The weight of a bird is a basic measurement that can be used as a measure of size in cross-species comparisons. However, as it incorporates variation in both size and condition (e.g. fat reserves), it should not be used alone as either a measure of size or condition in intra-specific studies. Weight is readily determined either with a lightweight precision spring-balance (e.g. manufactured in Switzerland by Pesola®), or on an electronic, digital, pan balance. Again, these are now available as small, precise, highly portable battery or solar-operated units (e.g. Tanita® TPK100). Birds of up to 50 g should typically be weighed to a precision of 0.1 g, though greater resolution is desirable for the smaller hummingbirds. For birds between 50 and 300 g weight to the nearest gramme should suffice, and for even heavier birds to the nearest 10 or 100 g as appropriate. The bird should be immobilized in some form of restraint (a polythene funnel or cone weighing c.0.2 g is ideal for smaller birds), and this should be clipped onto the spring balance or, for digital balances, closed with a bulldog clip of known weight and laid on the pan of the digital balance. When weighing in the field, windy conditions can distort readings, but one way to avoid this problem is to suspend the bird in its cone within a larger windproof container such as a large jar (or a plastic or cardboard tube).
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