# Achromatic Markings as Male Quality Indicators in a Crepuscular Bird

## TL;DR

This study shows that white markings on male European Nightjars indicate quality traits like age and health, suggesting their role in sexual selection among nocturnal birds.

## Contribution

The study demonstrates that white, non-pigmented markings in nocturnal birds can serve as quality indicators under sexual selection.

## Key findings

- Larger white markings in male Nightjars correlate with age, better body condition, and site fidelity.
- Variation in marking size exceeds that of other body traits, suggesting sexual selection.
- Marking size varies across study sites and years, indicating environmental influences.

## Abstract

Many animals use conspicuous body parts in communication, often as indicators of individual quality. While this has been extensively studied in the colourful plumage of diurnal songbirds, little is known about the role of contrasting white markings in nocturnal species. Over 15 years, we captured European Nightjars, nocturnal birds with distinct white markings on their wings and tail in males. We measured these markings and examined whether they show greater variability than other body parts, suggesting sexual selection pressures on their size. We also investigated the associations between the size of these markings and indicators of individual quality, such as age, body condition, site fidelity, and whether these patterns varied across study sites and between years. Our findings revealed that larger markings, particularly in the tail feathers, were associated with older males, those in better condition, and males returning to the same breeding site. The marking size varied across study sites. Given the visibility of these markings during breeding and territorial displays of Nightjars, we propose that they act as quality indicators. This study highlights the potential role of white markings in the communication of birds active at night, advancing our understanding of sexual selection in these enigmatic species.

Secondary sexual traits, such as specific body parts or colouration, play an important role in mating interactions. It has been proposed that they function as quality indicators driven by sexual selection. In birds, much attention has been paid to the study of feather pigmentation, especially in diurnal passerines. However, recent research demonstrates that structural achromatic colours are likely to be of similar importance for communication, especially for species inhabiting poorly lit environments and that are active at night. Using 15 years of capture–recapture data from a long-term study on adult European Nightjars (Caprimulgus europaeus), we investigated the role of males’ white tail and wing markings as secondary sexual traits. We show that the inter-individual variation in marking size exceeds that of the other morphometric variables, suggesting that wing and tail markings could be subject to sexual selection. Older males, individuals with a higher body condition index, and long-term territory holders had larger markings, while these effects were particularly pronounced in terminal tail feather markings. The importance of markings for signalling is likely related to their observed use in social displays. Pronounced site differences in tail marking sizes and annual variation suggest environmental factors acting on the ornaments that remain to be further examined.