Effects of cold winters and roost site stability on population development of non-native Asian ring-necked parakeets (Alexandrinus manillensis) in temperate Central Europe – Results of a 16-year census

Asian ring-necked parakeets (Alexandrinus manillensis, formerly Psittacula krameri, hereafter RNP) first bred in Germany in 1969. Since then, RNP numbers increased in all three major German subpopulations (Rhineland, Rhine-Main, Rhine-Neckar) over the period 2003–2018. In the Rhine-Neckar region, the population increased to more than fivefold within only 15 years. Interestingly, there was no significant breeding range expansion of  RNP in the period 2010–2018. In 2018, the total number of RNP in Germany amounted to >16,200 birds. Differences in RNP censuses between years were evident. Surprisingly, cold winters (extreme value, −13.7 °C) and cold weather conditions in the breeding season (coldest month average, −1.36 °C) were not able to explain between-year variation. This finding suggests that in general winter mortality is low – with exceptions for winters 2008/2009 and 2009/2010, and a population-relevant loss of broods is low in our study population. Surprisingly, the social behaviour in terms of spatio-temporal stability of roost sites could well explain positive and negative population trends. Years of spatially stable and regularly used roost sites seem to correlate with increasing population sizes. In contrast, known shifts of RNP among different roost sites or the formations of new roost sites by split are related to population stagnation or a decrease in numbers. Climate change may lead to further range expansion as cities not suitable yet for RNP may become so in the near future.

Properties of a subfossil oyster bed (Ostrea edulis) offshore Helgoland (German Bight)

A recently encountered fossil oyster bed (Ostrea edulis) located in the sublittoral zone offshore the island of Helgoland (German Bight, Europe) was investigated. During a ship campaign in June 2019 onboard RV „Mya II“, the local extent and surface properties of the oyster bed were determined by submersible underwater drift video and grab sampling. The aim of this study was to facilitate the first description of a sublittoral oyster bed in the area and to understand its past ecological state and the reasons of its demise. Ostera edulis and associated habitats disappeared from the North Sea at the beginning of the last century and are a current focal point of restoration initiatives across Europe. Oyster shells were collected from the grab samples, cataloged and systematically analyzed. The weight of the valves was determined on a laboratory scale in grams with a precision of two decimals. The length (mm), width (mm), and degree of infestation (%) by the boring sponge Cliona sp. were digitally determined for each valve on the base of photographic images in the open-source software ImageJ (https://imagej.nih.gov/ij/)