Running the hummer numbers

I just ran across a post from last August on The Fat Finch Bird Brain Blog that addresses the frequently asked question about how to calculate the numbers of hummingbirds visiting your feeders. The bloggers based their formula on a book that they absolutely gush over, Hummingbirds of North America by Dan True. True is a meteorologist by profession and was the weatherman for an Amarillo, Texas TV station when my husband Tom was growing up there. Tom has fond memories of True rushing through his weathercast to have time to show his amateur wildlife films.

Nostalgia aside, I don’t share the TFFBB bloggers’ enthusiasm for True’s hummingbird book. It’s full of half-baked ideas, misinterpretations of other people’s work, and out-of-date information such as the sugar-consumption figure on which the TFFBB bloggers based their feeder-usage formula. They said they’d love to hear from anyone with different methods of estimating hummingbird populations around feeders, so I’ll share my take on the issue plus a couple of others. (Warning to math and chemistry geeks: Biology is messy business, so the measurements and calculations used below will be a bit sloppier than you might like.)

The actual statement from True’s book is: “Hummingbirds eat an amount of 25% content nectar that is equal to their body weight, daily (Skutch 1973).” The source cited is Alexander Skutch’s Life of the Hummingbird, which I no longer have in my library and so can’t check to be sure the number was cited correctly (the clumsy sentence structure suggests it’s not a direct quote). This is a pretty sweeping generalization to start with, and whoever came up with this figure miscalculated by a pretty large margin.

Studies of field metabolic rates (the average rate at which an organism consumes energy as it goes about its daily life) indicate that small hummingbirds such as Black-chinned and Ruby-throated are going to need 45% to 50% of their body weight in sucrose (a.k.a. white sugar, the dominant sugar in the nectar of hummingbird flowers) to get through an ordinary day, so they would actually need 180% to 200% of their weight in a 25% sucrose solution.

A 25% solution is much stronger than most people use in their feeders. The generally recommended proportion is 1 part table sugar to 4 parts water by volume, which comes out to about 18% sugar by weight. Converting to this recipe, it would take approximately 250% to 280% of the bird’s weight in ordinary 1:4 feeder solution to meet each bird’s daily energy requirements.

So, how do you use these data to estimate numbers of feeder visitors? The simplest way is to convert grams to fluid ounces so that you can measure the volume consumed (you can even mark your feeder and estimate usage on the fly).

According to my postal scale, one fluid ounce of 1:4 sugar water weighs about 35.5 grams (approximately 20% more than its plain water counterpart). We’ll average the weight of the birds to 3.5 grams, or about 10% of the weight of a fluid ounce. Multiply that times by 265% for average consumption and we get 0.265 fluid ounce of 1:4 feeder solution per bird per day, which we’ll round down to 1/4 fluid ounce per bird per day. This multiplies out to around 32 smallish hummingbirds per 8 ounces of 1:4 sugar water, 128 per quart, and 512 per gallon. This is higher than the TFFBBB estimate, which is not surprising considering the differences between our figures for weight and consumption rates of the birds and weight/volume ratio of the sugar solution.

Of course, there are a lot of factors that can skew this already crude estimate. The amount of sugar water each bird consumes may be greatly reduced when natural nectar sources are available and greatly increased when the birds are under stress from cold, drought, courtship, fighting, nesting, and/or migration. A given volume will supply the needs of more birds if you make your feeder solution a little stronger than 1:4, as many people (myself included) do in winter and migration, and fewer if you make it a little weaker. Size figures in as well, so a given volume of sugar water will feed fewer Anna’s than Black-chinneds.

The late William A. Calder III Stephen M. Russell, professor emeritus at the University of Arizona and the world’s authority on Rufous and Broad-tailed co-author of the Birds of North America life history account on Black-chinned Hummingbird, estimated that a gallon of sugar water would feed about 750 Black-chinneds for a day. Based on published field metabolic rates, this number would be quite high unless you are a) using a ratio of sugar to water much higher than usual for feeders (almost 28%) or b) assuming that the birds are getting a significant fraction of their calories from other sources. I wish I knew how Bill Steve arrived at this figure, but since I don’t I’ll stick with the more conservative figures I derived from the metabolic studies.

Fellow hummingbird researchers Nancy Newfield and Bob and Martha Sargent have suggested a method for estimating hummingbird numbers that has nothing to do with sugar consumption. They recommend counting the number you can see at one time and multiplying this number by six. The nice folks at Bird Watcher’s Digest used this method to estimate their population of Ruby-throateds, but they also kept track of their sugar-water usage. Based on a half gallon of feeder solution per day, I’d estimate that they’re feeding over 250 hummingbirds, not the 139 they estimated by counting and multiplying. Of course this method would never work at really busy feeders like the ones at the home of my friends Bruce and Sue in Arizona’s White Mountains (below), but it’s one more approach to a challenging question.

Sitko hummers in the White Mountains, July 2007

One additional nitpick about the TFFBBB entry: It assumes that Ruby-throated Hummingbirds migrate south across the Gulf of Mexico when the evidence strongly suggests that the majority take an overland route around the Gulf in fall migration. They didn’t get this from Dan True, whose book thoroughly documents his skepticism about trans-Gulf migration in either direction (he’s apparently unaware of the many Ruby-throateds that take refuge on offshore oil rigs and fishing boats each spring, along with other trans-Gulf migrants, or eyewitness accounts of them skimming the waves on approach to coastlines).