Inbreeding depression depends on the body size and environmental conditions in a threatened songbird, the aquatic warbler Acrocephalus paludicola

While inbreeding is known to affect individual fitness and thus extinction risk in populations, studies have under-represented non-model species of conservation concern and rarely sought conditionality of inbreeding depression. Here, using SNPs identified with RAD-seq, we determined inbreeding depression in a threatened passerine, the aquatic warbler Acrocephalus paludicola , and whether its magnitude depends on phenotypic and environmental factors. We found no support for an association of the inbreeding coefficient ( F ) with the long-term return rate to breeding grounds and seasonal breeding success in adult males, and with the clutch size, hatch failures, nestling mortality, and fledged brood size in adult females. However, the F of adults with small tarsi (a proxy for structural body size) was negatively associated with the seasonal breeding success (in males) and clutch size (in females). We found no support for a relationship between nestling or parental F and nestling scaled-mass index (SMI) on days 2, 5 and 9 post-hatch. Nestling tarsus was negatively related to father F on days 5 and 9, and less strongly to mother F on day 5, but not to nestling F . The mother F effects on nestling tarsus and SMI were more negative in higher temperatures and the father F was more negatively related to nestling tarsus on later laying dates. The nestling F was more negatively associated with SMI under lower temperatures and rainfall. Neither the parental, nor the chick F effects on tarsus and SMI were found to interact with nestling sex, brood size and prey abundance. The inbreeding load on the adult male long-term survival rates was low for the mean covariates under study, but the decrease in fitness in the most inbred individuals, relative to the least inbred ones was high (∼76%) for the breeding success in small-tarsus males and moderate (∼10%) for the clutch size in small-tarsus females. We conclude that in the aquatic warbler (1) while inbreeding depression for the average phenotypic and environmental variables studied is not supported, (2) it is moderate to strong in small-bodied adults, (3) for chick condition, it depends on the weather, and (4) parental inbreeding effects could carry over to the next generation.