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The Answer

             Male Mourning Warbler         Female Kentucky Warbler

                   (Oporornis philadelphia)                          (Oporornis formosus)      



How the parents of Junkin’s Warbler were diagnosed from its DNA

by Dr. Irby Lovette, director
Fuller Evolutionary Biology Program
Cornell Lab of Ornithology

All birds, like all humans, inherit some of their DNA from their mothers and some from their fathers. Depending on the type of DNA, however, the pattern of inheritance can differ, and we can use these differences to our advantage to determine which two species interbred to produce a hybrid individual like the Junkin’s warbler.

At work in the DNA lab

Lab scientist Amanda Talaba started by extracting DNA from a single feather plucked from the bird when it was captured a second time. Extracting DNA is a fairly straightforward process, and even one feather usually has ample DNA for lots of molecular tests.

Once she had the DNA purified, Amanda used a process called the Polymerase Chain Reaction to copy short and very specific regions of the bird’s DNA. This copying process is necessary to give us enough of the target DNA to determine the DNA’s sequence, the series of chemical building-blocks that varies among species. As part of a different project, we had already sequenced a number of genes from every one of the world’s more than 100 wood-warbler species, so to identify the parents of Junkin’s warbler, all Amanda had to do was sequence its own corresponding genes and compare those sequences to our existing warbler gene database.

Dr. Irby Lovette banding a warbler

To determine the mother, Amanda sequenced a gene encoded in the mitochondrial DNA. This type of DNA is unusual in that it is passed only from mothers to their offspring; fathers don’t make any contribution at all to the next generation. It also is highly variable among species, so much so that mitochondrial DNA has been used in hundreds of studies of bird evolution. The mitochondrial DNA sequence from the Junkin’s warbler individual was a perfect match to sequences we had obtained previously from Kentucky Warblers (Oporornis formosus), and quite different from sequences of all other warbler species.

To determine the father, Amanda sequenced part of a gene from the bird’s nuclear genome. Nuclear genes come in pairs in birds: one copy from the mother, one from the father. The sequence of one copy of this gene in Junkin’s warbler matched the database sequences of Kentucky Warbler, just as expected based on the mitochondrial DNA evidence. The other copy, the one that must have come from the father, matched the database sequences of Mourning Warbler (Oporornis philadelphia).  Mystery solved.

To our knowledge, this is the first recorded hybridization between these two warbler species.

Why Hybridize?

by Dr. Rachel Vallender
Fuller Evolutionary Biology Program
Cornell Lab of Ornithology

Dr. Rachel Vallender with a Blue-winged Warbler captured in a mist net. Photo by Kevin Fraser

Hybridization can be defined as the mating between organisms from different races, breeds, varieties, or species. Nearly 10% of the total number of bird species in the world are known to hybridize on a regular basis. Even so, most people assume that hybridization events occur only rarely, and for the most part that is probably true. But what do we miss? Are hybrids actually more common that we think? Why does hybridization even happen in the first place? And are some species more likely to hybridize than others? These might be some questions you’re asking yourself, and if so, you’re not alone. 

With the advent of sophisticated genetic techniques, like those employed to determine the ancestry of the Junkin’s warbler, there has been a recent resurgence of interest in hybridization by researchers in a variety of fields. 

"Brewster's Warbler" is a cross between Golden-winged and Blue-winged Warblers. Photo by Rachel Vallender

Within the wood-warblers, there are two particularly well known and well studied hybridization situations. The first of these is in Western North America between Townsend’s and Hermit warblers. Hybridization between these species is common in the places where both species occur, and has been taking place for at least a century.

The second system, occurring in the Northeast, is between Golden-winged and Blue-winged warblers. These two species have been hybridizing regularly since the 1870s and the hybrid forms are encountered regularly enough that they were initially mistaken for new species and given their own names (i.e. “Brewster’s” and “Lawrence’s” warblers). Both of these hybridization situations offer excellent opportunities for research exploration. They’re also convenient to study because hybridization events are frequently encountered and occur in all areas where the species’ breeding ranges overlap. 

