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Migrating birds chill to fatten up

September 11, 2009
Journal of Experimental Biology
Most migrating birds can't carry enough fuel to reach their destinations, so refuel en route. However the aviators expend twice as much energy during stopovers as they use in transit. Wondering whether migrating blackcaps save energy by dropping their body temperature during stopovers, researchers measured the bird's temperatures as they refuelled and found that they drop their body temperatures at night by up to 30 percent to conserve energy and fatten faster.

Blackcap (Sylvia atricapilla) male.
Credit: iStockphoto/Andrew Howe

Marathon runners are famed for pasta packing in the days before a big run but when tiny passerine birds set out on their epic migrations, the distances are too great to cover on the energy reserves with which they embark.

Michał Wojciechowski and Berry Pinshow explain that most birds stop off en route to their destination to refuel. One of the Eurasian blackcaps' preferred refuelling stops is Midreshet Ben-Gurion, Israel, where the birds fill up on fruit and insects before setting off again. Knowing that birds expend twice as much energy during stopovers than they use in transit, the duo wondered whether the tiny aviators drop their body temperature at night during stopovers to save energy and build up their reserves faster.

They publish this discovery on 11 September 2009 in The Journal of Experimental Biology at

Collecting migrating blackcaps at their stopover site on the Sede Boqer Campus of Ben-Gurion University and near Toron, Poland, Wojciechowski and Pinshow weighed the birds and monitored their body temperatures and metabolic rates as the birds stocked up on fruit supplemented with mealworms. During the day the birds' body temperatures hovered around 42.5°C, but as dusk fell, their temperatures began to drop. The average normal body temperature at night was about 38.8°C, while one particularly skinny individual's temperature plummeted to 33°C. And when the team plotted the birds' body masses against their nocturnal temperatures, the smaller birds' (< 16.3g) temperatures correlated with their body masses.

Finally, the team looked at the relationship between the birds' temperatures and their metabolic rates and found that the heavier birds dropped their metabolic rates least, while the lightest birds dropped their metabolic rates most. Some conserved a remarkable 30% of their energy by becoming hypothermic.

Knowing that small birds also conserve energy by huddling together for warmth, Wojciechowski and Pinshow suggest that migrating birds may combine both strategies to shorten refuelling stopovers to fatten up fast before hastening on their way.

Story Source:

Materials provided by Journal of Experimental Biology. Original written by Kathryn Knight. Note: Content may be edited for style and length.

Journal Reference:

  1. Wojciechowski, M. S. and Pinshow, B. Heterothermy in small, migrating passerine birds during stopover: use of hypothermia at rest accelerates fuel accumulation. Journal of Experimental Biology, 2009; 212 (19): 3068 DOI: 10.1242/jeb.033001

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Journal of Experimental Biology. "Migrating birds chill to fatten up." ScienceDaily. ScienceDaily, 11 September 2009. <>.
Journal of Experimental Biology. (2009, September 11). Migrating birds chill to fatten up. ScienceDaily. Retrieved May 23, 2017 from
Journal of Experimental Biology. "Migrating birds chill to fatten up." ScienceDaily. (accessed May 23, 2017).