The diets of tropical birds are not influencing population changes

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Miami University

Source: Boyle W.A., Sigel B.J. 2015. Ongoing changes in the avifauna of La Selva Biological Station, Costa Rica: Twenty-three years of Christmas Bird Counts. Biological Conservation 188: 11-21

Summary: Contrary to previous research, recent research has found that the diets of tropical birds are not a main factor that affect affect bird communities. Instead, body size was found to be strongly associated with population changes. This correlation indicates physiological mechanisms that could have resulted from climate change.

Human activities have lead to habitat loss and forest fragmentation of various species across the globe. These impacts have led to population changes in many organisms including birds. Research by Boyle and Sigel conducted at La Selva Biological Station in Costa Rica analyzed the changes in tropical birds communities. Previous studies had shown that insectivorous birds were particularly sensitive to the changes caused by human activities.The authors analyzed 23 years worth of data collected by bird lovers who go La Selva and spend two weeks counting the bird species they see within a defined area. The population sizes were then categorized in different dietary groups to see if there was a particular pattern between a specific diet and population changes. To explore other possible correlations with population changes they also looked at other variables such as flocking, body mass, habitat, and rarity.

The analyses indicated that from 1989 to 2011, 63 species’ populations out of 202 have decreased and 44 have increased. However there was no correlation between any of the dietary categories with population change. What was found to have a strong correlation with population changes was body size. Specifically, small birds were more likely to experience in population decline in La Selva. The unexpected findings of this study suggest that there should be further research to determine what is the major drive of the population declines of tropical birds. With a better understanding of this issue, better management plans can be made to prevent biodiversity loss in the tropics. The physiological mechanisms that seem to cause population declines of small tropical birds may have resulted from climate changes. If this is the case it would be one more reason to battle the ongoing change of Earth’s climate.

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