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This study analyzes the influence of egg laying order on egg dimensions (especially on egg volume) and on the size of the last egg laid in a Blackcap clutch. The research was carried out in deciduous woodland in NW Croatia during 2002 and 2004. The analyses covered only clutches with five eggs (the dominant size of the first clutch in the study area), where the first eggs were laid on the same day (5 May 2002 and 11 May 2004). The present study suggests that the Blackcap may employ a strategy of increasing the egg volume of each successive egg laid, which may reduce size hierarchy between nestlings and prevent competition between them.
Small solitary open nesting passerines, such is the Blackcap Sylvia atricapilla that builds nest in the undergrowth, have little chance of successfully scaring off a predator to defend a nest. The aim of our study was to determine if parental care by Blackcaps can reduce the risk of depredation of its nests. We compared the survival of natural clutches with artificial clutches (plasticine and independently both plasticine & quail eggs). The artificial clutch was placed in a nest after the natural clutch had been concluded, and the results were analysed as matched pairs of data. We assumed that significantly higher survival rates of natural clutches than of artificial clutches deprived of parental care, might indicate a significant positive effect of parental care on reducing depredation risk of Blackcap clutches. Losses caused by birds, rodents and larger mammals were 49%, 41% and 9%, respectively. The differences in survival rates of artificial clutches (plasticine as well as quail & plasticine) and natural clutches were not statistically significant. This might show that parental care is not strong enough to significantly reduce depredation risk of Blackcap clutches. Although this conclusion should be treated cautiously because it was difficult to assess the influence of using artificial clutches on our results.
Plumage is regularly replaced over the lifetime of birds, and, consequently, variable conditions experienced during plumage development can affect feather growth rate or the expression of feather characteristics. We assessed intra-individual constancy in the length, mass, and growth rate of tail feathers between complete moults of 13 free-living individual Blackcaps Sylvia atricapilla. Between-moult repeatability was high for the length and mass, but not for growth rate of individual tail feathers. The feathers produced in the later moult grew at a slower rate, which shows that the time to produce similar feathers increases with age.
One of the hypotheses to explain a lack of relationship between nest location and breeding success of passerine birds predicts that in an environment characterized by a high diversity of predators that use a variety of foraging strategies, clutches in different locations and degrees of concealment may be equally subject to destruction. In this study I assumed that the impact of nest location on breeding success of the Blackcap Sylvia atricapilla would differ in habitats characterized by significantly different species richness and, consequently, population density and species diversity of potential predators. Thus, I expected that the influence of the nest location and particularly its concealment on breeding success of the Blackcap would be more significant in a habitat characterized by the overall low biotic diversity and low vegetation density. The study was conducted in two forested areas located in central Poland – managed pine forest characterized by low biodiversity and vegetation density and floodplain forest consisting of much higher biodiversity and having higher stand density. I assessed the influence of Blackcap nest placement (concealment, height above the ground, nest plant height, distance from the nest to the closest edge of the nest plant) on breeding success. Among the analyzed parameters, only nest concealment had a significant impact on breeding success. This conclusion, however, pertains only to the pine forest, where successful nests were better concealed than nests with clutch losses. Similar relationships were found in the floodplain forest area, but they were not statistically significant.
Many authors have demonstrated use of plants and animals as sensitive biomonitors for climate changes. This research was conducted in the Mokrice area (mixed agricultural landscape and small deciduous woods) in northwestern Croatia and studies cover the period from 1979 to 2007. This paper assesses evidence that year and local air temperature changes influence first arrival dates of the migratory passerine Blackcap, Sylvia atricapilla. The mean first arrival date of the Blackcap was 2 April (1979–2007). Correlation between timing of arrivals and year was significant (P <0.001). Blackcap arrival dates have become 9.3 days earlier during the research period (slope of linear regression = –0.32 ± 0.01). The relationship between spring temperatures (March-April) and year (P = 0.019), and spring temperatures and timing of arrival (P <0.001) was also significant. This result suggests that Blackcap respond to spring temperatures by earlier arrival at breeding grounds.
Recent climate change has a major impact on the sizes and distribution of bird populations, the phenology of their breeding/migration and migratory behaviour (migration distance, migration strategy). We documented changes in the numbers of juvenile Blackcaps migrating in autumn through the S Baltic that were paralleled by changes in wing length of captured individuals during a 43-year study period (1967-2009). We suggest that the observed trends may indicate changing population composition of migrating birds. In the Blackcap, wing length distinguishes among different populations and increases with increasing migration distance of a given population. Available published data show that long-distance and short-distance Blackcaps pass the study region. Hence, we assumed that shorter-winged birds are short-distance migrants wintering in the southern Europe, and that longer-winged individuals are long-distance migrants wintering in the sub-Saharan region. It seems that in 1967-1980 most Blackcap populations declined, but, as wing length slightly increased, the rate of this decline has been higher in the shorter-winged/short-distance Blackcaps. Over the subsequent 24 years alongside with rapidly growing numbers of birds, we noted a remarkable decrease in wing length. This indicates a pronounced increase in the number of short-distance individuals compared to long-dis- tance migrants. Both groups may benefit from improved conditions at their breeding grounds, but the shorter migration route and favourable conditions at wintering sites north of Sahara could favour short-distance migrants over the longer distance and longer-winged Blackcaps.
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