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The Baja California Pacific Islands (BCPI) is a seabird hotspot in the southern California Current System supporting 129 seabird breeding populations of 23 species and over one million birds annually. These islands had a history of environmental degradation because of invasive alien species, human disturbance, and contaminants that caused the extirpation of 27 seabird populations. Most of the invasive mammals have been eradicated and colonies have been restored with social attraction techniques. We have recorded the number of breeding pairs annually for most of the colonies since 2008. To assess population trends, we analyzed these data and show results for 19 seabird species on ten island groups. The maximum number of breeding pairs for each nesting season was used to estimate the population growth rate (λ) for each species at every island colony. We performed a moving block bootstrap analysis to assess whether seabird breeding populations are increasing or decreasing. San Benito, Natividad, and San Jerónimo are the top three islands in terms of abundance of breeding pairs. The most widespread species is Cassin’s Auklet (Ptychoramphus aleuticus) with 14 colonies. Thirty-one populations of 14 species are significantly increasing while eleven populations of seven species are decreasing. We did not find statistical significance for 19 populations, however, 15 have λ>1 which suggest they are growing. Twelve of the 18 species for which we estimated a regional population trend are significantly increasing, including seven surface-nesting species: Brandt’s Cormorant (Phalacrocorax penicillatus), Brown Pelican (Pelecanus occidentalis), Caspian Tern (Hydroprogne caspia), Double-crested Cormorant (P. auritus), Elegant Tern (Thalasseus elegans), Laysan Albatross (Phoebastria immutabilis) and Western Gull (Larus occidentalis), and five burrow-nesting species: Ainley’s (Hydrobates cheimomnestes), Ashy (H. homochroa) and Townsend’s (H. socorroensis) Storm-Petrels, and Craveri’s (Synthliboramphus craveri) and Guadalupe (S. hypoleucus) Murrelets. The BCPI support between 400,000 and 1.4 million breeding individuals annually. Our results suggest that these islands support healthy and growing populations of seabirds that have shown to be resilient to extreme environmental conditions such as the “Blob”, and that such resilience has been strengthen from conservation and restoration actions such as the eradication of invasive mammals, social attraction techniques and island biosecurity.

San Benito Archipelago is internationally important for the conservation of 13 species of seabirds. San Benito Oeste, the largest and only inhabited island, was declared mammal-free in 2000 after a series of eradications conducted in collaboration between the fishing cooperative Pescadores Nacionales de Abulón, the Mexican conservation organization, Grupo de Ecología y Conservación de Islas, A.C., and the Mexican Government. The archipelago remained mammal-free until 2006, when an unusual invader, the Cedros island cactus mouse (Peromyscus eremicus cedrosensis), was accidentally introduced to San Benito Oeste island. The same collaboration scheme involving locals, conservationists, and authorities was once again put in motion, delivering tangible results. Research informed the mouse eradication strategy, the local community supported the operation, and the mouse eradication was successfully implemented in December 2013. To date (8 years later), no mammals have been recorded in the archipelago, which suggests community-led island biosecurity is working. In addition, this collaborative restoration work contributed to the creation of the Baja California Pacific Islands Biosphere Reserve, protecting 21 islands, including the San Benito Archipelago, and 97 islets in the Mexican Pacific.

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