Do migratory birds need stopovers for physiological recovery?

• Applicant: Cas Eikenaar, Ph.D.
• Subject Area: Sensory and Behavioural Biology ; Animal Physiology and Biochemistry
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 433146546

• Final Report Year: 2023

Final Report Abstract

Most of the billions of migrating birds cannot reach their final destination (wintering or breeding grounds) in a single flight and have to make so-called stopovers in between flights. Stopovers have various functions of which efficient fuel accumulation has received the most attention. Fuel, mainly in the form of fatty acids, is evidently required for migratory endurance flights, and, accordingly, many studies observed that lean individuals are less likely to depart from stopover than fat individuals. Other aspects of a migrant’s physiological state, their dynamics at stopover, and their potential effects on stopover behaviour (departure decisions) have received far less attention. This is somewhat surprising as migratory flights are performed at incredibly high metabolic rates for hours to even days on end, and thus are physiologically extremely demanding. One physiological consequence of elevated metabolic rates is an increase in reactive oxygen species (ROS). Vertebrates, including birds, have evolved a complex antioxidant defence system to avoid or minimize the damaging effects that ROS can have on lipids, proteins and DNA. Yet, even though antioxidant defences are upregulated during migration, endurance flight has been shown to cause oxidative damage to lipids and proteins. With the negative effect of endurance flight on birds’ oxidative balance between ROS production and antioxidant defense, it seems plausible that migrants need to recover this balance at stopover. We have shown in the current project that migrants indeed can rapidly reduce the amount of oxidative lipid damage while at stopover, thereby probably recovering their oxidative balance. Our studies also indicate that this reduction may be incompatible with fuel accumulation. The role of the antioxidant defense system in recovery of the oxidative balance at stopover is currently unclear; we observed no changes in birds’ antioxidant capacity during stopover. Although, we could not show that migrants’ stopover departure decisions are associated with their oxidative damage or antioxidant defense system, we believe that our studies have provided evidence for the long existing, but seldom tested idea that stopovers serve physiological recovery. Additional studies on birds’ fatty acid fuel composition indicated that migrants have a preference for polyunsaturated fatty acids (PUFAs) as fuel. This makes sense because PUFAs deliver most energy. PUFAs, however, are very susceptible to oxidative damage, and accordingly migrants making long flights appear to rely less on PUFAs than migrants making short flights.

Publications

• Migrant blackbirds, Turdus merula, have higher plasma levels of polyunsaturated fatty acids compared to residents, but not enhanced fatty acid unsaturation index. Ecology and Evolution, 10(18), 10196-10206.
  Jensen, Johan Kjellberg; Isaksson, Caroline; Eikenaar, Cas & Andersson, Martin N.
  
• Oxidative damage to lipids is rapidly reduced during migratory stopovers. Functional Ecology, 34(6), 1215-1222.
  Eikenaar, Cas; Winslott, Erica; Hessler, Sven & Isaksson, Caroline
  
• Changes in fat mass affect the motivation to migrate in northern wheatears. Journal of Avian Biology, 52(6).
  Eikenaar, Cas; Karwinkel, Thiemo & Hessler, Sven
  
• Can differential fatty acid composition help migrating birds to limit oxidative lipid damage?. Physiology & Behavior, 249, 113768.
  Eikenaar, Cas; Winslott, Erica; Schmaljohann, Heiko; Wang, Hong-Lei & Isaksson, Caroline
  
• Do departure and flight route decisions correlate with immune parameters in migratory songbirds?. Functional Ecology, 36(12), 3007-3021.
  Brust, Vera; Eikenaar, Cas; Packmor, Florian; Schmaljohann, Heiko; Hüppop, Ommo & Czirják, Gábor Á.
  
• Understanding the ecological and evolutionary function of stopover in migrating birds. Biological Reviews, 97(4), 1231-1252.
  Schmaljohann, Heiko; Eikenaar, Cas & Sapir, Nir
  
• Recovery of constitutive immune function after migratory endurance flight in free-living birds. Biology Letters, 19(2).
  Eikenaar, Cas; Ostolani, Alessia; Hessler, Sven; Ye, Ellen Y. & Hegemann, Arne
  
• The oxidative balance and stopover departure decisions in a medium- and a long-distance migrant. Movement Ecology, 11(1).
  Eikenaar, Cas; Ostolani, Alessia; Brust, Vera; Karwinkel, Thiemo; Schmaljohann, Heiko & Isaksson, Caroline