With at least 18,000 species across our planet, birds are never that far away. Yesterday morning, one bird sought me out during my morning routine.
I was sitting on the stairs outside my dingbat apartment, drinking a stovetop americano, and reading. This has become a treasured morning ritual, a way of centering myself before another day of work. (Plus, it prevents me from starting the day with an endless doomscroll.) As I was reading, I became distracted by a low hum. The noise caused the anxious kid in me to flinch.
When I looked up, I was greeted by hummingbird. It stared at me, as if it wanted to tell me something. Suddenly, I become aware of the cacophony of birds calling to each other in my neighborhood. To my (admittedly naïve) ear, there were at least three distinct ones: a harsh caw (probably from a nearby crow), a light tweet (perhaps a warbler?), and an abrupt chrip (likely a mockingbird).
I took that moment to listen to what they had to say.
For the last 60 years, scientists have known that some birds demonstrate divergence between their sex chromosomes (termed Z and W as they are distinct from mammalian X and Y chromosomes) and their gonads (i.e. ovary, testis, or a blended organ with features of both). However, the prevalence of these traits remained unknown until mid-August when new research was published in Biology Letters.
An Australian research team examined 500 birds from a total of five local bird species (the Australian magpie, laughing kookaburra, crested pigeon, rainbow lorikeet, and the scaly breasted lorikeet) to estimate how frequently the chromosomes of a wild bird are unable to predict its gonads. This described 3-6% of individual birds in each species — most commonly seen in the crested pigeon (6.3%) and least common in the Australian magpie (3.0%).
At the the level of avian genetics, ZZ chromosomes most commonly lead to the development of male-associated anatomy, and ZW chromosomes result in female-associated anatomy. Yet, the development of sex traits in birds (as well as humans, albeit through different mechanisms) is sensitive to both hormones and life history.
During embryonic development, elevated estradiol levels promotes the formation of female traits regardless of chromosomal composition. The inverse is true when estradiol levels are depressed.
In adult birds, decreases in estradiol (due to, for example, damage to an ovary) can cause the animal’s ovaries to transdifferentiate into testes. In doing so, the cells reorganize and reidentify themselves as part of testicular tissue.
This avian phenomenon where chromosomal and gonadal sex diverge is known to as “sex reversal.” (I would be remiss if I did not point out that this phrase is rooted in assumptions of assigned sex as a binary lever that can exist as only male or female and is determined gonads. Personally, I prefer “sex discordant.”)
Of the sex discordant birds observed by the Australian researchers, the vast majority had ZW chromosomes (considered “female”) and had testes (considered “male”), although the researchers identified one Australian magpie and one laughing kookaburra that had ZZ chromosomes and ovaries.
The researchers told Science that they were surprised at how common this phenomenon is in the wild. Yet, their ultimate conclusion is decisive: “Our findings suggest that sex reversal is a common and potentially widespread phenomenon in avian species.”
More work still needs to be done to expand these conclusions to other species and other regions of the world. It is possible that environmental factors (such as local diets or pollutants) influence the prevalence of sex discordance.
Additionally, the authors did not assess the fertility of sex discordant wild birds. But, previous work using lab domesticated birds suggest that fertility can be preserved. To test this, scientists isolated sperm from a sex discordant chicken and injected it into an unfertilized quail egg. In this experiment, 20% of the sperm was able to fertilize the egg and induce early embryonic development.
Today, as I sit outside my apartment enjoying the cool October morning, my ears immediately attune to the chrips, whistles, and tweets all around me. I smile to myself and wonder: Perhaps the neighborly birds and I have something in common.
from the archive
