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Why were the extinct Mancallinae more specialized for underwater swimming than modern auks?

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Illustration of a crab plover (Dromas ardeola), showing a slender wading bird with long legs, a long pointed black bill, white head and underparts, and contrasting black wings and back. — Charadriiform Crab plover (Dromas ardeola)

The premise that the extinct Mancallinae were more specialized for underwater swimming than modern auks is supported by the fossil record, which reveals that these birds possessed highly flattened wing bones (humeri) featuring characteristics similar to those of penguins.1

While modern alcids must balance the dual requirements of using their wings as both aerial flight appendages and aquatic paddles, this evolutionary compromise limits their specialization for underwater propulsion. Because the Mancallinae were flightless, they were not constrained by the need to maintain a wing size suitable for aerial lift. This allowed them to evolve more specialized, efficient structures for underwater wing propulsion, surpassing the aquatic capabilities of even the extinct great auk.21