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Using underwater videography, we developed a subsurface ethogram of humpback whale (Megaptera novaeangliae) calf behavior among mother/calf resting groups in their Hawaiian Islands birthing/breeding grounds. Data collected from 2001 to 2018 in waters between the islands of Maui, Moloka’i, Lana’i, and Kaho’olawe (the Four-Island area), support the assumption that humpback whales are more vulnerable to small-vessel collision during their first year(s) of life. This, along with four incidents of first-hand observation of calves with significant wounds likely resulting from propellers were the impetus for further investigation into calf vulnerability to vessel collision in Hawaiian waters. We examined underwater video footage of mother/calf groups to investigate calf-surfacing events with or without the mother, as well as the calf behavior displayed while at or near the surface. We defined three depth zones: Depth A (<5 meters [m]), Depth B (5-10 m), and Depth C (>10 m). We considered Depth A the small-vessel collision vulnerability zone based on the approximate body length of a calf, and the depth at which a small-vessel draft (i.e., hull or propeller) could make contact with the calf at or below the surface. We analyzed 3,022 30-second samples of underwater video footage from 226 mother/calf resting groups. It was possible for the calf to be in more than one Depth Zone during an interval when moving from deeper water to surface water or surface water to deeper water which resulted in a total of 3,499 samples of which 1,422 (40.6%) the calf was in Depth A, 592 (17.0%) the calf was in Depth B and 1485 (42.4%) the calf was in Depth C. Of the samples having calves in Depth A, there were 509 samples (36%) of calves surfacing alone, 532 samples (37%) of calves surfacing with the mother and 381 samples (27%) where calves did not break the surface. In Depth A, calves were found to exhibit rest behavior significantly more than other behaviors (χ2 = 1,430.1, df = 3, p < 0.001, n = 1,422). Our results provide insight into the vulnerability of humpback whale calves to small-vessel collisions in the coastal waters of Hawaiian Island birthing grounds.