Each one has been meticulously measured and recorded. 

But when researchers started analyzing those measurements in 2019, they noticed something strange.  

"These birds are pretty different in their behaviors: where they nest, where they winter, where they migrate.

But we found that everything — all of the species — had declined in size over the past 40 years."

One of the body parts declining in size was a lower leg bone called the tarsus. 

And when they lined up the tarsus measurements with summer temperature records, changes in temperature were followed one year later by a change in leg size.

"When temperatures started to cool down the next year, birds started to get bigger, when temperatures started to warm up, birds started to get smaller."

On average, warming temperatures were making birds shrink. 

But it's not just birds.

The same effect has been observed among salamanders in Appalachia, red deer in Norway, woodrats in New Mexico, fish in the Atlantic and Gulf of Mexico, and beetles around the world. 

These changes might seem small so far, but they could eventually push many species closer to extinction.

So why is this happening? 

Researchers are still in the process of answering that question, but there are two broad hypotheses. 

"The mechanisms really fall into two different categories. There's selection and developmental plasticity."

Selection pressure consists of changes that pass from one generation to the next because they're advantageous to survival, while developmental plasticity describes changes that happen during an animal's life due to environmental conditions.

For a long time, researchers believed that shrinking was caused only by selection pressure because of a principle in ecology called Bergmann's Rule.

"There's a really well known rule in ecology that individuals tend to be smaller in the warmer parts of their ranges.

So there is this long standing hypothesis that as we warm the world, things will get smaller."

That's because of warm-blooded animals' ratio of body size to surface area, which makes it easier for large animals to conserve heat when it's cold, and easier for small animals to cool off when it's hot.

"You could think about an ice cube melting into glass.

If you have a bunch of small ice cubes, they melt faster than one big ice cube. It's the same idea. "

But cold-blooded animals don't generate body heat, and they're shrinking too, which means that selection pressure for conserving body heat isn't the only thing at play.

Instead, cold-blooded animals are particularly susceptible to plastic changes: changes that occur during the individuals' lifetime and aren't passed on to the next generation.

With frogs, for example, warmer temperatures increase metabolism and make the transitions between life cycle development phases speed up.

But because their rate of growth doesn't change, they're smaller by the time they arrive at adulthood. 

What researchers are still trying to figure out is how much these two factors are shaping warm and cold-blooded species, and whether they can make predictions about what those changes will look like.

Plus, this trend isn't uniform: warming temperatures are making some species bigger.

In high latitudes, increasing temperatures and precipitation have given shrews, otters, and martens more time and resources to grow before winter.

But we do know that warming results in smaller animals on average, because Earth has been through periods of warming that have shrunk animal bodies before. 

In a warming event roughly 56 million years ago, temperatures increased between 5 to 8 degrees Celsius over 10,000 years.

And we can see a noticeable dip in animal size in the fossil record.

But today, we're warming the planet at an unprecedented rate — about 10 times faster than the average warming following historic ice ages, giving animals little time to adapt. 

"That's the problem with human-driven climate change.

It's the rate of change that's just orders of magnitude faster than what the natural world has had to deal with in the past. 

Size is really important to survival, and you can't just change that indefinitely without consequence.

For one thing, I don't think it's feasible that species are going to be able to continue to get smaller and maintain things like a migration from one hemisphere to another."

And since smaller bodies can hold fewer eggs, they result in fewer offspring, and a lower population size in the long run. 

For amphibians who need to keep their skin wet in order to breathe, shrinking can mean higher chances of drying out in a drought because their bodies absorb and hold smaller quantities of water.

But the more concerning consequences have to do with how this could destabilize relationships between species.  

Because shrinking plays out at different rates for different species.

Predators might have to eat more and more of shrinking prey, for example, throwing a finely-tuned ecosystem off balance.

It's that mismatch that's particularly dangerous.