlated with prolonged seasons in multi-brooded species, but not in single-brooders. This might be a result of higher ecological flexibility of multi-brooded birds, whereas single brooders may have problems with synchronizing their reproduction with the peak of food resources. Sedentary species and short-distance migrants prolonged their breeding seasons more than long-distance migrants, which probably cannot track conditions at their breeding grounds. We conclude that as long as climate warming continues without major changes in ecological conditions, multi-brooded or sedentary species will probably increase their reproductive output, while the opposite effect may occur in single-brooded or migratory birds.

A cuckoo Cuculus canorus dummy was exposed at 24 nests of great reed warbler Acrocephalus arundinaceus (GRW) and 34 nests of reed warbler Acrocephalus scirpaceus (RW) during the egg-laying stage. The eight GRW pairs attacked the cuckoo directly, striking the dummy, but such a behaviour was not recorded in RWs. Also, other behavioural measures (closest distance from the model, duration of distress calls and number of excitement calls) indicated a lower level of defence by RWs compared to GRWs. In the study area, the parasitism rate was much lower in GRWs (1.7% of nests) than in RWs (11.3%). We suggest that one of the reasons for the lower level of cuckoo parasitism on GRWs is its stronger nest defence and hence higher risk of injury or even death for the cuckoo during egg dumping.

Predation, the most important source of nest mortality in altricial birds, has been a subject of numerous studies during past decades. However, the temporal dynamics between changing predation pressures and parental responses remain poorly understood. We analysed characteristics of 524 nests of European reed warblers monitored during six consecutive breeding seasons in the same area, and found some support for the shifting nest predation refuge hypothesis. Nest site characteristics were correlated with nest fate, but a nest with the same nest-site attributes could be relatively safe in one season and vulnerable to predation in another. Thus nest predation refuges were ephemeral and there was no between-season consistency in nest predation patterns. Reed warblers that lost their first nests in a given season did not disperse farther for the subsequent reproductive attempt, compared to successful individuals, but they introduced more changes to their second nest sites. In subsequent nests, predation risk remained constant for birds that changed nest-site characteristics, but increased for those that did not. At the between-season temporal scale, individual birds did not perform better with age in terms of reducing nest predation risk. We conclude that the experience acquired in previous years may not be useful, given that nest predation refuges are not stable.

Weather is an important factor affecting many aspects of avian ecology, yet its importance for survival during various periods of the avian annual cycle has received relatively little attention and remains poorly understood. We have investigated the effect of weather conditions at the breeding and wintering grounds and during migration on the survival probability of Eurasian Reed Warblers Acrocephalus scirpaceus, a long-distance Palaearctic-African migrant species. We found that annual survival was significantly affected by precipitation at the autumn stopover sites in Spain and Morocco, where Reed Warblers accumulate energy reserves prior to crossing the Sahara desert: low rainfall was associated with decreased survival. We suggest that arid conditions at these crucial stopover sites may cause limitations in food availability, prevent proper refueling and hence result in lower survival.Survival estimates were not related to weather conditions in Africa during the bird wintering period or the preceding wet season. Likewise, meteorological conditions at the breeding area did not influence survival. Survival estimates for males and females did not differ, although recapture probability was significantly lower for females than for males. Our results indicate that weather significantly influences Reed Warbler survival, although its effect may vary for different stages of the annual cycle. Our findings suggest that marked climatic changes occurring along migratory routes, in particular at important stopover sites, may have far-reaching consequences on bird survival and population size.

Published information relating to changes in the chemical element content of avian eggs caused by embryonic development is extremely scarce, although it may be crucial for understanding both the presence of anthropogenic pollutants as well as physiological levels of micronutrients. We assessed the variation in concentrations of calcium (Ca) and magnesium (Mg) and nine trace elements: seven essential (chromium (Cr), copper (Cu), nickel (Ni), manganese (Mn), iron (Fe), cobalt (Co) and zinc (Zn)) and two non-essential (lead (Pb) and cadmium (Cd)) in shells and contents (both egg yolk and egg white) of embryonated and non-embryonated eggs. We investigated the eggs of the Eurasian Reed Warbler Acrocephalus scirpaceus, a large proportion of whose eggs are infertile in our study population (almost 43% of clutches contain unhatched eggs) as well as significant embryo-induced eggshell thinning at the equator of embryonated eggs. We found significantly higher concentrations (≥ 22.7%) of all the focal elements in the contents of embryonated eggs in comparison with non-embryonated eggs, and a very pronounced one for Ca (nearly twice as high). The shells of embryonated eggs contained significantly higher concentrations of Zn (104.1%), Fe (56.5%), Pb (32.8%) and Cu (28.0%) but significantly lower ones of Co (8.9%) and Ca (9.3%) than the shells of non-embryonated eggs. The simultaneous higher concentrations of all elements in the content of thinnershelled embryonated eggs suggest the parallel transfer of these elements along with Ca resorption from the shell into the egg interior during embryo formation. The higher concentration of most elements in the thinner shells of embryonated eggs may be indicative of the maternal deposition of some of these elements in a shell layer not subject to embryonic depletion, or in the eggshell membrane. Our results highlight the need for the careful selection of egg samples, which should differentiate between embryonated and non-embryonated eggs in the analytical treatment of eggs and eggshells.

We examined long-term responses in the breeding performance of the Great Reed Warblers Acrocephalus arundinaceus to climate change. The study took place in various years from 1970 to 2007. During the study period, mean temperatures in the breeding season of the species increased and precipitation decreased significantly. We found evidence for the significant advancement in both earliest and annual median first-egglaying dates. This advancement correlated with temperature increases early in the season. The latest first-egg laying dates, however, remained unchanged. Other breeding statistics: clutch size, nest losses, and production of young per nest, did not change significantly over the study period. Precipitation did not affect any of the analysed measures. It is important to note, though, that during dry seasons, the production of young per successful nest was higher. In contrast to some woodland species, the Great Reed Warbler seems to adapt well to climate change by shifting laying dates. The reason for this is probably to optimise food resources.

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