Birds in the Anthropocene: Scientific evidence of rapid change

The world is changing at an unprecedented pace. Warming of two to four degrees Celsius appears to be inevitable by the end of this century, at which time the human population will exceed 10 billion. This is the Anthropocene, a new geological epoch in which humans are the dominant ecological force. Emerging scientific evidence shows that our impacts are dramatically affecting birds.

We need not look further than Albuquerque backyards to see that bird populations respond quickly to environmental change. Before the post-war expansion of the city, most of it was desert grassland, with Horned Larks aplenty. Now the larks have been replaced by species that subsist wholly on urban subsidies: water, trees, and birdseed. The birdseed feeds White-winged Doves and House Sparrows, which are then eaten by Cooper’s Hawks and Greater Roadrunners, respectively. Native species such as Bullock’s Orioles, Black-headed Grosbeaks, and Black-chinned Hummingbirds have expanded their populations into the mulberries and Chinese elms that line the streets.

Consistent with our local experience, ornithologists measuring bird populations are reporting evidence of rapid change throughout the world. Here are four striking examples:

  1. Humans are directly eliminating one of the world’s most intelligent bird species, the African Gray Parrot, from its wild habitats in west Africa. A new study has shown that populations have declined by 90-99% in Ghana over the past two decades (Annorbah, Collar, & Marsden, 2015). The primary cause is trapping for the pet trade. Like wild elephants, wild African Gray Parrots cannot withstand unfettered market forces.
  2. Indirect human impacts are also contributing to bird population declines. In a pristine Amazonian rainforest, the entire avifauna has declined in abundance by 40-50% since 2008 (Blake & Loiselle, 2015). The study site in northeastern Ecuador has been monitored since 2001 by Bette Loiselle and John Blake of the University of Florida. The leading hypothesis for the decline, according to Loiselle and Blake, is climate change.
  3. Long-term declines in survival rates of North American birds seem to have both direct and indirect anthropogenic causes. A recent study of data from breeding-season banding stations revealed persistently reduced survival rates following the invasion of West Nile Virus, ~15 years ago (George et al., 2015). Particularly hard-hit species include Purple Finch, Swainson’s Thrush, Wrentit, and White-crowned Sparrow. The authors found significant declines in survival rates for about half of the North American species that they examined (23 out of 49). They argued that disease effects could be magnified by habitat destruction and climate warming.
  4. Climate change may help some bird populations, at least temporarily. A new study showed that understory forest birds are thriving in high-altitude forests on Kilimanjaro, where both low and high elevation species increased in abundance over a 20-year period (Dulle et al., 2015). Even this news could be seen as ominous, however, as anyone can see that the cone-shaped Kilimanjaro gets smaller towards the top. Other recent studies have shown that bird species are moving upslope in New Guinea, Costa Rica, and Peru. The ‘climate escalator’ seems to be a global phenomenon.

Figure for Bosque Tracks 2015 Anthropocene articleWhat can we do to help bird populations survive the Anthropocene?

First, we can think globally. Our leading climate scientist, Dr. James Hansen, argues that preventing catastrophic global warming is only possible if we put a price on fossil fuel consumption that accounts for its true cost. This suggests that those of us who abhor the prospect of avian mass extinction should advocate a carbon tax, an idea that has support from both the left (Bernie Sanders) and right (Gregory Mankiw) sides of the political spectrum.

Second, we can act locally. It is possible to reduce bird mortality by our direct actions. For example, we can keep our cats inside. The outdoor cats that we feed and the feral cats that our communities tolerate are major sources of mortality for birds. A 2013 study scientifically estimated the impacts of domestic cats on birds (Loss, Will, & Marra, 2013). Out of ~84 million pet cats in the contiguous United States, about half are allowed outside, and these kill ~1 billion birds per year; feral (unowned) domestic cats kill another ~1.8 billion. This year, Australia launched an effort to cull ~2 million feral cats to reduce stress on its native wildlife populations. In the United States, public awareness of the wildlife carnage inflicted by free-ranging domestic cats is relatively low. Compared to rising carbon dioxide and melting icecaps, lack of awareness seems eminently solvable.

Annorbah, N. N. D., Collar, N. J., & Marsden, S. J. (2015). Trade and habitat change virtually eliminate the Grey Parrot Psittacus erithacus from Ghana. Ibis, in press. http://doi.org/10.1111/ibi.12332

Blake, J. G., & Loiselle, B. A. (2015). Enigmatic declines in bird numbers in lowland forest of eastern Ecuador may be a consequence of climate change. PeerJ, 3, e1177. http://doi.org/10.7717/peerj.1177

Dulle, H. I., Ferger, S. W., Cordeiro, N. J., Howell, K. M., Schleuning, M., Böhning-Gaese, K., & Hof, C. (2015). Changes in abundances of forest understorey birds on Africa’s highest mountain suggest subtle effects of climate change. Diversity and Distributions, in press. http://doi.org/10.1111/ddi.12405

George, T. L., Harrigan, R. J., LaManna, J. A., DeSante, D. F., Saracco, J. F., & Smith, T. B. (2015). Persistent impacts of West Nile virus on North American bird populations. Proceedings of the National Academy of Sciences of the United States of America, 112(46), 14290–14294. http://doi.org/10.1073/pnas.1507747112

Loss, S. R., Will, T., & Marra, P. P. (2013). The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications, 4, 1396. http://doi.org/10.1038/ncomms2380

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