The inspection of Haemosporidian parasites of the genera Plasmodium, Haemoproteus and Leucocytozoon, combining microscopic screening of blood smears and a broadly used PCR-based detection method, suggests that adult male and female blue-footed boobies breeding on Isla Isabel are likely free of these blood parasites. Our results support previous findings by Clark and Swinehart based on blood smears 20 who recorded the absence of these parasites in 19 blue-footed boobies from islands off the North Pacific coast of Mexico (not including Isla Isabel boobies). These negative results reflect the overall scarcity of blood parasites in seabirds, which is often attributed to unsuitable conditions for vectors in seabird habitats 5,21.
In addition to the occurrence of potential vectors in the area, Haemoproteus parasites have been previously detected in frigatebirds from Isla Isabel (prevalence ranging from 16 to 54%; 19,22), thus lack of blood parasites in Isla Isabel boobies probably implies that parasite infections vary among avian host species that coexist on the island. Differences in the prevalence of blood parasites between boobies and frigatebirds could be due to differences in their habitat use or in the abundance and activity of vectors 23,24. Infected frigatebirds were sampled in the southwestern end of the island, in a shady, vegetated area with large trees located a few ten of meters from an unfinished building where rainwater accumulates, and 150 meters from the only pool of alkaline water on the island 19,25. This area is protected from the wind and suitable for development and proliferation of some potential vectors 26. In addition, the parasites detected in frigatebirds are transmitted by louse flies (Hippoboscidae) that spend most of the time on their vertebrate hosts and present higher host specificity than mosquitoes, Culicoides and black flies. Boobies breed mostly where we sampled them in the wind-exposed northeastern end of the island, ~ 1.0–1.5 km from the source of alkaline water pool and the unfinished building where rainwater accumulates. The northeastern area is mainly covered by stunted woody vegetation, since big trees have been recurrently damaged by hurricanes 27,28. Windy, dry, and hot conditions limit the abundance and activity of potential vectors of blood parasites in marine environments 6,23, and their prevalence in the Isla Isabel booby colony may explain the absence of blood parasites in boobies.
Lack of blood parasites in seabirds has been attributed to evolutionary and ecological factors other than spatial heterogeneity in the occurrence of suitable vectors 5,8, but it is unlikely that these alternative mechanisms explain the results reported here. Earlier screening of blood parasites in boobies from the Galápagos archipelago revealed the infection by a tentatively identified Leucocytozoon parasite, and especially, by parasites of Haemoproteus genus 14,18. Thus, it is unlikely that blood parasites are absent from Isla Isabel boobies because they exhibit high immunological capacities, have short exposure times to parasitic infections or present physiological incompatibility with haemoparasites that prevents the latter from completing their life cycles 8,9. In addition, differences in host numbers 8 may not explain differences in the occurrence of blood parasites between the booby populations of Isla Isabel and the Galápagos. The booby colony of Isla Isabel has up to 1769 breeding pairs in our study area alone, which covers 26, 889 m2 and contains approximately 65% of all breeding pairs that are established annually on the island 29. The breeding population of blue-footed boobies in the whole Galápagos archipelago was estimated at 3,200 pairs at the time when high prevalence of blood parasites was detected in Galápagos boobies 14,30. Thus, host numbers are not different enough between the two populations to explain the apparent lack of blood parasites in Isla Isabel boobies.
In conclusion, we failed to find evidence of blood parasites in one of the largest colonies of blue-footed boobies of the North Pacific coast, after microscopic examination of blood smears and state-of-the-art molecular analysis for detection of avian blood parasites 9,31. Apparent absence of blood parasites in Isla Isabel boobies indirectly adds to the growing evidence of variation in parasite infections among avian host species that coexist locally 23,32, and highlights the relevance of performing evaluations of the prevalence of blood parasites in different populations of widespread host species 21.