A Rare Jay Hybrid and The Complex Consequences of Climate Change
In late May 2023, a unique bird made an appearance in an online birding community. It had striking blue feathers with white spots, characteristic of a blue jay. But its face had telltale green jay signs — a black patch around its eye with a blue ‘eyebrow’ spot, and no feathery crest upon its head. Scientists discovered that this bird was a rare hybrid, the result of a male blue jay mating with a female green jay. This hybrid was the first of its kind ever observed in the wild.
Blue jays are temperate birds, and green jays are their distantly related tropical cousins. Their ancestors split around 7 million years ago, about the same time humans and chimpanzees diverged. For millennia, the two jay species lived apart. Now climate change has allowed their ranges to overlap.
With warming temperatures, the tropical green jays have been expanding their range northward from their original habitat deep in south Texas. At the same time, blue jays are moving further south and west, possibly following human settlers and the ample resources their bird feeders provide. This has led to individuals from both species coexisting around San Antonio, Texas, which is where the blue-green jay hybrid was found.
Hybridization between species is not uncommon, in captivity or in the wild. The most popular domesticated hybrid is probably the mule: the offspring of a female horse and a male donkey. Mules have been bred by humans for centuries, offering the best of both horse and donkey worlds. They’re longer lived than either parent species, physically stronger and healthier.
The mixing of genes from distinct sources in a hybrid can lead to a more robust, fitter individual, a phenomenon termed hybrid vigor. But due to mismatches in genes, hybrids, such as mules, can often be infertile.
Fertile hybrids, however, can thrive. In several animals and plants, hybridization is a way through which new species have formed and continue to form. In fact, natural hybridization can sometimes help species adapt to environmental change.
Scientists studying closely related species of rainbowfish in Australia, for example, found that natural hybrids occurred between a widely spread, warm-adapted species and several other cold-adapted species with smaller populations. The cold-adapted species are at risk as the climate warms, and some parts of their range have begun to overlap with the warm-adapted fish, allowing them to produce fertile hybrids. Genetic testing has shown that the hybrid populations have acquired necessary genes to adapt to warmer climates from the warm-adapted parents, and are therefore more likely to survive climate change than their non-hybrid cold-adapted parents. In this case, natural hybridization has reduced the overall climate change vulnerability of these rainbowfish.
So why does the emergence of the blue-green jay hybrid worry scientists?
First, this event highlights a broader pattern of ecological disruption and shifting species ranges due to climate change. The increasing frequency of hybrid zones — regions where species ranges overlap and hybrid individuals are found — is an indicator of the environmental stress facing these species.
Hybridization can be harmful to one or both of the parent species, especially if they are already threatened. Hybrid individuals, with their increased vigor, may outcompete a threatened parental species, potentially leading to its extinction.
Moreover, hybridization between distantly related species often wastes reproductive effort, producing infertile hybrid offspring instead of viable offspring that could add to the parent species’ numbers.
Breeding between species might also reflect dwindling options due to reducing population sizes or habitat fragmentation. Individuals from the parental species, being unable to find a suitable mate, might resort to mating with any available individual, regardless of whether it belongs to another species.
A case in Borneo illustrates this. Deforestation due to logging and palm oil plantations has broken up the habitat of two distantly related monkey species: the proboscis monkey and the silvered leaf monkey. This has led to greater competition, fewer potential mates, and less room to roam. In this fragmented habitat, a male proboscis monkey mated with females of the silvered leaf monkey. The result was a “mystery monkey” offspring, later identified as a hybrid. With larger, connected habitats, both species might have had enough mates and greater ability to disperse to find mates. Both of these monkey species are threatened, and this hybrid monkey is evidence of their struggle to survive.
Scientists expect more such hybridization events will occur as the climate changes, with species shifting their ranges or having fewer options for suitable mates. While hybridization might hasten extinctions, it could also confer resilience through novel gene combinations. Unfortunately, which of these scenarios will occur is hard to predict and will depend on the species involved.
The future of the blue-green jay hybrid is uncertain. It is not known if the hybrid is fertile, better adapted than either parent, or if there is more than one of them flying around. In an extreme scenario, continued hybridization might lead to a new blue-green jay species emerging in the hybrid zone. But one thing is certain — the consequences of altering the climate can be wildly unpredictable, a lesson worth remembering as we try to navigate this environmental crisis.