1.	Hummingbirds are very small. Typically their mass is between 3 and 5 g. They are able to hover at a fixed point in the air by beating their wings very rapidly. The rufous hummingbird, Selasphorus rufus, is a migratory species. It breeds in Canada and Alaska in the summer, migrates south to Mexico in the autumn and returns to high latitudes in spring after completing its annual moult (loss of feathers, which are then re-grown).
(a)	Suggest why the rufous hummingbird has a very high requirement for energy.
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[4]

	In order to save energy, rufous hummingbirds can enter a state called torpor during the night. This is when their metabolic rate and body temperature both drop to a very low level. An investigation into how rufous hummingbirds use, save and store energy at different times of year was carried out. Key findings of the study are given in Figs. 1, 2 and 3 below.

	Fig. 1	Fig. 2

	Fig. 3
© Sara Hiebert, Hummingbird Torpor and Body Mass, from The Auk, vol. 110, October 1993.
Reproduced by kind permission of Sara Hiebert
•	Fig. 1 shows how use of torpor by the birds varies according to season.
•	Fig. 2 compares the oxygen consumption of birds resting at normal body temperature with that of birds resting in a state of torpor.
•	Fig. 3 shows how body mass of the birds changes over the course of a year.

(b)	Use Figs. 1, 2 and 3 to describe and explain the results for the birds in the September-October (autumn) period.
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[4]
 
(c)	Suggest how the low body mass of the birds in spring may be related to enhancing the birds’ survival during the moulting period, when the feathers are lost and regrown.
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[2]

(d)	It is suggested that smaller birds, which have a larger surface area to volume ratio when compared to larger birds, require more oxygen per gram of their body mass.
	Discuss whether the data given in Figs. 3.1, 3.2 and 3.3 support this hypothesis.
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[3]
[Total: 13 marks]