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Changes in biometric characteristics of a migrating bird population at a given capture site could reflect that the migration strategies of the species may be changing. A robust data set (15,520 records) was used to analyse the changes in biometric characteristics of the Eurasian Blackcap Sylvia atricapilla in western Hungary between August and October during the study period (2001–2019). All age and sex classes displayed similar phenology of monthly captures and biometric change: the average wing length, fat score and body mass increased in the migrating population from August to October. This could be explained by changes in the morphometric distribution of different migrating populations, in later months more birds might arrive from a larger distance, primarily from the Baltic regions and the Czech Republic. The biometrics of juvenile birds captured in August did not change significantly from 2001 to 2019, while the wing lengths decreased and fat scores increased significantly between 2001 and 2019 in September and October in both sex classes. This could be explained by a change in the migration distance or different morphology of individuals from the north which stopped over at the study area. Due to the global warming and habitat changes in the last decade, the shorter migration route and favourable conditions at overwintering areas north of the Sahara or around the Mediterranean could favour Blackcaps which migrate a shorter distance. [ABSTRACT FROM AUTHOR]

The article discusses the results of a study that examines the relative contributions of brood size, hatching asynchrony and date and feeding rate in determining the growth development of Eurasian Blackcap Sylvia atricapilla nestlings. The study shows a significant effect of brood size, hatching order and date on the nestling development but the effect was observed to be significantly only for young nestlings and gradually disappeared as they age. The brood size of four is the optimal number for Blackcaps breeding in a good year.