Hermaphroditism and Culling Adaptations of the Blue-Banded Hawk

An evolutionary anomaly, the blue-banded hawk (Parabuteo discinctus) has long flown under the radar. Sexually dimorphic in adulthood yet displaying hermaphroditism as a fledgling, there is much to examine concerning the social and reproductive behaviours contributing to their decline. 

Blue-banded hawks are raised in kettles consisting of a single adult female, adult males, and hermaphrodite youth. Whereas the female labors considerably to nurture her offspring,¹ adult males only interact with the brood when they are being fed, snatching insects from the mother’s mouth.² As a consequence of this confounding adaptation, many chicks do not reach adulthood. 

A female hawk may be identified by two iridescent blue-grey bands on her rectrices. Such distribution of visual attractiveness is atypical in species displaying anisogamy, with females also producing macrogametes. Though biologically greater in size, females often display fasting behaviors.³ This stereotypic behavior is hypothesized to be a stress response.

Until maturity, fledglings exhibit simultaneous hermaphroditism. Protandry, defined by the increased secretion of female sex hormones and the suppression of male sex hormones, is induced when the sole female in the kettle is unable to provide.⁴ These sex changes mark adulthood, and have been observed to occur only once in a hawk’s lifetime. Protogyny, the counterpart to protandry, has no clear causative factors. Under the care of an adequate female, fledglings mature primarily into male individuals. If an adequate female is already present in a kettle, protandrous fledglings are cast out.⁵ Lone females become vulnerable to predators ⁶and the aggression of other hawks.⁷ 

As hermaphrodites, fledglings simply mimic adult traits.⁸ However, upon reaching maturity, males cease allopreening and social behaviors beyond feeding and physical conflict.⁹ Such violence over the possession of a lone female mirrors the female’s imperative to mate and raise a kettle for survival.¹⁰ This violence, contradictory to Darwinian evolution, extends within a kettle, lowering the survival rates of both offspring and adults. 

Blue-banded hawk populations are fragmented and facing extinction, as a consequence of stress, ostracism, and aggression in adult individuals. Hermaphrodite offspring appear to have greater fitness for survival, performing social behaviors for the benefit of the kettle. Fledglings have been observed to ensure smaller individuals are first provided for. There is emerging evidence of fledglings attempting hunting and feeding behaviors out of curiosity, rather than duty.¹¹ It is perhaps in the best interest of the population, and in accordance with Darwinian evolution, for the blue-banded hawk to retain the hermaphroditism of its youth.¹² 

1. I’d wake every morning to smoke and searing oil—my mother frying eggs on the stove. She’d ask me to set the table. My brother would wake at noon and complain that the eggs had gone cold. ↩︎
2. Before she’d moved out, my sister and I would cook on the weekends. Our dad ate his dinner with one hand, sitting at the table on his laptop. As always, he was writing emails or cursing shouldn’t have hired her or on a work call. 
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3. My mother would watch her wrinkles grow in the mirror and moan about the fat she couldn’t get rid of. She’d serve herself half a bowl of rice and serve my brother two. ↩︎
4. When my mother came down with the flu, I found her in the kitchen, heating the stove, before I’d told her to go to bed. My brother scarfed down the eggs after I’d scrambled them and asked why mum had changed her recipe. ↩︎
5. My sister invited me first to her new home—a closeted apartment paid off with her own money. Rent was at $530 a week. We laid on her bare mattress under the window, side by side, birds disappearing into clouds. ↩︎
6. I’d dropped my jacket on the road and a car honked to alert me. I ignored it at first, certain it was another man rolling down his window to yell blow us a kiss, babe. ↩︎
7. Give us a kiss before we go. My uncle opened his arms, waiting at the door for his embrace. His wife’s arms were laden with bags, her right hand seized by their child. ↩︎
8. My sister invited me first to meet her child. The curtains in the nursery were closed and I huddled with her, in the dark, beside the cot. She broke down in my arms. The baby, hearing his mother cry, began crying too. ↩︎
9. Since and long after her second trimester, my sister had been reliant on her husband’s income. My mother, when invited over, criticized her for the unwashed dishes. I began washing the dishes. My brother, when offered, held the baby like a wet dishrag. ↩︎
10. Again, my mother asked when I would give her another grandchild. My worst nightmares are her waking moments. But I couldn’t tell her that. Instead, I told her I did not like men. She paled, almost fainting. She told me not to joke about that. 
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11. I brought my sister her favorite meal—an entire plate of honey chicken, two bowls of rice. My sister laughed in delight. The baby, hearing his mother laugh, began laughing too. ↩︎
12. At noon, my brother wanders into the kitchen, peers at the empty pan on the stove, and asks if we’re running low on eggs. “No,” I say. “I didn’t cook any for you.” ↩︎