Local Superstition: Hubulkoks augurs death.
How can the harmless hubulkok signal deaths?
The islanders here are mostly agricultural and close knit, mostly related one way or another. They have their own religion, traditions and superstitions. One such superstition their grandparents passed on to them is the relation between death and hubulkoks based on actual sea death incidences observed through the centuries. Many islanders have also lost their fishermen folks to the notorious whirlpools. This too happened after sighting the hubulkoks nesting near their villages. They accused hubulkoks to be death itself.
The reality is that: it is the time of year when the islanders' produce are brought to the old market ten miles off the beautiful sandy white beach near the Great Pillar cliffs. Having made their sell many of them decide to take a day off and go swimming or boating. Normally it is quite safe but not if it is the sea mood is as explained above.
Rationality: Hubulkoks sightings on that island may signal dangerous seas nearby.
Musings: Thankfully, for this particular case we can observe and understand the whole scene on a macro level. It can be explained with appropriate scientific principles we know. But imagine a situation where it involves sciences beyond our understanding or maybe even those we haven't yet formalized. Then superstitions and omen readings could just be reliable.
Morals of the story: Don’t ignore everything you can’t explain today, there are more things in heaven and earth than are dreamt of in your philosophy.