Plasma melatonin concentrations in most animals investigated so far increase at night regardless of whether individuals are day or night active. Nevertheless, daily melatonin amplitudes are often seasonally adjusted to ecological conditions, with birds that breed at high latitudes and migrate during the night showing lower daily amplitudes. Here we investigate whether nocturnal seabirds, gulls that feed at night, also show a low melatonin amplitude because they have to be active predominantly during the night but also intermittently during the day. We sampled free-living nocturnal-foraging swallow-tailed gulls (Creagrus furcatus) on two Galapagos islands every ~4 h and compared their plasma melatonin concentrations with those of related black-headed gulls (Larus ridibundus) sampled in the Netherlands. Like most seabirds, the black-headed gulls showed generally low melatonin concentrations, but clear diel cycles. The swallow-tailed gulls, on the other hand, had similarly low absolute melatonin concentrations, but no detectable diel changes. Despite problems inherent in comparisons between two species and field/lab setups, our data lend support to the hypothesis that the lack of a diel melatonin rhythm allows animals to be active at any time.