Experimental manipulation

As tree swallows acquire all of their insect food resources for self-maintenance and offspring provisioning on the wing (Robertson et al., 1992), we used a feather-clipping manipulation that alters flight performance and foraging profitability to induce an extended increase in workload in females. Similar feather-clipping manipulations have been shown to result in a decreased ability to acquire food resources in this species (Winkler and Allen, 1995), and the manipulation leads to a decrease in the number of foraging bouts compared with control birds in our population (Madliger et al., 2015). Thus, feather clipping in this species provides a good means through which to mimic declines in food supply that would be expected to occur following environmental degradation. When females were captured for banding and blood sampling at day 10 of incubation (immediately before hatching), we clipped every other primary flight feather (four feathers on each wing) at the base of the wing with scissors (Winkler and Allen, 1995; Ardia and Clotfelter, 2007) on a subset of females (n = 33). Control females (n = 40) were handled identically, but their feathers were left intact. Control and manipulated females were matched spatially across sites and temporally by date over the season. Our previous work confirmed that control and feather-clipped females do not differ in baseline CORT concentrations prior to the manipulation, but clipped females have significantly higher baseline CORT than control birds at the mid-nestling provisioning period 2 weeks later (Madliger et al. 2015). Feathers remain clipped until natural moult occurs after breeding (Stutchbury and Rohwer, 1990); therefore, this manipulation alters female foraging ability for the entire period of nestling provisioning.