Why Birds Are Getting Bigger; Or Are They?

This post is a response to the post by L.A. Times “Greenspace”: Are birds getting bigger because of global climate change?

A banded Golden-winged Warbler
Golden-winged Warbler

The article linked above suggests that birds may be “bulking up in body weight to ride out severe storms related to global climate change.”

While this is an interesting idea it goes against Bergmann’s rule (as the article also points out).

“Bergmann’s rule: Birds and mammals tend to be larger at higher latitudes, perhaps to conserve body heat. Under this reasoning, birds and mammals would get smaller as they adapted to rising global temperatures.”

But it also only looks at one area that can effect birds (and other animals). The amount of food available has changed greatly over the years with people and entire cities planting more food-producing plants that birds benefit from. Plus, there are many more backyard bird feeders so an overall greater amount of easy-to-access food.

However, my theory of why birds might increase in size is that, along with better food, the water they drink and the chemicals they encounter on plant material and the insects they eat is causing them to grow larger.

Plus, it would make sense that bigger birds would be better at surviving adverse (localized) weather and fighting for better territories and then brooding more chicks. It all comes back to habitat loss: with less prime habitat available, the stronger (bigger) birds would have the better habitat and the weaker (smaller) birds would have the “lesser” habitat.

Of course, all of this is just my thoughts and the article only points out a study in central California and size increases of 2% to 5%. What about elsewhere? Is that significant?

Definitely some interesting research but more is needed. We’d love to hear other birder’s thoughts on this study.

The data collected is from banded birds at Point Reyes National Seashore and the southern end of San Francisco Bay. We love the use of banding data for studies as it is one of the best methods to collect great amounts of data.

One thought on “Why Birds Are Getting Bigger; Or Are They?

  1. Spring 2022, NJ, definitely have several Robins that are larger than I’ve seen over the past 40 years. I also have many more Robins than in the past. The ‘larger’ robins are a bit ‘chunkier’’ than Morning Doves. I still have Robins that appear w/in the ‘normal’ size range so the larger ones stand out. Other birds seem a bit larger too…though the overall numbers are way down due to the sudden death of several birds over the past year (due to something strange)….they literally just fall out of the sky. I’ve had 5 dead birds in the past year…previously, I don’t recall seeing one.

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