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AnimalsClimate ChangeGlobal WarmingNatureScience

The Exception to Bergmann’s Rule

 

photo of birds

The birds in California are growing. Nobody is exactly sure why, as it goes against the existing theory known as “Bergmann’s Rule,” which states that the colder the environment or the higher the latitude, the larger the bird or mammal will be (most likely to preserve body heat). Birds and mammals of the same genus tend to be significantly smaller when found in warmer climates. Bergmann’s Rule has been accepted in the science community for over 150 years, so by all logic, birds in California should be getting smaller in size due to global warming, not bigger.

Yet, according to a study conducted over the last 40 years, birds in central California have increased between 2 and 5 percent in body weight and wingspan. Data have been collected from a variety of birds by researchers at San Francisco State University since 1971. Almost 15,000 birds have been tracked near the northern tip of the bay at the Palomarin Field Station at Point Reyes National Seashore since 1971. From 1983 to 2009, an additional 18,052 birds have been tracked from the southern end of the bay at Coyote Creek Field Station.

A wide variety of bird species were used in the study. Each bird that was used in the study was caught, banded with an identification tag, and weighed and measured before being released. Many of these birds were recaptured over the following years, which contributed to building a database that tracked the changes in the birds.

In a similar study conducted in Pennsylvania, the birds shrunk, following Bergmann’s Rule. Other researches throughout Europe have also studied patterns in bird growth over the decades and have found that the birds tend to have decreased in size, as well. So why are California birds the exception to the rule?

Researchers in California expected to find that the birds they were studying would decrease in size. They were quite surprised to discover the opposite was true. One theory as to why California’s birds are increasing in size, though, is that they are storing more body fat in order to survive extreme weather conditions, which scientists say are connected to climate change. Simply put: larger birds have higher chances of surviving a storm than smaller birds. By this theory, natural selection is selecting birds with larger bodies and wingspans.

Another theory is that the birds’ diets have changed. With the warming of the climate, plant and insect populations change, meaning the local birds would have access to different types of foods that might be causing the birds to pack on the pounds and grow bigger in size.

So, what does the increase in bird size mean? Scientists don’t know yet, and state that more research will have to be done in order to determine the consequences of these changes.

One thing’s for sure, the next time you step outside your garage door in California, keep an eye out for birds. They shouldn’t be hard to spot.

Join in the discussion in the comments below and share this piece with your California friends.

Photo credit: AttributionNoncommercialNo Derivative Works Some rights reserved by Macorig Paolo




2 comments
  1. Michael R.

    There my, in actuality, be no violation of Bergman’s rule. Birds are some of the very ‘first responders’ to climate changes…with many species adapting to warming by migrating north and ‘up-slope’ to higher elevations and cooler temperatures (note that California has many montane environments/ecosystems.

    See my early PS post:

    Movin’ On Up – Global Warming Forcing Plants, Animals
    North & Up-Slope

    http://planetsave.com/2011/08/22/movin-on-up-global-warming-forcing-plants-animals-north-up-slope/…thus, the increasing size of some birds may reflect, in part, this general move to higher elevations (where temps are cooler) by many species of bird.

  2. Michael R.

    There my, in actuality, be no violation of Bergman’s rule. Birds are some of the very ‘first responders’ to climate changes…with many species adapting to warming by migrating north and ‘up-slope’ to higher elevations and cooler temperatures (note that California has many montane environments/ecosystems.

    See my early PS post:

    Movin’ On Up – Global Warming Forcing Plants, Animals
    North & Up-Slope

    http://planetsave.com/2011/08/22/movin-on-up-global-warming-forcing-plants-animals-north-up-slope/…thus, the increasing size of some birds may reflect, in part, this general move to higher elevations (where temps are cooler) by many species of bird.

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