Size matters – at least for the Nightjars

A male European Nightjar, showing its prominent white wing and tail ornaments during a territorial display. Photo: Ervín Hrtan

A study on how white patches convey the quality of male European Nightjars

By Richard Schnürmacher

• Many avian species sport bright, contrasting colours, but what about the birds active at night?

• Variation in the white markings of adult male European Nightjars suggests sexual selection on their size.

• Age and body condition strongly influence how big these ornaments are.

• Site fidelity and local environmental conditions also affect ornament size.

• White wing and tail patches occur in most nightjars, hinting that this function may be widespread across the family.

Background

Animals communicate through an astonishing variety of signals. From elaborate bird songs to complex chemical cues in insects, all communication ultimately serves to transmit information to another individual. The message may be simple: “Hey, I’m here!” or “Let’s forage over there.” In other contexts, typically when the sender tries to woo a new partner or defend its territory against a potential rival, signals can advertise the sender’s quality. They may reflect social status, experience, condition or even parasite load. For receivers, these signals provide valuable information to accurately assess whether the individual would be a suitable, caring parent and a formidable opponent who is better not to be confronted with, or one to avoid for mating and a rival that can be easily overpowered. Because individuals differ in their ability to develop and maintain such traits, variation is common within populations, and the partners tend to select for more elaborate traits. Such communication frequently comes at a cost for the bearer (like stag antlers or peacock train) and it is hard or even impossible to fake, therefore it has been termed “honest”.

Most honest signalling in birds involves acoustic or visual displays – think of complex songs or vibrant plumage. But what about species active at twilight or during the night? While sound travels regardless of light levels, colourful plumage becomes ineffective in darkness. Surprisingly, many crepuscular and nocturnal birds still rely heavily on visual cues. They often concentrate their displays at dawn, dusk or on bright moonlit nights. Under low-light conditions, contrast matters more than colour, and many species rely on white, pigment-free patches on a dark background. These can be hidden when not in use, reducing predation risk, but highly conspicuous during display. Still, when they are visible, they should serve an analogous role as in diurnal species, meaning to attract mates and repel the rivals.

Although this phenomenon is well studied in some owls, such as the Eurasian Eagle-owl, relatively little is known about visual signalling in nightjars. This is striking, as most species in the family have conspicuous white patches on their wings, tails or throat; and some, like the Pennant-winged or Standard-winged Nightjar, carry some of the most spectacular feather ornaments in the bird realm.

What was our goal?

To understand the potential role of these white, achromatic markings in nocturnal birds, we selected the European Nightjar (Caprimulgus europaeus) as a model species. We aimed to test whether the markings are more variable than other body traits – a first indicator that they function as ornaments under sexual selection. Furthermore, we wanted to identify which factors determine ornament size, particularly traits linked to individual quality such as age, body condition and site fidelity. Finally, we explored whether marking size differs between study sites, which hints at possible environmental effects.

How did we gather the data?

Like most nightjars, European Nightjars have contrasting patches on their three (occasionally four) outer primaries and two (rarely three) outer tail feathers. In males these patches are pure white, while in females they are duller brownish or sometimes absent. As part of a long-term study running since 2009 in Flanders, Belgium (Prof. Evens’ lab - Nox’alis group), we measured the size of these patches in more than 700 adult males caught using mist nets within their territories. Early in the project, markings were measured manually; since 2019 they have been photographed and analysed digitally. Alongside marking measurements, we recorded biometric traits such as wing length, body mass, sternum length and tail length. These allowed us to estimate body condition. Age was determined in the field, and recaptures across years provided minimum known age and site fidelity information. Using statistical models, we were able to investigate which of these factors had a significant impact on the size of the male markings.

What are our findings?

