Niche separation in 3D: foraging behaviour and nice stratification in sympatric insectivorous birds

• Applicant: Dr. Wiebke Ullmann
• Subject Area: Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450380116

Species interactions and movements shape population dynamics and community assemblages by fostering niche differentiation between ecologically similar species. The niche overlap between species may temporarily be adjusted to events, such as short-term food shortages. However, these ecological processes may be challenged by recent, rapid changes due to anthropogenic stressors such as permanent food shortages. Permanent, low food abundance can increase competition and affect niche differentiation processes and fitness of sympatric species, potentially leading to local species loss. I propose to investigate the processes underlying this species loss by studying changes in species interactions and niche differentiation under permanently reduced food availability and as a control under enhanced food availability. Within this context, I will investigate changes in niche overlap between ecologically similar species, their use of alternative movement processes to increase niche differentiation when necessary, and whether these changes and adjustments influence reproductive success. To achieve this, I will use one of the most severe human-mediated changes causing permanently low food abundance: the currently observed decline in insects. I will investigate the influence of the constantly reduced insect food abundance on two sympatric insectivores: barn swallows (Hirundo rustica) and house martins (Delichon urbicum). In the past, these species segregated their niches along gradients of prey size and hunting height. During short-term food shortages they temporarily hunted within the same air layer, while their prey-size niche overlap decreased. However, a more permanent decrease in prey-size niche overlap might not be sustainable under the synergistic effects of constant low food abundance and climate change. In a first step, I will test the niche overlap between the two insectivore species along gradients in hunting height and prey size under constant low food availability, as presented by the intensively used agricultural study area and under an enhanced food availability treatment (i.e. planting flower strips supporting preferred prey items). Second, I will study fine-scale movement interactions between the species to test for alternative movement processes that are used to avoid heterospecifics when niche overlap is high. Third, I will investigate whether the degree of niche overlap in hunting height and prey size as well as the level of fine-scale avoidance affect the reproductive output of individual insectivores. Hunting height and fine-scale movements will be determined by tagging large numbers of barn swallows and house martins simultaneously with ultra-light, high-resolution radio tags. Prey size will be analysed by collecting faecal samples from the tagged individuals for prey size analysis. With this project, I expect to shed light on processes underlying the global biodiversity decline.

• DFG Programme: Research Grants