Avian Adventures: Guide to Hybrids




With the arrival of spring comes the start of many mating seasons, including that of the local avian populations.

In Pennsylvania, up to ten percent of bird species do not mate only with members of their own species, but with related species. This phenomenon is known as hybridization.

“It often occurs when one species is absent or rare, which leads to mixed pairings between different species,” said Christoph Randler in his article Extrapair Paternity and Hybridization in Birds.

Irby J. Lovette, director of the Fuller Evolutionary Biology Program at the Cornell Lab of Ornithology, said “Many birds occasionally mate with members of other bird species, producing hybrid offspring.”

In a study performed by Peter R. Grant and B. Rosemary Grant entitled Hybridization of Bird Species, they concluded that “approximately one in ten species is known to hybridize, and the true global incidence is likely to be much higher.”

Locally, two species that hybridize are the Black-capped Chickadee and the Carolina Chickadee. Though the Black-capped Chickadee has a smaller body size and a longer tail, both species contain similar black, white, and grey markings.

The Carolina Chickadee can be distinguished from the Black-capped Chickadee by greater wing coverts, more grey and cinnamon-buff coloration, and a higher and faster song. Coverts are the small feathers that cover the base of the feathers of the wings and tail.

Though the two species generally cannot be found in similar areas, there is a small overlapping zone in the northern United States, including as close as southeastern Pennsylvania. Carolina Chickadees, which are typically found in the southern United States, have recently been moving north because of warmer climates.

Black-capped Chickadee females prefer to mate with male Carolina Chickadees, though the reason behind this has not yet been discovered.

Hybridization has the potential to lead to increased survival and fitness, as the offspring may gain the best features from both intermixing species. In the past, however, hybrid chickadees have shown lower survival rates than their non-hybrid counterparts.

Because of their willingness to interbreed and their similar features, many scientists and birdwatchers may have trouble distinguishing the two species. The favorability of mating with members of different species has the potential to increase the frequency of hybrids in the future.

However, studies have shown that the hybrid chickadees may be at a disadvantage when it comes to producing their own offspring. According to data collected by C. L. Bronson, Thomas C. Grubb, Jr., Gene D. Sattler, and Michael J. Braun of Liberty University, “A decrease in productivity of breeding pairs in the hybrid zone…is positively related to their probability of producing homozygous offspring at each autosomal or sex-linked locus.”

Hybrid chickadees do not always contain genes suitable for reproduction, according to the study. This means that the hybrids’ offspring are more likely to have genetic mutations and deformities, leading to unsuccessful chicks.

Though these birds may be less fit, some have been shown to be able to reproduce and raise their young, though the population of hybrids is small. According to a report by Robert Curry of Villanova University, “In Pennsylvania, eggs of mixed pairs have a much lower hatching rate than those of pure pairs.”

Genetics may be one of the reasons behind this, and Curry is still working toward understanding the hybridization of Black-capped Chickadees and Carolina Chickadees, as well as their ecology and social behavior.

“Hybridization presents challenges to the reconstruction of phylogenies, formulation of biological species concepts and definitions, and the practice of biological conservation,” said Peter and Rosemary Grant in their essay.

When two separate species mate and form a hybrid offspring, the chick contains features of both parent species, leading to difficulties when identifying the species. DNA testing has shown that in some instances, a hybrid cannot be distinguished from a pure species, as there are no outward differences. This may cause difficulty when researching the evolution of an organism.

Hybrids appearing identical to the parent species have other implications for hybridization. If a hybrid exists but seems similar to a preexisting species, it may not have been discovered yet, as it would be labeled as the existing species.

As hybridization of Black-capped and Carolina Chickadees is relatively new to the United States, a new species name has not been developed to describe these individuals. If the hybrids cannot support their own population successfully, they may never receive their own species name.

Chickadee mating season typically lasts from April to July, and so any birdwatchers can keep their eyes open for the new feathered friends to arrive later in the season.


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