Why do frigatebirds lack wing slots?

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Unlike other seabirds like albatrosses, which use dynamic soaring, frigatebirds are thermal soarers, riding on air collumns that form above the sea. Thermal soaring is, however, more often associated with continental flyers like storks, large raptors, ravens and cranes, which makes sense as continental settings tend to produce collumns of hot air.

These flyers also have a thing frigatebirds lack: wing slots.

Wings of the American Black Vulture Coragyps atratus.

The only other marine soarers with wing slots I know off are pelicans, and these tend to just as easily soar in continental settings. So what gives?

I don’t think an actual study has been conducted as to why frigatebirds lack these slots otherwise fundamental for thermal soarers. Closest I’ve found was Liu et all 2021, and that doesn’t explore the aerodynamics of frigatebird wings (even lumping them with dynamic soarers, which is flat out wrong).

My best guess is that, unlike most thermal soarers, frigatebirds are rather acrobatic. They routinely chase after other birds in flight and pursue flying fish; this might mean that a sharper wing shape is more desirable, much as in falcons, skuas and other aerial predators. The sheer size of frigatebird wings – some of the proportionally largest among living birds – might compensate for this, and as you can see in the above picture they do angle them forward, likely further macking up for the lack of slots.

What does this means for pelagornithids?

Pelagornis sandersi silhouette by corvarts. Note wing slots.

Another lineage of sea birds, the massive pelagornithids, were also thermal soarers. Unlike frigatebirds, however, I doubt these were acrobatic flyers, seeing as they were much larger. Thus, I think wing slots might be more appropriate in future depictions of these birds.