Mechanisms of population dynamics in Red Kites

    After facing near extinction, the Red Kite has recolonized the Swiss Central Plateau with remarkable speed, continuously expanding its upper elevational range. One of the Swiss Ornithological Institute’s major research projects aims to find out how this charismatic species achieves this.

    Aims

    As a result of human persecution, the Red Kite was almost extirpated from Switzerland in the early 20th century. Similar negative trends were observed Europe-wide. Fortunately, the population of this nearly endemic European raptor has increased over the last years and is now considered as Least Concern at both a global and national level. Currently, Switzerland harbors almost 10% of the global breeding population. The Red Kite is, thus, one of 50 national priority species in the Swiss species recovery program. Since the 1970s, the Red Kite has recolonized the northern half of the country with remarkable speed. Moreover, the upper elevational range limit keeps expanding as the species increasingly colonizes alpine valleys. In addition, a new overwintering strategy has emerged; while winters were traditionally spent on the Iberian Peninsula, an increasing fraction of the population nowadays overwinters in central Europe. As a result, Switzerland is currently the country with the highest density of overwintering Red Kites. The species often attends large communal roosts.

    Our goal is to investigate the factors that affect survival rate, reproduction, dispersal, and space use in general, to explain the population growth and distribution of the Red Kite in Switzerland and Europe.

    Modules

    The project has seven modules: 

    Diet composition and anthropogenic feeding (several Master theses, Postdoc Dr. Pietro Milanesi): In the context of several Master theses, we investigate the diet composition along the elevational gradient. Private feeding of Red Kites seems a widespread phenomenonin Switzerland, and a special focus is therefore given to the role of anthropogenic food sources for Red Kite’s reproductive success and space use in summer and; winter. Apart from direct anthropogenic feeding, we also examine the role of the Red Kite in the Swiss scavenger community.

    Departure and early prospecting phase (PhD thesis Patrick Scherler): The environmental conditions at the place of birth affect survival and body condition. They might even affect post-fledging behavior of juvenile Red Kites. Patrick investigated the interplay of environmental factors shaping the reproductive output of a Red Kite population and how natal conditions affect the timing of departure from the natal home range. This crucial time in the life of a young bird marks the start of dispersal. Patrick also disentangled dispersal and migration movements to investigate how natal environment effects carry-over to affect prospecting movements during different dispersal phases.

    Nestling stress physiology and carryover effects (PhD thesis Benedetta Catitti): Benedetta analyses the factors affecting Red Kites during development in the nest and assesses whether their effect carries-over to later life stages. Specifically, a focus is set on sibling rivalry and stress physiology at the developmental stage and their link to social behaviour and movement patterns during the post-fledging and adult phases.

    Drivers and consequences of partial migration (PhD thesis Stephanie Witczak): Stephanie aims to investigate the drivers of partial migration in terms of the individual characteristics of migrants and residents, as well as local environmental conditions. Furthermore, the consequences of the chosen overwintering strategy (migration, residence) are determined on both an individual level (reproduction, survival) and on a population level (effect of overwintering strategy prevalence on population growth and decline).

    Development of Migration (Postdoc Ginny Chan): Using a large dataset of GPS-tracked juvenile Red Kites, Ginny investigates the ontogeny of migration routes and sex differences in migration patterns. Further, several Master students determine important stopover sites and migratory connectivity at a European scale.

    Settlement decisions (Postdoc Florian Orgeret): Florian builds on our extensive settlement data to investigate the drivers and consequences of natal dispersal. We focus on the decision-making process during the transience phase and settlement and investigate potential dispersal syndromes in the species.

    Genomics (Dr. Reto Burri): Capitalizing on a large network of collaborators across Europe, we study the population structure of this species throughout its global range. The results of this study will help elucidating the phylogeographic history and demography on a European scale. We aim to combine genomic data with empirical dispersal data to determine the mechanism that govern effective gene flow in birds of prey.

    Approach

    Since 2015, extensive territory mapping and nest controls have been conducted in the Cantons of Fribourg and Bern. Approximately 450 juveniles and 70 adult birds were tagged with solar-powered GPS transmitters, generating long-term information on space use. We also conducted food supplementation experiments during different seasons to identify the effect of food supply and anthropogenic feeding on reproduction, survival, post-fledging behavior, dispersal and overwintering strategy. Nest cameras monitor incubation and recorded food delivered to the young.

    Significance

    Given the significant fraction of Europe’s breeding and overwintering Red Kites occurring in Switzerland, the country has a large responsibility for the species. Understanding the mechanisms and factors influencing population trends is pivotal for its conservation; this project will contribute to identifying the driving forces and underlying mechanisms that shape the population trajectory of this conservation-relevant raptor. Furthermore, the unprecedented sample of GPS-tracked juveniles will help resolving questions about the mechanism of dispersal in general, and dispersal-shaping range margins in particular. Our findings in combination with our international collaborations enable informing management decisions regarding Red Kites in Switzerland and at a European level, thus contributing to a pan-European effort towards the species’ conservation.

    https://www.youtube.com/watch?v=yQASAQImjZk

    Project management

    Dr. Urs KormannDr. Martin Grüebler

    Project Team

    Benedetta CatittiDr. Ginny ChanDr. Pietro MilanesiDr. Florian OrgeretDr. Patrick ScherlerValentijn van BergenStephanie WitczakDr. Reto Burri 

    Partners

    Dr. Adrian Aebischer

    AG Naturschutz, Universität Marburg
    www.uni-marburg.de

    Anne-Katleen Malchov und Damaris Zurrel, University of Potsdam
    damariszurell.github.io

    Rainer Raab, Technisches Büro für Biologie, Deutsch-Wagram, Austria
    www.tbraab.eu

    Eurokit
    www.life-eurokite.eu

    Further partners for the genomic sampling:

    Grzegorz Maciorowski (POL), Alfonso Godino (ESP), Julio Blas (ESP), Javier de la Puente (ESP) Max Steinmetz und Lisa Glesener (LUX), Fabienne David FRA)

    Publications

    Nägeli, M., P. Scherler, S. Witczak, B. Catitti, A. Aebischer, V. van Bergen, U. Kormann & M. U. Grüebler (2021):
    Weather and food availability additively affect reproductive output in an expanding raptor population.