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Some fascinating questions are raised by Kurt and William Burnham and Ian Newton’s findings, reported in this issue of Ibis, that 14C dating has shown some Gyrfalcon Falco rusticolus nests to have been occupied for around the past 2500 years. As the authors demonstrate, this finding could make an important contribution to our understanding of climate change in the high arctic. And stable isotope analysis also has the potential to record changes in prey selection and foraging habitat over long periods of time. Other detritus from nest-sites can also achieve this.

Remains of bones, feathers and eggshells from abandoned Adelie Penguin Pygoscelis adeliae colonies has revealed clear changes in diet over tens of thousands of years (Emslie et al. 2007), and Hiller et al. (1988) used deposits of solidified stomach oil from Snow Petrel Pagodroma nivea sites to prove occupancy in the Antarctic back to 33 900 years ago. It is perhaps not surprising that sites occupied by colonial seabirds show such site fidelity. Some other colonial species, such as some species of swift and cave swiftlets (Apodidae), oilbirds Steatornis caripensis, herons and egrets (Ardeidae) and weaverbirds (Ploceidae) show similar site fidelity within historical times, because the location is generally accepted to be a reflection of food resources around the colony (Furness & Birkhead 1984); if these remain unchanged, colony location will also probably persist.

The Gyrfalcon study raises a separate question, and is an extreme example of the well-known feature of some birds of prey, where a solitary nesting species makes repeated use of exactly the same nest location over long periods of time: what is it about certain raptor sites which makes them so very attractive?

Greenland has an abundance of rocky ledges, and the finding that specific sites have been so strongly chosen over millennia implies that there is strong selection pressure for only a very few places. We can measure a limited number of variables that might influence this choice, such as ledge size, shape, shelter, aspect and cliff features (Newton 1979). And species undoubtedly differ in their requirements. Walter (1979) showed that in a small species, Eleonora’s Falcon Falco eleonorae, visual isolation was an important factor, and perhaps one that is given lower priority by larger species. Figure 1 shows a nest of a Ruppell’s Griffon Vulture Gyps rueppellii in the Serengeti region of northern Tanzania (see Houston 1976), where the extent of the guano deposits suggests centuries of occupation, although we agree with the authors of the Gyrfalcon paper that subjective assessments are unreliable without 14C dating. These vultures feed on the Serengeti ungulate populations, which archaeologists know to have been present for tens of thousands of years. This particular nest-site was always among the first to be occupied in the colony, and we suspect that for species such as the larger falcons, eagles and vultures, wind conditions in the vicinity of the nest-site are a critical factor in site selection. But because they are almost impossible to measure, they have received little study. These species all have a wing morphology that prevents precise control of flight at slow speeds (Pennycuick 1989). This means that accurate and controlled landing on the nest can be difficult if the wind conditions are turbulent, unpredictable or from the wrong direction. Watching young birds making their first flights from the nest always makes us anxious, because some inexperienced fliers make repeated unsuccessful attempts. A young fledgling that fails to return to the nest will probably fail to survive. In species with a low rate of reproduction, this must place strong selective pressure on adult birds to select breeding sites extremely carefully for their wind conditions. Just because these factors are difficult for us to measure, does not mean they are unimportant. Tree climbers will also know that unpredictable wind conditions are definitely not confined to rock ledges. Tree nest-sites also differ considerably in the predictability, strength and direction of their air flow. We suspect that for many large raptors these considerations may be critical in determining an optimum nest location, and explain why so few sites are chosen. And these factors are probably not confined to the immediate vicinity of the nest-site. Virtually all raptors use soaring flight to some extent, and the wind conditions and sites of lift along major cliff faces, hill ranges and woodland edges within the foraging range of their nest-site may be important factors in the speed and energy efficiency with which they can hunt. Greenland must experience the most harsh weather conditions that any predatory bird has to withstand, and maybe these factors are particularly relevant for the Gyrfalcon in the extreme weather conditions with which it has to contend.

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Figure 1.  Nest-site of a Ruppell’s Griffon Vulture Gyps rueppellii on cliffs of the Gol Mountains, northern Tanzania, showing extensive guano deposits at the site. Photo: David Houston.

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References

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  2. References
  • Emslie, S.D., Coats, L. & Licht, K. 2007. A 45,000 year old record of Adelie Penguins and climate change in the Ross Sea, Antarctica. Geology 35: 6164.
  • Furness, R.W. & Birkhead, T.R. 1984. Seabird colony distributions suggest competition for food supplies during the breeding season. Nature 311: 655656.
  • Hiller, A., Wand, U., Kämpf, H. & Stackebrandt, W. 1988. Occupation of the Antarctic continent by petrels during the past 35,000 years: inferences from a 14C study of stomach oil deposits. Polar Biol. 9: 6977.
  • Houston, D.C. 1976. Breeding of the White-backed and Ruppell’s Griffon Vultures. Ibis 118: 1439.
  • Newton, I. 1979. Population Ecology of Raptors. Berkhamsted: T. & A.D. Poyser.
  • Pennycuick, C.J. 1989. Bird Flight Performance. Oxford: Oxford University Press.
  • Walter, H. 1979. Eleonora’s Falcon: Adaptations to Prey and Habitat in a Social Raptor. Chicago: University of Chicago Press.