All white markings showed much higher variance than other morphological traits, on average more than four times higher (~26% vs ~6%). This strongly supports the idea that these patches are under sexual selection and function as ornaments. Older males had larger ornaments than males born the year before, or non-juvenile individuals that we could not determine their age group based on moult. This pattern was particularly strong for tail markings. In recaptured birds, yearlings (second calendar-year birds) had smaller tail ornaments than males aged two years, so in their third calendar year, or older. Males in better body condition had larger ornaments. Site-faithful males had larger tail ornaments than males that did not return to the same breeding area (after accounting for age). Ornament size varied between study sites. Males in Bosland NP had significantly larger ornaments than those in the Military Domain of Meeuwen, with additional seasonal fluctuations.

Variation of the markings compared to other body traits from doi.org/10.3390/biology14030298 licensed under CC BY 4.0. Using both methods (manual calliper and digital measurements), we found that the size of the markings varied to a larger degree than other morphological traits measured (shown in grey) that represent individual size differences, i.e. the variation of size of P8 Digital around the mean value was over 34%, while it varied only roughly 2.5% in wing length. Still, there were considerable differences among the respective markings, which indicates that some may be “more important” for conveying quality. The colour of the bars corresponds to the position of the markings in the feathers on the Nightjar silhouette.

What is the impact?

Our study provides clear evidence that honest visual signalling can be important even in night-active species. In the European Nightjar, males are more conspicuous than females, and their prominent white patches appear to act as sexually selected ornaments. Male display behaviour reinforces this interpretation: during territorial flights they fan their wings and tail to maximise exposure of the white patches to rivals and potential mates. Interestingly, the most variable markings were those on the inner feathers (P8 and T4), which are better protected from wear than outer feathers. We observed greater abrasion in the more exposed outer tail feathers. When the birds are not displaying, all markings are covered by their cryptic, bark-reminding plumage on the rest of the body.

The positive relationship between body condition and ornament size from doi.org/10.3390/biology14030298 licensed under CC BY 4.0. The colour of the curves corresponds to the position of the markings in the feathers on the Nightjar silhouette.

Age-related patterns suggest that ornament size peaks around four years of age, with a slight decline in males five years or older, consistent with the species’ typical lifespan. Thanks to our recapture data, we also found that most of the birds aged in the field as >1CY were actually yearling birds that manage to moult all their feathers during their first complete moult in Africa. These individuals tended to have slightly larger ornaments, which may be linked to higher-quality young males being able to moult all their juvenile feathers. Therefore, it would be interesting to investigate the conditions right there, at their wintering grounds, to see if the body condition and other factors determine the marking size, since that is the period of the feather growth. As expected from a quality indicator, the body condition was reflected in the ornament size, but again, this link is only indirect, since their feathers developed some half a year before they utilised their ornaments in social interactions at their Belgian breeding sites. However, if they did return to the same site, they tended to have larger ornaments compared to their fellows who did not show such site fidelity (corrected for potential age effects) – perhaps since they were successful in claiming these territories and breeding within them. Lastly, two of our four study sites showed marked differences in ornament sizes. Bosland NP, a relatively recently restored heathland habitat with smaller clearings surrounded by pine plantations, hosted males with overall largest tail ornaments, especially compared to males inhabiting the vast heathland in the military domain. We speculate that the darker habitats in Bosland could have favoured birds with larger, more conspicuous ornaments, compared to light, open habitats of Meeuwen. Still, much of the potential environmental impacts remain to be uncovered.

This study provides the first robust evidence that the white markings common across nightjars convey information about individual quality. The findings open exciting avenues for future work: Do females actively prefer males with larger ornaments? How do these visual traits interact with other quality indicators such as song rate or territory size? Do similar patterns occur in other nightjar species? We look forward to investigating these questions and hope to see some more evidence in other nightjar species that this is indeed not an isolated case in the widespread Eurasian species, “The” Nightjar.

We hope that you have enjoyed reading this post about nightjars. It is our aim to help spreading the word of these enigmatic birds and to encourage communication and collaboration within the global community of nightjar enthusiasts. We are aiming to post regularly about nightjars, please contact us if there is a story you want us to write about.