Birds like this American tree sparrow are declining rapidly, shows a study which looks at huge declines in North American bird populations (Image Credit: Ryan Hodnett, CC BY-SA 4.0, Image Cropped)

Decline of the North American avifauna (2019) Rosenberg et al., Science, DOI: 10.1126/science.aaw1313

The Crux

When we talk about species loss, we generally focus on extinctions. Too often, when we start to rally around a species, it’s because there are a particularly low number of that species left. In many cases, they’ve often crossed a threshold, from which it’s impossible to pull them back from the brink of extinction.

Often this draws attention away from non-threatened species. Often that’s fine – they’re non-threatened right? But downward population trajectories in these species can still damage ecosystems by lessening the impact of their ecological function, lead to local (if not total) extinctions, and of course, leading them to eventually be threatened.

This week’s authors wanted to look at bird population declines in America, but from the perspective of total abundance, as opposed to a more species-specific view.

What They Did

Birds are a great source of data, as there’s generally plenty of funding for bird-monitoring programs, as well as masses of community science data from avid birdwatchers. The authors used data from a wealth of bird-monitoring datasets across the US and Canada, obtaining information for over 500 different species.

They also used a technique which (subjectively) is cool as hell, using data from weather monitoring stations to obtain an estimate of the reduction of biomass (total “weight” of birds) migrating across the US at nighttime.

Did You Know: Data Integration Bringing together data from different source is no simple task. Different researchers have collected the different information with different goals in mind, meaning the results could be formatted in all sorts of ways. Luckily organisations like GBIF (the Global Biodiversity information Facility) have been compiling this data over recent decades to the point where vast swathes of online data on different species is now easily downloadable. There are still considerations that have to be made when using such data sources, of course, but it’s a huge step forward for people studying large scale trends in species populations. Read more about it at the link below. The Modern Biologist’s Challenge: Data Management

What They Found

Both techniques showed massive declines in North america’s bird life. The long-term surveys showed a loss of between 2.7 and 3.1 billion birds, equating to a reduction of about 30% since 1970. The weather station data indicates a loss of around 14% in avian biomass since 2007. When categorised by what sort of habitat the birds prefer, grassland birds are the worst off, although every habitat type except wetlands have lost significant numbers of individuals.

Problems

Whilst the use of weather stations to monitor bird biomass is pretty inventive, it obviously lacks a lot of nuance. It’s hard to tell how closely a drop in bird migrations relates to a drop in abundance. Additionally, the confidence interval on that 14% is pretty high at 9% – meaning the actual figure could theoretically be anywhere between 5% and 24%.

Additionally, any paper that makes claims using large numbers like this will make headlines. Headlines that generally focus on those big numbers (as, admittedly, I have). So for those who want to go into more depth, I’d highly recommend digging into Brian McGill’s critique of the paper, linked below.

Did America Really Lose 3 Billion Birds?

So What?

This is another reminder that our understanding of species declines needs to go beyond the concept of species extinctions. The fact that we are picking up these trends in such a well-monitored group of animals means they are likely present in a whole lot of other groups. The factors that are causing the bird declines, such as pollution and habitat loss, are just as likely to negatively affect other species.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.