New Hybridization section offers unparallelled visibility into avian hybrids

BOW Team March 20, 2023
Red-billed StreamertailTrochilus polytmus

Birds of the World continues to evolve. We recently added a new section of content to species accounts called “Hybridization.” This section – which can be found under the Systematics menu –  dynamically integrates multimedia of known hybrids from the Macaulay Library wildlife media archive into the account, so the appearance, location, and ecology of hybrid crosses are available for study and comparison.

Hybrid section in BOW

As time goes on, these sections will be filled out by Birds of the World authors to include narrative content about the implications of hybridization on the ecology, evolution, and conservation of the parent species (for an general review of those topics, keep reading below). 

Some great examples of completed avian hybrid accounts include the following (click Systematics/Hybridization once you review the introduction). 

Let’s review some examples of how hybridization is important to the study of birds: 


Range Changes and Introductions: The distributions of species are rarely static, whether that is due to natural expansions and contractions associated with climate change, changes in prey and food availability, habitat succession, or population growth, or to human mediated introductions. When distributions do change, a species may come into contact with new species, and if those species are closely related, they may interbreed and produce hybrid offspring. Hybridization in these instances can be limited to a few random occurrences, or it could be more widespread. Geese, and waterfowl in general, are well-known for their proclivity to hybridize with other species, and the Barnacle Goose (Branta leucopsis) is no exception. Barnacle Goose has been documented to have hybridized with almost every other species in both the Anser and Branta genera. Most of these are the result of Barnacle Goose interacting with these duck species as a result of accidental introductions, either of captive individuals or domesticated individuals. Two species Barnacle Goose hybridizes with relatively frequently are both Cackling (Branta hutchinsii) and Canada Goose (Branta canadensis). Hybrids with Canada Goose are particularly common in northern Europe, where Canada Goose has established feral populations. Hybrids with Cackling Goose are also common in the Netherlands and Sweden, where there are introduced populations of Cackling Goose. Natural hybridization with Cackling Goose is also relatively frequent in Greenland, where hybrids are encountered in the northeastern United States. Hybridization may have increased in Greenland with the huge growth of the Greenland breeding population of Barnacle Goose.


Hybrid Zones: How and why different species evolve are questions evolutionary biologists are always trying to answer. Studying hybrid zones, or locations where two (or more) species’ ranges come into contact and interbreed to form hybrids, is one way that scientists go about trying to answer these questions in nature. Things like the frequency of hybrid offspring relative to the two species, where those hybrids are found on the landscape, and whether the area where hybrids occur changes over time, are all clues that can help scientists to understand how the two species may have evolved in the first place. Even if hybrid populations?s outnumber the parental species, if those hybrids are found over a relatively small area, that means that hybrids likely have lower fitness than either of the two species, and in the long-term, the two species will likely continue to evolve on their own separate trajectories. For example, in the recently split Red-billed (Trochilus polytmus) and Black-billed Streamertail (Trochilus scitulus) hummingbirds, hybrids are found across a very narrow band 2.2–3.7 km wide. What is more, this narrow area where hybrids occur has not moved over a period of over 50 years. All of this suggests that hybrids are not as biologically successful as the two parental species, and the two species have evolved enough differences to prevent them from fusing back into a single species.   


Conservation: Hybridization also has real-world implications for conservation. Endemic to New Zealand, for example, is the critically endangered Black Stilt (Himantopus novaezelandiae) or kakī (Māori). The exceedingly small population (<170 as of April, 2021) of the Black Stilt is threatened by predators and habitat degradation. Furthermore, the genetic integrity of the Black Stilt is threatened by its frequent hybridization with the native Pied Stilt (Himantopus leucocephalus). Pied Stilt colonized New Zealand naturally in the 1800s, but hybridization was not widespread until Black Stilt populations crashed in the mid 1900s. While Black Stilt populations crashed, Pied Stilt populations expanded in New Zealand, spreading across New Zealand as forests were cleared and open wetlands created. The increase in hybridization has been attributed to Black Stilts being unable to find Black Stilt mates, and therefore pairing up with the more numerous Pied Stilts. Hybrid offspring appear to show reduced fitness. Active management has included captive breeding and predator control, in the hopes that given a large and stable population of Black Stilt, hybridization will be minimal.


The new “Hybridization” content sections within Birds of the World will help shed much needed light on these and other complex hybrids.  Keep in mind that hybrids that exist in the wild but do not yet have a documentation mechanism within eBird are not yet featured within Birds of the World; they may be added as new data are provided by citizen scientists. 

We encourage birders and researchers to submit detailed information on hybrid species – with photos or recordings – on your eBird checklists so we can learn even more about hybrids and speciation. Here is some useful advice for how to submit eBird data on hybrids, intergrades, spurs, subspecies, and domestics.

And if you missed our BOW Discovery webinar on Hybridization, find it here.