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.

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.

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.

Over four study years we have investigated hatching success and failure, the presence of an embryo and other egg biometrics, and eggshell thickness of the Eurasian Reed Warbler Acrocephalus scirpaceus. Of 1354 eggs from 350 nests, 940 (69.4 %) survived until hatching, of which 159 (16.9 %) were incubated but failed to hatch, while 273 (20.2 % of all) were in nests destroyed during incubation. Of 161 intact unhatched eggs from 97 nests, 64 (39.8 %) had no visible embryo and 88 (54.7 %) had a visible embryo. Eggs without visible embryos were present in 42.7 % of normally incubated clutches; among deserted clutches over 50 % had no visible embryo. Eggshell thickness at the egg equator (the widest part) was negatively correlated with clutch size and the presence of an embryo. A similar relationship was found for the blunt end of the eggs but was not statistically significant. Shells at the equator of eggs with visible embryos were significantly thinner (on average 8.0 % less) than the shells of eggs without embryos. Our study clearly implies that shell thickness decreases in the course of embryonic development. It also indicates that comparison of eggshell thickness data without controlling for the presence of an embryo in eggs can produce biased results, and may, therefore, fail to identify the actual causes of eggshell thinning. Our findings also suggest that shell thickness at the blunt end is a reliable estimator of shell thickness, irrespective of embryonic development. Keywords Hatching failure Á Avian eggs Á Embryo development Á Infertile eggs Á Eggshell thinning Á Acrocephalus scirpaceus Zusammenfassung Verdü nnung der Eischale als Folge der Embryonalentwicklung bei einem kleinen Singvogel