Hybridization of the habitat: More admixed chickadees in urban areas

Hybrids are more common in heterogenous landscapes.

In 1948, the botanist Edgar Anderson published a paper in the journal Evolution, entitled “Hybridization of the Habitat.” Using his studies on several plant genera, such as Tradescantia and Iris, he explores why hybrids are often rare in nature. Apart from the difficulty of detecting hybrids (which has greatly improved since we have genetic tools), he noted that a suitable habitat for the resulting hybrids is generally missing. Because hybrids often show intermediate characteristics they cannot compete with the parental species and will thus not establish themselves. An important exception concerns heterogeneous landscapes, which tend to be scarce. He stated that “Such heterogeneous habitats are seldom or never met with, the only approach to them being found in places where man has greatly altered natural conditions.” These hybridized habitats might thus be the best places to find hybrids. In the decades following Anderson’s paper, this phenomenon has been observed in several animals and plants (as reviewed by Grabenstein and Taylor in 2018, and by me in 2021). A recent study in the Journal of Avian Biology adds another example to the growing list hybrids in heterogeneous, hybridized habitats.

Admixed Birds

According to a recent estimate of avian hybrids in North America, species in the family Paridae produce the most commonly observed passerine hybrids. It is thus no surprise that Brendan Graham and his colleagues focused on four members of this family: the Black-capped Chickadee (Poecile atricapillus), the Mountain Chickadee (P. gambeli), the Chestnut-backed Chickadee (P. rufescens), and the Boreal Chickadee (P. hudsonicus). Using microsatellites, the researchers determined the number of hybrid individuals between these species across North America. The genetic analyses revealed admixture among five of the six species pairs examined. There was no apparent admixture between Mountain and Chestnut-backed Chickadees. Interestingly, almost all of the admixed birds were phenotypically classified to one species, confirming Edgar Anderson’s claim that hybrids are often hard to find (without genetic tools).

Urban Areas

Next, the researchers focused on the locations where these admixed birds were observed. They found that “over 70% (81 of 112) of individuals identified as admixed were found in urban parkland or mixed forest habitat.” This finding is in line with the concept of “hybridization of the habitat” in which hybrids are more common in human-mediated or heterogenous landscapes. The exact mechanisms underlying increased hybridization rates in these areas remain to be determined, but could be related to higher food availability in urban settings (such as in hummingbirds). Nonetheless, this study is a nice example of avian hybridization in an increasingly anthropogenic world.

Top figure shows the distribution of 122 admixed individuals across the four habitat types. Bottom figure indicates the occurrence of admixed individuals within each species pair across the four habitat types. From: Graham et al. (2021) Journal of Avian Biology.

References

Graham, B. A., Gazeley, I., Otter, K. A., & Burg, T. (2021). Do phylogeny and habitat influence admixture among four North American chickadee (family: Paridae) species? Journal of Avian Biology, 52(5).

Featured image: Chestnut-backed Chickadee (Poecile rufescens) © V.J. Anderson | Wikimedia Commons