Blue-footed Booby Population Analysis
Blue-footed Booby Population Analysis
Dr. David Anderson, Wake Forest University
Began in May 2011; completed in April 2014
Blue-footed Boobies are on the decline in Galapagos. Photo by Daniel Boyce.
Blue-footed boobies (Sula nebouxii) are iconic elements of the biodiversity of the Galapagos Islands, but concerns that the booby population was declining on an island-wide scale prompted the need for a comprehensive study to assess the current population size and understand the cause of the decline. The latest research paper on this project in the online journal Avian Conservation and Ecology confirms the decline in population, and suggests that it may be closely tied to a decline in clupeid fish, especially sardines (Sardinops sagax), in the boobies’ diet.
The results of this project, which was completed in April 2014, suggest that the Blue-footed Booby population is having trouble breeding, resulting in a slowly declining population that is experiencing typical adult mortality but little replenishment from new young adults. Project data suggest that the decline may be long-term; however, data collection on an annual basis is needed to provide sufficient evidence to conclude that this is a permanent condition and not just a normal fluctuation that may last several years.
In May 2011, seabird biologists Dave Anderson of Wake Forest University, Kate Huyvaert of Colorado State University, and Ecuadorian Master’s degree student David Anchundia began the first comprehensive survey of Blue-footed Booby distribution and population dynamics in the archipelago with support from Galapagos Conservancy, Galapagos Conservation Trust, and Swiss Friends of Galapagos. This effort came in response to concerns of several long-time Galapagos observers that this iconic species seems to be declining in numbers. Traditional breeding sites seem to be largely unattended and without successful nesting; indeed, the large colony of hundreds of nests at Anderson’s research site on Española Island has been essentially unused since 1997.
The project began with a survey of the entire coastline of Galapagos, excluding the northern-most islands (Genovesa, Marchena, Pinta, Darwin, and Wolf), where Blue-footed Boobies almost never venture. That effort revealed a critical clue: only two birds in juvenile plumage were seen in the entire survey area. Blue-footed Boobies show the distinctive juvenile plumage from the moment that they become independent from their parents until around two years old. This was a change from the 1980s and 1990s, when birds in juvenile plumage were common throughout the archipelago, except in the northern tier of islands.
The absence of these young birds at the beginning of the study was an immediate indication of something noteworthy in the Galapagos population of Blue-footed Boobies.
Since May 2011, the research team completed four rounds of intensive searches of major breeding colonies in the archipelago at four-month intervals. During the first round of searches, large aggregations of birds were observed at most colonies, and the team successfully banded more than 700 birds. Since then, few of the banded birds have been sighted because attendance at breeding colonies has been very low. With few or no fledglings that can eventually recruit into the population, balancing natural losses of adults to old age and the other hazards of life will be impossible. The virtual absence of juveniles in May 2011 probably indicates that little successful breeding occurred during the previous two years (at least).
In early June 2012, a coastal survey of Blue-footed Boobies was conducted around the entire archipelago except the few northern islands mentioned above. This effort required ten people in 5 different boats, and occurred over 3 consecutive days. This method minimized double-counting and misses due to movements of birds among sites. Few juvenile birds were observed (<100), consistent with the previous year’s data, indicating little breeding. The preliminary estimate of the adult population was 6,000-8,000 birds. For comparison, the estimate from the 1960s and 1970s was 20,000-30,000 breeding birds; the population at that time also must have contained some pre-breeders. The contrast between even these admittedly loose numbers suggests a substantial decline in population size, and the failure to breed offers a demographic mechanism for the decline.
Why are they not breeding? Dr. Anderson suspected that food was behind the failure to even try to breed. Previous work on Española has shown that successful breeding occurs when Blue-footed Boobies have access to sardines. However, sardines have been largely absent from the Española area since 1997 (this has been shown from Nazca boobies there, who also prefer sardines but can breed using other prey). From 2012-2013, approximately half (54%) of >500 individual prey items were sardines. This is much less than their diet of essentially 100% sardines found during good times, suggesting that the birds find this diet sufficient to live but insufficient to breed. Other factors may be involved, but there is no evidence yet of effects of introduced predators, disease, and/or persecution by humans.
Monitoring of the population at four-month intervals continued through January 2013 to provide as clear a picture as possible of the population status of this trademark Galapagos species. However, further monitoring will be necessary to better determine the long-term status of the Blue-footed Booby population.
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