YOUR QUESTIONS ANSWERED
Q Gynandromorphism observations often cite Northern Cardinal as an example – for good reason: it’s spectacular! (bit.ly/3qHykjC). But I wonder how frequently it occurs in species without obvious male/female plumage differences? Harder to discern, or is sexual dimorphism part of equation? @gwobek, via Twitter
A Daniel Hooper, a post-doctoral research scientist at Columbia University, replies: “Great question! We do expect bilateral gynandromorphism – a phenomenon in which an organism exhibits both male and female traits on opposite halves of its body – to occur at the same (low) rate in species that don’t display sexual dimorphism as it does in species with clear sex differences, as in Northern Cardinal.
“It’s difficult to say how often the phenomenon occurs precisely due to the detection bias you refer to. Many species have such conspicuous differences in plumage between males and females that we easily notice when a bird is split right down the middle between the two sex-stereotypical plumage types. For those species where there isn’t a sex difference in appearance, we are unlikely to notice a gynandromorphic individual.
“Bilateral gynandromorphism is a by-product of the difference between males and females in the sex chromosomes their cells carry – something shared in all birds, regardless of whether there are plumage differences between the sexes. The genetic sex of individual cells (whether a cell carries two Z chromosomes in the case of males or one Z and one W chromosome in the case of females) plays a large role in determining sex-specific cell differentiation. If half the body is ZZ (genetic sex male) and half is ZW (genetic sex female) that bird will develop bilateral gynandromorphy. This is as true for a Northern Cardinal, with its dramatic plumage differences, as it is for a Carrion Crow, where males and females have the same black plumage.”
Q I was amazed to see this Common Starling in my garden (right). I dubbed it ‘the Golden Starling’ due to its plumage; in flight it reminded me of a mini Squacco Heron. I was wondering which plumage anomaly it is showing. It was truly beautiful, but I fear an easy kill for a Eurasian Sparrowhawk. Theo Campbell, via email
A Hein van Grouw, Senior Curator, Birds, at the Natural History Museum, replies: “The aberration causing the plumage of this ‘Golden Starling’ is known as ‘brown’. It means the production of melanin pigment in the feathers isn’t fully finished (incomplete melanin synthesis), which results in the melanin remaining dark brown instead of becoming black.
“Brown is the most common heritable aberration in birds, but also probably the most mis-identified. It is caused by a single recessive and sex-linked genetic mutation, which is the same in every bird species (in mammals it is not sex linked), and therefore in the wild birds with this mutation are mainly female. In records of observations, brown is known by many different, but also incorrect, names: albino, fawn, isabella, leucistic, schizochroistic, erythristic, cinnamon and pale morph are only the most common.
“A possible reason for the difficulty in identifying this mutation in wild birds is that the incompletely synthesised melanin is very light sensitive and rapidly bleaches in the sunlight. Within a couple of months, fresh, but aberrant, brown plumage can become almost white, making correct identification challenging. The much paler large wing feathers of this starling are the result of such bleaching.
“Contrary to popular belief colour aberrations are not significantly more likely to fall victim to predators for the simple reason that they prefer to avoid novel prey. So a Common Starling which doesn’t look like a Common Starling will be mainly ignored. The Golden Starling supports this as she has, despite being aberrantly coloured, survived for at least a year (and perhaps longer, as we don’t know how old she is).”
Q Are you able to identify these birds? The flock was perched in a tree on a quiet country lane. Chris Plumridge, via email
A Birdwatch Identification Consultant Andy Stoddart replies: “These birds can be identified as finches due to their dumpy proportions, short, stubby bills and clearly forked tails. Furthermore, finches are highly gregarious in winter and small flocks like this are typical. As for the species, the combination of relatively plain-looking greyish heads, rufous upperparts (particularly noticeable on the bird in flight) and bright white fringes to the primaries and tail feathers readily identify them as Common Linnets.”