But are there other cases where hybridization occurs in wood-warblers, albeit with much lower frequency? The answer is yes. To date there have been at least 69 known or suspected cases of hybridization between different pairs of North American wood-warbler species. Most of these involve species that are similar in behavior or plumage, and that are closely related. For example, species within the same genus appear to be more prone to occasional hybridization than are species that are more distantly related.

Golden-winged Warbler with some Blue-winged Warbler genes. Photo by Kevin Fraser

While remaining rare overall, there are certainly many instances of warbler hybridization that go undetected by humans. Because these events are so infrequently encountered, when hybrids are discovered in the field it tends to generate a lot of excitement. This is what happened when David Junkin discovered a hybrid warbler in his mist-net on June 27, 2006, in Wethersfield, New York. Now known as the “Junkin’s warbler,” this warbler was determined to be the result of a mating between a male Mourning Warbler (Oporonis Philadelphia) and a female Kentucky Warbler (O. formosus) – two members of the same genus with breeding ranges that only overlap to a very small degree in Pennsylvania.

As interesting as these events are to simply document, the bigger question is why do they occur? Why might a female warbler accept a different species as her mate? Of course, in the vast majority of cases individuals successfully pair with members of their own species, so why do mistakes sometimes occur? And are such choices always mistakes? Or, could a female make her mate choice decision based on some factor that makes pairing with an individual of another species a “good” choice?

One thing that we don’t yet know in the case of the Junkin’s warbler is the location where it was born. However, we think the individual may be a male, and male warblers tend to return to the same general region where they hatched in order to reproduce, so it is plausible that the Junkin’s warbler's parents interbred somewhere in the western part of New York State. (As an aside, this pattern is not as strong for female offspring, which tend to disperse somewhat farther from their natal areas; it has been suggested that this is an evolutionary response that helps them avoid pairing with relatives in subsequent generations.) If the Junkin’s warbler was hatched somewhere near the location where he was captured as an adult, then the hybridization event occurred in an area where Kentucky Warblers do not normally breed, but where Mourning Warblers are common. Therefore, we speculate that the Junkin’s Warbler was created when a female Kentucky Warbler found herself settling in an area where she found no possible mates of her own species.

Because many hybrid warblers appear to be fertile and go on to reproduce successfully, this may explain why she’d choose to mate with a Mourning Warbler male in the absence of Kentucky Warbler males. In fact, research has shown that hybridization events might occur most often because one species finds itself to be rare. When females arrive at their breeding destinations they must select a partner from the available pool of mates. In instances in which there are no members of their own species around, the best choice may be a male of a different species – effectively, “making the most of a bad situation.” This theory would explain why two species that don’t have overlapping breeding ranges might produce offspring in a situation where they do come into contact. 

The close evolutionary relatedness of the two parent species and their similar nesting behaviors (including the use of the same nesting materials, and similar nest placement) may also have encouraged their pairing in a region where Kentucky Warblers are typically absent.

Junkin's warbler: an uncertain future. Photo by Sandy Junkin

What will be the fate of the Junkin’s warbler? Hybrids are often less fit than their parental species, but this individual has already made it through at least a full annual cycle, almost certainly including a round-trip, long-distance migration. Breeding is another matter: some hybrids are sterile, whereas others have problems attracting mates. Since females often select males that sing a song similar to the one they heard their fathers sing while they were nestlings, it’s possible that the Junkin's warbler might attract a female Mourning Warbler (assuming that it learned its song from the father, which is typically the case, although this bird was never heard singing). And this is certainly more plausible than finding another female Kentucky Warbler in its breeding range. But would a female accept the hybrid, with unusual plumage that combines characteristics from two different species, and would this bird even be able to produce offspring? Unfortunately, we’ll likely never know, although the Junkin warbler's behavior will certainly be scrutinized if it returns this Spring.

Rachel Vallender is a postdoctoral associate in the Lab’s Fuller Evolutionary Biology Program. In her research she is investigating the genetic consequences of the Blue-winged/Golden-winged warbler hybridization, and applying the resulting information to conservation initiatives aimed at maintaining viable populations of both species.

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Top images: Mourning Warbler, by Charlie Eiseman; Kentucky Warbler, by Bill Hilton, Jr., Hilton Pond Center for Piedmont Natural History; Junkin's warbler, by Sandy Junkin.