I’ve always been fascinated by how much variation there is among bird species. Color, body shape, wing shape, nest type, social behavior, food types and vocalizations, to name a few attributes, vary enormously. Birds occupy every habitat on earth, except for the deep sea, and all that adaptation has led to a huge variety of forms. There are now 11,167 recognized species on Earth.
And how about size? An adult bee hummingbird — a Cuban endemic species — weighs only about 2 grams (g), while an adult male common ostrich — widespread in Africa — can weigh up to 157 kilograms. So, that ostrich is 78,500 times heavier than the bee hummer. That’s variety.
For the 658 bird species in North America for which I have data handy, weights range from 2.7 g for the calliope hummingbird to 10,500 g for the California condor. The average is 414 g, which is the weight of a typical black-legged kittiwake. If kittiwakes are not familiar to you, consider a typical northern harrier at 420 g.
The 11 lightest species in North America are all hummingbirds, which is probably not too surprising. The smallest non-hummer on the list is the black-tailed gnatcatcher at 5 g. But we’re not finished with hummingbirds yet. The blue-throated mountain-gem (formerly the blue-throated hummingbird) weighs in at whopping 7.6 g, heavier than both kinglets and the blue-gray gnatcatcher.
For more perspective, a lesser goldfinch weighs 9.5 g, an American robin 79 g, a mallard 1150 g, and a great blue heron 2300 g.
I emphasize that all these weights are averages. And as with the average family size in the U.S. (3.15 people), the average bird probably does not exist. Among the many qualities of the All About Birds website (allaboutbirds.org/news/) is that the site reports the range of weights for each species under the “ID Info” box. For the previous species, the lesser goldfinch ranges 8-11.5 g, the American robin 77-85 g, mallard 1000-1300 g, and the great blue heron 2100-2500 g.
There are several sources of this variation. First, all birds are different. Just as with humans, and likely most other species (I’ll leave parthenogenesis to you), each individual is a little different. That’s part of the variation that provides the raw material for evolution by natural selection.
Second, age is an obvious factor. Older birds are heavier than younger birds. But unlike humans, adult birds do not tend to keep gaining weight as the years go by. Here’s a billion-dollar idea if you can find the reason. I’ll take 10%.
Third, males and females differ substantially in many species. And this can go both ways.
In some birds of prey and in some flightless species, males are heavier. But it’s more common for females to be heavier. Female hawks, eagles, falcons, owls and those of many shorebirds tend to be larger than males. The difference can be 20–50% or more in some species.
The most familiar example around here is in Cooper’s hawk. When you see a nesting pair together, they almost look like two different species. Males weigh up to about 411g while females can reach 680g.
There have been a number of explanations proposed over the years for why females, in birds of prey especially, are larger than their mates. I have found all of these theories to be unsatisfying because most bird species get along just fine with males and females being about the same size. As far as I can tell, to this day there is no good explanation.
I’m tempted to conclude that the answer is the same as is the answer to why some birds are colorful and some are plain, some have incredible complex songs and some have only chips or squawks, and some lay only two eggs while others lay more than a dozen. There are many ways to be a bird. Countless strategies have evolved, and they all work.
An individual bird’s weight can change by 10-30% over relatively short periods of time and by even more over the course of a year. Birds preparing for long migrations can accumulate large fat reserves. A small songbird might gain 20-50% of its body mass before a major migratory flight.
Males of lekking grouse, like greater sage-grouse, lose up to 15% of their weight over the spring season because they are displaying for many hours a day in “hopes” of getting to mate with most of the females on the lek. The stakes are very high. Losing weight is their last concern.
Females of most species gain weight before laying eggs. Their pre-laying condition can be critical to the number and quality of eggs, as well as to their health. The ability of a male to provide food to his mate during this period has been shown to be an important variable. There are various theories on how females judge males on their health and robustness before they choose to mate with that male. Sound familiar?
After the young have hatched, both sexes of adults in most species lose weight. The period of feeding chicks is intense. Food has to be constantly sought, procured and delivered to hungry young while the parents put their needs second. Because many species spend all of their days looking for food at any time of year, it’s amazing they can find yet more to feed their nestlings during the nesting period.
That’s one reason the availability of insects is so critical and that many species’ nesting period is tied to insect abundance. The volume of food and its content of protein, fat and micronutrients is essential.
One of the documented costs of climate warming is that insect emergences get out of synch with the timing of some bird species’ nesting cycle. Nestlings may hatch before or after their main insect food appears and then the young do not have adequate resources. This mismatch of resources can occur in other ways, such as when leaf or grass growth is not adequate to provide cover for nests.
One of the questions I often get asked by young audiences (K-6) when I talk about common birds in their area is something like, “how much does a magpie weigh?” I’m never ready for this because there are so many possible questions along those lines.
The correct answer is, “it depends” (on all those things I just covered). But that is unsatisfying to second-graders. My best response is, “What do you think? Is it more or less than a raccoon?” That gets us talking about the relative weight of various animals, and I can forget about the original question. Way more fun than grams and kilograms.