A red-footed rarity

November 26, 2018

Red-footed Boobies, the smallest and widest-ranging booby, are a tropical seabird that forms large breeding colonies and feeds on flying fish and squid. Despite being a highly pelagic, nomadic species, they have for decades remained the rarest booby in California, with the exception of the recently-split Nazca Booby of the Galapagos. The first two occurrences of Red-footed Booby in California were both in the fall of 1975 on Southeast Farallon Island; since then, records from the state as a whole have been sparse.

On 5 September 2018, the first Red-footed Booby since 1975 appeared on Southeast Farallon Island. On 7 November 2018, another Red-footed Booby appeared. What are the odds that the first two Red-footed Boobies in over forty years would arrive in the same fall season? While two data points are insufficient to establish a pattern (there were two in 1975 after all—and that was it), a glance at what’s happening in the rest of California reveals that 2018 is an exceptional year for Red-footed Boobies.

This graph shows the number of records of Red-footed Boobies accepted by the California Bird Records Committee. The 2018 total is provisional, as the committee has yet to evaluate most of the records from this year, many of which could pertain to the same bird in different parts of the state.

The origin of these boobies is probably unknowable. While they breed as close as the Islas Revillagigedo in Mexico, they have a pantropical distribution, and are prone to wandering large distances. Three subspecies exist, and while these are likely of the eastern Pacific race websteri, all groups around the world appear similar. In addition, there are several color morphs across all subspecies, but, as with the much larger invasion of Blue-footed Boobies in 2013, virtually all of the Red-footeds in 2018 pertain to immatures, which all appear brown regardless of their color morph as an adult. Interestingly, the two 1975 records from the Farallones pertain to light-morph adults, one with a white tail and one with a dark tail – but this doesn’t tell us much about their origin either.

Also poorly understood is why these boobies are here. It’s tempting to say climate change…and it almost certainly is related to climate change. As ocean temperatures rise, records of all boobies are increasing in California. Fall 2018 also saw the second-ever record of a Masked Booby for the Farallones, as well as another booby in October that was either Masked or Nazca but was too distant to be identified. A recent paper in PLOS1 carefully documented a post-El Niño diet shift in Nazca Boobies from nutrient-rich sardines to nutrient-poor flying fish – a change that has adversely affected their ability to successfully nest and find food. Authors of the Fourth Report of the Alaska Checklist Committee suspect that this is related to the rapid increase in records of Nazca Booby on the west coast of North America, including Alaska’s first record. It seems likely that similar diet shifts are affecting all boobies, including the Red-footeds we are seeing here. Given the improbability that global warming will reverse any time soon, we can expect to see more boobies in the coming years.