24 January 2020

Climate’s Changing, So Are Birds

Welcome back. In an earlier blog post, I wrote that, though I grew to 5 foot 10-3/4 inches (“5-11” for high school basketball and dating), I’ve been getting shorter for years (see I’m Shrinking!).

Shrinking with age is common--spinal discs can compress, loss of bone density can cause spine curvature and collapsed vertebrae, loss of muscle and gain of fat can contribute.

The possible reasons vary, but they don’t include climate change. That can’t be said for North American migratory birds as shown in a study by a team of researchers affiliated with the University of Michigan and The Field Museum, Chicago.

Bird Sample and Measurements

Reflective windows can
be bird magnets. Imagine
how many strike buildings
during bird migrations.
North American migratory birds, whose breeding ranges span North America and winter ranges extend to South America, regularly collide with buildings in Chicago during their fall and spring treks. Since 1978, personnel and volunteers of The Field Museum have retrieved some 87,000 bird carcasses of more than 200 species.

One person (the same person) made the following morphological measurements on each fresh or thawed bird carcass: (1) tarsus (part of lower leg) length and bill length using digital calipers, (2) relaxed wing length using a wing rule and (3) mass using a digital scale. After the measurements, the carcasses were prepared as specimens, and skull ossification (bone formation, indicative of age), fat levels, sex and molt were recorded.

Although the measurements were made to assess annual and seasonal variation of birds, subtle changes in bird size were observed which led to the team’s analysis of longer-term trends and possible causes.

Shortening of tarsus length (millimeters) of two bird species over the years; dashed lines have zero slope, intercepts equal the mean tarsus length for each species and n equals number of specimens sampled (from onlinelibrary.wiley.com/doi/abs/10.1111/ele.13434).
Chief among the possibles was climate change. Body sizes of animals, especially birds and mammals, are often tied to climate. Within a species, individuals living in colder environments tend to be larger than those in warmer areas. The trend, referred to as Bergmann's rule, helps animals in cold places stay warm.

Linking Bird and Climate Changes
The dataset selected for analysis consisted of the measurements of 70,716 birds, collected 1978 to 2016. The birds represented 52 species (11 families, 30 genera), reflecting diverse ranges, habitats, migratory distances, life histories and ecologies.

The researchers employed two statistical modeling approaches (a form of regression and a mixed-effects model) to test hypotheses on causes of change in adult body size and wing length.

Body size changes focused on three indices: tarsus, mass and a statistical combination (first principal component) of tarsus, wing, bill and mass. These were related to species-specific estimates of climatic and environmental variables at the breeding and wintering grounds over time--temperature, precipitation and a proxy for resource availability (Normalized Difference Vegetation Index).

Wrap Up
The researchers found a near-universal reduction in body size for the 52 species over the four decades. Average body mass decreased by 2.6% and tarsus length by 2.4%.

Modeling showed the variable that accounted for the greatest change--by an order of magnitude--was increased summer temperature at the breeding grounds. There was no evidence that precipitation or resource availability drove the trend.

Lines represent all bird species, with group mean centered by bird species (70,716 specimens of 52 species). Mass, tarsus and a statistical combination of tarsus, bill, wing and mass (PC1) all decreased over the years, while wing length increased (from onlinelibrary.wiley.com/doi/abs/10.1111/ele.13434).
While body size decreased, wing length consistently increased across species. Average wingspans grew by 1.3%, a change which was not explained by environmental variables on the breeding or wintering grounds. There is evidence, however, to suggest that increased wing length could be a compensatory adaptation, allowing birds with smaller bodies to produce the energy needed to make the long migrations.

Stay tuned as climate continues changing, and thanks for stopping by.

Study of North American migratory bird changes in Ecology Letters journal: onlinelibrary.wiley.com/doi/abs/10.1111/ele.13434
Article on study on EurekAlert! website: www.eurekalert.org/pub_releases/2019-12/fm-bas112719.php
Bergmann's rule: en.wikipedia.org/wiki/Bergmann%27s_rule

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