Why do not all insectivores birds suffer from the collapse in insect biomass?
The recent finding that invertebrate populations have collapsed in western Europe (Hallmann et al. 2017) raises the question to what extent this also has had consequences higher in the food chain. A logical reasoning is that when, in general, biomass of insect populations has strongly declined, insectivores like many bird species that feed their offspring with insects will suffer because of lack of food. Failure to raise sufficient offspring will ultimately lead to declines in population size.
Picture not consistent
Indeed, studies from abroad have found that population declines dominate among insectivore bird species (e.g. Moller 2019). However, if we summarize the population trends for Dutch insectivore breeding bird species the overall picture is much less consistent. Some insectivores show severe declines, whereas others increase.
I.e. during 1990-2017 skylarks have lost more than 90% of their population, while yellow wagtail seems to hold on or even increases in the same habitats. Likewise, typical heathland species like tawny pipit and wheatear (almost) went extinct, while stonechat and nightjar recently recovered and now have strongly increasing heathland populations.
We believe that a better understanding of these contrasting patterns contributes to (1) increasing knowledge of underlying driving forces and their relative importance (2) better conservation strategies and (3) better societal support for policy and plans to reverse the insect decline.
Why do not all insectivore birds suffer from insect declines? We have some hypotheses. There are signals that the number of broods that a species is able to raise within a breeding season is a flexible trait, and changes under (other) environmental pressures such as climate and land-use changes. Skylarks and turtle doves are unable to raise a high proportion of second broods nowadays, and this is a highly likely explanation for their poor performance.
Nightjars and possibly stonechats may be able to raise more broods than in the past and this might explain their increase. In this sense climate change may counteract the potentially negative effects of declining food availability. This of course only works when offspring is able to survive.
Focus on quantitative information
To test this and other hypotheses, this internship focusses on gathering quantitative information on demography and diet composition, preferably including changes over time.
In this internship the student needs to formulate clear working hypotheses on mechanisms that mitigate or compensate negative effects of insect declines, and to conduct a thorough literature search focussing on (changes in) breeding biology and diet composition for insectivore bird species in the Netherlands.
The ultimate goal is to conduct a meta-analysis for insectivores in order to unravel the contributions of phenology (length breeding season), breeding biology (number of broods) and diet composition to explain why there are winners and losers among Dutch insectivore bird species.
We aim to publish the results in a peer-reviewed paper.
Start of internship: ASAP
Length of internship: >3 months