Migration distance does not predict blood parasitism in a migratory songbird.

Marjorie C Sorensen,Jason Newton,Claire N Spottiswoode,Keith A Hobson,Susanne Jenni‐Eiermann,Staffan Bensch,Tanmay Dixit,Kevin J Kardynal

doi:10.1002/ece3.5404

Abstract

Migration can influence host–parasite dynamics in animals by increasing exposure to parasites, by reducing the energy available for immune defense, or by culling of infected individuals. These mechanisms have been demonstrated in several comparative analyses; however, few studies have investigated whether conspecific variation in migration distance may also be related to infection risk. Here, we ask whether autumn migration distance, inferred from stable hydrogen isotope analysis of summer‐grown feathers (δ 2Hf) in Europe, correlates with blood parasite prevalence and intensity of infection for willow warblers (Phylloscopus trochilus) wintering in Zambia. We also investigated whether infection was correlated with individual condition (assessed via corticosterone, scaled mass index, and feather quality). We found that 43% of birds were infected with Haemoproteus palloris (lineage WW1). Using generalized linear models, we found no relationship between migration distance and either Haemoproteus infection prevalence or intensity. There was spatial variation in breeding ground origins of infected versus noninfected birds, with infected birds originating from more northern sites than noninfected birds, but this difference translated into only slightly longer estimated migration distances (~214 km) for infected birds. We found no relationship between body condition indices and Haemoproteus infection prevalence or intensity. Our results do not support any of the proposed mechanisms for migration effects on host–parasite dynamics and cautiously suggest that other factors may be more important for determining individual susceptibility to disease in migratory bird species.

Highlights

The seasonal movements of migratory animals have important ecological consequences (Bauer & Hoye, 2014) including animal host–parasite interactions and the spread of disease (Altizer, Bartel, & Han, 2011)
While there have been several comparative analyses of migration effects on levels of parasitism, few studies have investigated whether conspecific variation in migration dis‐ tance may be related to infection risk
We found no relationship between migration distance (δ2Hf) and either avian malaria prevalence or intensity in male willow warblers sampled on their nonbreeding grounds in Zambia

Summary

| INTRODUCTION

The seasonal movements of migratory animals have important ecological consequences (Bauer & Hoye, 2014) including animal host–parasite interactions and the spread of disease (Altizer, Bartel, & Han, 2011). Migration may influence host–parasite dynamics by removing infected individuals from the population during strenuous journeys (mi‐ gratory culling; Bradley, 2005) or by allowing migrants to escape harmful parasites (Hall & Bartel, 2014) These interactions are complex, and comparative studies from various taxa have shown that migratory species can both have higher (Koprivnikar & Leung, 2014; Leung & Koprivnikar, 2016; Teitelbaum et al, 2018) and lower (Satterfield et al, 2015) levels of parasitism than resident species. We investigated whether migration distance, inferred from stable hydrogen isotope values in summer‐grown feathers (δ2Hf), was correlated with malaria prevalence and intensity of infection for willow warblers (Phylloscopus trochilus) wintering in Zambia. Understanding the factors that predict individual infection risk is an important com‐ ponent of effectively predicting the population dynamics of spe‐ cies, especially given that global change is rapidly affecting some of the mechanisms hypothesized to affect infection risk in migra‐ tory animals (Garamszegi, 2011; López‐Calderón et al, 2018)

| METHODS

Findings

| DISCUSSION