Size and immune function as predictors of predation risk in nestling and newly fledged jackdaws

• Published in: Animal Behaviour
• Main Authors: Aastrup, Christian, Nilsson, Jan Åke, Hasselquist, Dennis, Hegemann, Arne
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2023
• Subjects: Ecology; Zoology; ecophysiology; immunity; predator–prey interaction; prey selection; structural body size; Accipiter gentilis
• Online Access: https://lup.lub.lu.se/record/a84d880a-25de-4500-85cc-b376ea60d96a; https://doi.org/10.1016/j.anbehav.2023.01.012

Prey choice by predators may be based on the potential prey's condition, for example resulting in substandard individuals running a higher risk of being predated. Over 5 years, we studied young jackdaws, Corvus monedula, to determine whether size and innate baseline immune function may predict predation risk by goshawks, Accipiter gentilis, during the nestling and early fledging phases. We measured body mass, wing length, tarsus length and four innate immune indices two to four times when nestlings were 12–29 days old. To determine which individuals had been predated during the nestling phase and shortly after fledging, we searched for metal rings of the jackdaws in the only goshawk territory close to the jackdaw colony. Nestling mortality before 12 days of age was entirely due to starvation, whereas between 12 days of age and fledging, mortality was mainly due to predation. Nestlings with smaller size (mass, wing, tarsus) and low lysis titre and haptoglobin concentrations were at a higher risk of being predated before fledging. Directly after fledging, individuals with short wings were preferentially predated, with no effects of body mass, tarsus length or any of the four immune indices measured at day 29 (i.e. shortly before fledging). That lower immune function and smaller size predict predation risk in nestlings may reflect that these individuals are of poor quality and/or lag behind in development. We hypothesize that hunger makes these nestlings sit closest to the entrance hole and hence become the first to be predated. For fledglings, our results suggest that poor flight ability makes individuals with short wings the easiest targets for avian predators.