Docosahexaenoic acid (DHA), a long-chain omega-3 fatty acid (n3-LCPUFA), is integral to vertebrates’ brains. Mammals require continuous DHA replenishment to maintain cerebral function and structural integrity; evidence points towards birds having similar requirements. Vertebrates maintain their DHA levels by endogenously synthesizing DHA using alpha-linolenic acid (ALA; omega-3 precursor) or by consuming preformed DHA and other n3-LCPUFAs. Given n3-LCPUFAs’ abundance in marine organisms, marine predators are presumed incapable of biosynthesizing n3-LCPUFAs; this assumption remains seldom tested. Alarmingly, many marine predators, including generalist seabirds, now forage on anthropogenic resources deficient in n3-LCPUFAs. Whether they can offset such deficiency by bioconverting ALA into DHA remains unknown. Here, we tested whether ring-billed gulls (Larus delawarensis), a generalist seabird now thriving in cities, can biosynthesize n3-LCPUFAs, including DHA, from ALA. We brought into captivity 24 hatchlings from two breeding colonies, including 12 from an urban colony and 12 from a natural colony. Nine hatchlings from each colony received an oral 13C-enriched ALA tracer to follow the potential bioconversion of ALA into n3-LCPUFAs, in addition to being fed 490 μL of unlabelled ALA daily for three days. The control groups (N=3 urban hatchlings and 3 natural hatchlings) received a caloric equivalent (free of omega-3s) in place of the ALA tracer and supplement. All chicks received a diet deficient in all omega-3s throughout the 72 hours of captivity. We analyzed their brains’ and livers’ fatty acid content and established their 13C signatures for each omega-3 using compound-specific isotope analysis. We found evidence of incomplete ALA bioconversion where, compared to controls, experimental chicks from both colonies accumulated more of all ALA derivates in their tissues except for DHA. We demonstrate for the first time that a seabird shows some omega-3 bioconversion abilities, albeit at levels insufficient to mitigate DHA deficiencies associated with urban foraging and shifting marine ecosystems.