Energetic and hormonal costs and benefits of stopover territoriality at a Saharan oasis and its influence on reproductive output.
Territoriality in migratory songbirds is a common occurrence at breeding and wintering sites, but it is rare during migratory stopover. Migration is the most energetically intensive time of a songbird’s yearly cycle, therefore it is logical for birds to avoid establishing and defending ephemeral territories during this period. Additionally, territoriality is time intensive and may delay an individual’s arrival at a breeding site. However, the benefits to stopover territoriality may include increased fuel deposition rates, as well as heightened hormonal levels prior to arriving at a final breeding site. Increased hormone levels, specifically testosterone, have benefits to muscle retention and increased aggression, which may increase the competitiveness of these individuals upon arrival at breeding sites. I am testing the hypotheses that territory defense increases stopover efficiency and increases competitiveness at breeding site. I do this by calculating and characterizing home ranges of birds making stopovers at a Saharan oasis. I also conduct simulated territory intrusions and make repeated measures of testosterone and gonadal state. Additionally, I will capture birds as they arrive at their breeding sites on the Iberian Peninsula, measure physical condition and gonadal state and make a test of the energy-minimizing strategy, that individuals in the most competitive conditions acquire the best territories, not necessarily those that arrive earliest. I will achieve this by tracking individuals and monitoring their nests. This work will provide pertinent information linking behaviour at stopover sites with reproductive output.
The influence of habitat-specific adaptation and water on stopover strategies in a Saharan oasis.
Billions of songbirds migrate across the Sahara Desert where they make stopovers to rest and refuel depleted energy stores. However, because this region is characteristically hot and dry, benefits to making stopovers are variable and not well understood. Some species that make Sahara Desert stopovers are better adapted to xeric conditions because they experience these conditions at breeding and wintering sites, while others only briefly (and maybe haphazardly) experience xeric conditions as stopover migrants. Therefore, in order to maximize stopover efficiency, behavioral strategies should reflect habitat-specific adaptations. I am testing the hypothesis that stopover activity is dependent on habitat-specific preferences, evaporative water loss rates and water availability. To achieve this, I compare species in the Acrocephalidae family that range in wintering habitat preferences on a xeric to mesic gradient. By measuring stable isotopes reflective of their wintering grounds I can assess each individual’s position on a xeric-mesic habitat gradient. Because the threat of dehydration is the greatest limiting factor to a successful migration, I am testing if water input and output predict activity levels. I test this by provisioning some birds with drinking water (because water is absent at this site), measuring evaporative water loss with respirometry equipment, and relating these metrics to activity levels measured by radio-telemetry.
Funded by the Austrian Academy of Sciences, DOC Fellowship 25133.