Birdwatching Calculators

Optics, feeding, timing and list keeping. 10 calculators for birders. The same optics arithmetic turns up under Astronomy.

Binocular specifications explain why 8x42 became the birding standard, and it comes down to one number. Exit pupil is objective diameter divided by magnification, so an 8x42 delivers 5.25 mm while a 10x42 gives only 4.2 mm. Your own pupil opens to around 5 mm at dusk and up to 7 mm fully dark-adapted. Once the binocular’s exit pupil is narrower than your pupil, you are throwing light away. 5.25 mm matches a typical adult at dusk without carrying a 50 mm objective around all day.

Hummingbird nectar is four parts water to one of sugar and nothing else. No dye, no honey, no raw or brown sugar. By weight that lands near 17 to 18% sugar, a little thinner than real flower nectar at 20 to 25%, which is deliberate: it is easier to drink and easier to keep clean. Go weaker than 4:1 and the birds work out that flowers pay better than your feeder.

Dawn chorus and migration pages run off your latitude, so set it once.