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Parasites are globally widespread pathogenic organisms, which impose important selective forces upon their hosts. Thus, in accordance with the Adjustment to parasite pressure hypothesis, it is expected that defenses among hosts vary relative to the selective pressure imposed by parasites. According to the latitudinal gradient in diversity, species richness and abundance of parasites peak near the equator. The uropygial gland is an important defensive exocrine gland against pathogens in birds. Size of the uropygial gland has been proposed to vary among species of birds because of divergent selection by pathogens on their hosts. Therefore, we should expect that bird species from the tropics should have relatively larger uropygial glands for their body size than species from higher latitudes. However, this hypothesis has not yet been explored. Here, we analyze the size of the uropygial gland of 1719 individual birds belonging to 36 bird species from 3 Neotropical (Peru) and 3 temperate areas (Spain). Relative uropygial gland volume was 12.52% larger in bird species from the tropics than from temperate areas. This finding is consistent with the relative size of this defensive organ being driven by selective pressures imposed by parasites. We also explored the potential role of this gland as a means of avoiding haemosporidian infection, showing that species with large uropygial glands for their body size tend to have lower mean prevalence of haemosporidian infection, regardless of their geographical origin. This result provides additional support for the assumption that secretions from the uropygial gland reduce the likelihood of becoming infected with haemosporidians. [ABSTRACT FROM AUTHOR]

Long-distance bird migration is expensive, and individuals in prime condition are therefore expected to arrive at the breeding grounds earlier than the average individual in the population. We tested whether arrival date during spring migration was associated with low parasite burden and strong T-cell-dependent immune response in populations of the transequatorially migrating Barn Swallow Hirundo rustica from Denmark, Italy, and Spain. Male Barn Swallows with heavy infestations of the chewing louse Hirundoecus malleus L. arrived later than other males, whereas that was not the case for females. This negative relationship remained after controlling the relationship between parasite load and arrival for tail length. Infection with blood parasites of the species Haemoproteus prognei was associated with delayed arrival in the Spanish population, which was the only one with a high prevalence of blood parasites. High intensities of infestation with a commensal feather mite were associated with early arrival by females, but also by males when the independent relationship between tail length and intensity of mite infestation was controlled statistically. Experimental manipulation of nest infestation with the hematophagous mite Ornithonyssus bursa Berlese affected the arrival date of adult males, but not of females in the following year, with arrival date being inversely related to the experimental mite population size. Males with strong T-cell responses arrived earlier than males with weak responses; independent of their tail length in two of three populations, but there was no significant relationship in females. These findings are in accordance with predictions based on the hypothesis that bird migration is dependent on condition, with individuals in prime condition arriving early. [ABSTRACT FROM AUTHOR]