Parental morph combination does not influence innate immune function in nestlings of a colour-polymorphic African raptor

Abstract

Conditions experienced during early life can have long-term individual consequences by influencing dispersal, survival, recruitment and productivity. Resource allocation during development can have strong carry-over effects onto these key parameters and is directly determined by the quality of parental care. In the black sparrowhawk (Accipiter melanoleucus), a colour-polymorphic raptor, parental morphs influence nestling somatic growth and survival, with pairs consisting of different colour morphs (‘mixed-morph pairs’) producing offspring with lower body mass indices, but higher local apparent survival rates. Resource allocation theory could explain this relationship, with nestlings of mixed-morph pairs trading off a more effective innate immune system against somatic growth. We quantified several innate immune parameters of nestlings (hemagglutination, hemolysis, bacteria-killing capacity and haptoglobin concentration) and triggered an immune response by injecting lipopolysaccharides. Although we found that nestlings with lower body mass index had higher local survival rates, we found no support for the proposed hypothesis: neither baseline immune function nor the induced immune response of nestlings was associated with parental morph combination. Our results suggest that these immune parameters are unlikely to be involved in providing a selective advantage for the different colour morphs’ offspring, and thus innate immunity does not appear to be traded off against a greater allocation of resources to somatic growth. Alternative hypotheses explaining the mechanism of a low nestling body mass index leading to subsequent higher local survival could be related to the post-fledgling dependency period or differences in dispersal patterns for the offspring from different morph combinations.

Integument colouration and circulating carotenoids in relation to urbanisation in Eurasian kestrels (Falco tinnunculus)

“…This could be due to an improved absorption of carotenoids as well as an accumulation over time and was also reported in other raptor populations (Casagrande, Costantini et al 2007 ; García-Heras, Arroyo et al 2017 ). However, no association between carotenoids and body mass was seen, which could indicate that integument coloration and blood concentration depend on the quality (carotenoid content) of the ingested prey items rather than the quantity of food, as mentioned in other studies (Eeva, Sillanpää et al 2009 ; Sternalski, Mougeot et al 2010 ; Sternalski, Mougeot et al 2012 ; Nebel, Amar et al 2021 ). This could also be one reason why no relationship between carotenoids and hatch rank was found either, because hatch rank is directly informed by nestling size and was assigned in the field.…”

Reduced ectoparasite load, body mass and blood haemolysis in Eurasian kestrels (Falco tinnunculus) along an urban–rural gradient

“…This finding is in accordance with other urban-dwelling avian species (see Liker et al 2008 ; Meillère et al 2015 ; Herrera-Dueñas, 2018 ; Isaksson et al, 2005 ; Chamberlain et al, 2009 ). However, a recent urban raptor study showed that offspring with lower body mass indices also had higher local apparent survival rates (Nebel et al 2021 )—unfortunately we do not have data on post-fledging survival in our population from which to infer potential long-term fitness consequences.…”

Innate immune function and antioxidant capacity of nestlings of an African raptor covary with the level of urbanisation around breeding territories

“…One would expect high‐‘quality’ nestlings to show stronger antioxidant capacity, lysis and bactericidal capacity, but curiously, none of these indices were correlated with body mass index. In a related study (Nebel et al, 2021 ), nestlings with lower body mass index showed higher survival rates—an index of energy reserves and physical well‐being. This finding raises questions about the suitability of avian body morphometrics (Green, 2001 ; Peig & Green, 2010 ) as proxies of individual condition.…”