Röhrichtsterben beeinflusst Brutverhalten des Haubentauchers (Podiceps cristatus) am Dümmer (Niedersachsen, Deutschland)

Abstract

Reed die-back affects breeding biology of the Great Crested Grebe (Podiceps cristatus) at Lake Diimmer (Lower Saxony, Germany)In the beginning of the 90s the breeding population of the Great Crested Grebe on the hypertrophic shallow Lake Dtimmer decreased from almost 400 pairs to about 40-120 pairs. We addressed the question of whether this decrease was due to changes in prey availability or to changes in nesting sites. Fishing catches by gillnet and electrical fishing gear throughout the breeding season indicated that prey availability was not a limiting factor. In particular, lake whitefish were found to occur in high densities. In the pre-laying period the whitefish stock consisted of individuals with 17.1 cm total length on average. In the following periods of hatching and rearing the young, the fish stock was dominated by juveniles of small size. Observations of breeding pairs and parents feeding their young support the hypothesis that prey availability was high: (1) We found no evidence that nowadays clutches are smaller than in the 80s. In 1998 clutch size did not decrease with laying date and first clutches were not significantly larger than second clutches. (2) In hunting for prey fishes for their chicks, adults grebes were very successful. The mode time interval between two feedings did not exceed 20 seconds. With regard to prey size we found no evidence for active or passive selection by the Grebes. Obvious changes in reproductive behaviour were related to the nesting sites. In the 80s most pairs nested in flooded reed beds, whereas in more recently most clutches have been found in water-lilies and flooded beds of calamus and bulrush. This change in nesting sites was apparently caused by the dying off of large areas of flooded reed beds in the last 30 years. In some inlets flooded reed beds completely disappeared. In 1998 the start of laying was clearly related to the nesting site: reed-breeders started significantly earlier than birds breeding in water-lilies and beds of calamus and bulrush. O~ving to the change in nesting sites during the last 10 years, we observed a time-shift in the reproduction: today Grebes start laying almost 50 days later than in the 80s. Moreover, we recorded a significant difference in the stability of nests related to the nesting site: nests in reed beds and bulrush had a higher stability than those in water-lilies and beds of calamus. The present findings are discussed in the light of reproductive decisions and reproductive successes. ZusammenfassungAm Dfimmer, einem hypertrophen Flachsee in Nordwestdeutschland, sank der Brutbestand des Haubentauchers von fast 400 Paaren Mitte der 80er Jahre auf 40-120 Paare in den 90er Jahren ab. In der vorliegenden Studie wurde der Frage nachgegangen, ob dieser Rtickgang auf Ver~inderungen im Nahrungsangebot oder einem Mangel an geeigneten Nistpl~itzen zurtickzuffihren ist. Sowohl Kiemennetzf~inge als auch Elektrobefischungen w~ihrend der Brutzeit lassen den Schluss zu, dass an Nahrungsfischen kein Mangel besteht. Es wurden insgesamt 13 F...

Comparison of spring and summer clutches of Great Crested Grebes (Podiceps cristatus)

“…Besides, the composition of the littoral vegetation and the condition of the stands could contribute to the later nesting of Grebes in the monitored area as well (Rajchard et al 2013). This outcome is in agreement with the findings of Marxmeier and Duettmann (2002) on the later start of Grebe nesting in declining littoral stands.…”

“…This happens mainly through two mechanisms: variable clutch size and asynchronous hatching (Vlug 2005). Changes in reproductive behaviour are also linked with changes in the nesting environment (Marxmeier & Duettmann 2002, Zaynagutdinova & Mikhailov 2019.…”