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\begin{document}

\title{The nightjar and the ant: a tale of a lose-lose game}



\author[1]{Carlos Camacho}%
\author[1]{Manuel Vidal-Cordero}%
\author[2]{Pedro Sáez-Gómez}%
\author[3]{Julio  Rabadán-González}%
\author[4]{Paula Hidalgo-Rodríguez}%
\author[5]{Carlos Molina}%
\author[6]{Pim Edelaar}%
\affil[1]{Estación Biológica de Doñana}%
\affil[2]{Universidad Autónoma de Madrid}%
\affil[3]{Observation.org}%
\affil[4]{Universidad Pablo de Olavide}%
\affil[5]{Sociedad Española de Ornitología}%
\affil[6]{Universidad Pablo de Olavide Departamento de Biologia Molecular e Ingenieria Bioquimica}%


\vspace{-1em}



  
  \date{May 27, 2024}


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\selectlanguage{english}
\begin{abstract}
Birds and ants co-occur in most terrestrial ecosystems and engage in a
range of interactions. Competition, mutualism and predation are
prominent examples of these interactions, but there are possibly many
others that remain to be identified and characterized. This study
provides quantitative estimates of the frequency of toe amputations
resulting from ant bites in a population of migratory red-necked
nightjars (Caprimulgus ruficollis) monitored for 15 years (2009-2023) in
S Spain and identifies the attacker(s) based on taxonomic analyses of
ant-mandible remains found on injured toes. Less than 1\% of examined
adults (N = 369) missed one or more toes. The analysis of ant remains
identified African army ants (Dorylus sp.) as the primary cause of toe
amputations in nightjars and revealed that body parts of the attacker
may remain attached to the birds even after intercontinental migration.
No cases of severe damage were observed in juveniles (N = 269), apart
from the mandible of a Messor barbarus --a local ant species-- attached
to one of the teeth of the characteristic comb of the medial toe of
nightjars. The incidence of ant-bite damage may appear unimportant for
nightjar populations, but this might not be true if only birds that
manage to survive their injuries and potential complications (e.g.
severe bleeding and sepsis from opportunistic infections) return from
the tropics. More field studies, ideally in tropical areas, that
incorporate routine examination of ant-induced injuries into their
protocols are needed to understand the true incidence and
eco-evolutionary implications of antagonistic ant-bird interactions.%
\end{abstract}%



\sloppy


\textbf{INTRODUCTION}

The interactions between birds and insects have been the subject of
extensive research (Morse, 1971; Willis \& Oniki, 1978). Bird-insect
interactions in the form of predation, parasitism, and mutualism have
received the most attention because of their ubiquitous occurrence and
importance as study systems for understanding coevolution (Holmes et
al., 1979; Maziarz et al., 2021; Price, 1997; Quinn \& Ueta, 2008).
Other less conventional or more cryptic interaction types also occur
along the antagonism-commensalism gradient, such as competition (e.g.
for food resources; Smith \& Balda, 1979) or `anting' (i.e. the use of
ants as a treatment against parasites; Camacho \& Potti, 2018), but they
often go undetected or underreported and their true nature and
biological significance remain to be elucidated in most cases.

Of all insects, ants (Formicidae) are the most abundant in most
terrestrial ecosystems (Del Toro et al., 2012). There is ample
literature documenting antagonistic ant-bird interactions, primarily in
the form of bird predation on ants (reviewed in Avil\selectlanguage{ngerman}és, 2023). There are
also studies reporting negative effects of ants on aspects such as bird
abundance, behaviour, and reproduction, primarily through predation on
eggs and chicks (Alvarez-Blanco et al., 2020; Davis et al., 2008;
Haemig, 1999). However, assessing the impact of ants on more mobile
segments of a population (e.g. juveniles and adults) is challenging and
our understanding of the nature and magnitude of these effects is
therefore limited.

This study takes advantage of a long-term capture-recapture program to
examine the nature and frequency of injuries (toe amputations) of a
migratory insectivorous bird --the red-necked nightjar
(\emph{Caprimulgus ruficollis} )-- in their breeding area in S Spain.
The study was motivated by observations of nightjars missing toes and
particularly of an adult bird recorded in 2015 that had what appeared to
be the mandible of an ant embedded in the medial toe of one of their
feet. Six years later, in 2021, we found another adult that had the full
head of an ant embedded in a central toe. Since then, we engaged in a
systematic revision of the feet of all captured nightjars to evaluate
the frequency and severity of toe injuries. Observations of insect
remains attached to injured toes in three nightjars provided an
opportunity to identify the agents of damage based on a taxonomic
analysis. We close by discussing the incidence of such injuries on
nightjar populations and identify further research needs.

\textbf{METHODS}

\textbf{Study system}

Data on toe damage were collected during three breeding seasons
(2021-2023) as part of a long-term (2009-present) study of red-necked
nightjars in the Doñana National Park, S Spain (see Camacho et al.,
2014, 2022 for details on the study site and sampling protocols). Most
nightjars arrive in the study area in early May and generally postpone
their migration departure until mid October (i.e. approx. 5½-month stay
in the breeding area; Camacho, 2013a). GPS tracks of adult nightjars
from the study population indicate that most individuals migrate through
Morocco and Mauritania to spend the non-breeding season (November-April,
approx. 5½ months) in S Mali and SE Guinea (authors' unpubl. data).

Nightjars spend much of their time on the ground: they are
ground-nesting birds and both members of the pair share incubation and
chick-rearing duties for over 35 days, the female taking the larger
share (Camacho, 2013a). During the day, they remain motionless on the
ground in the shade of trees or shrubs (Camacho et al., 2014). During
the night, they use open spaces on the ground as perches to facilitate
detection of aerial insects (mainly moths), although they can also
occasionally pick up terrestrial insects that are running around
(Jackson, 2003). In spending so much time on the ground, nightjars are
exposed to accidental contact with all sorts of diurnal and nocturnal
ground-dwelling creatures.

\textbf{Field and lab procedures}

Nightjars are attracted to roads at night (Jackson, 2003), so road
transects are one of the most commonly used methods for nightjar
monitoring (Camacho, 2013b; De Felipe et al., 2019). From late March to
late October, we conducted nocturnal surveys for red-necked nightjars on
a regular (3-7 days) basis along a 24-km road transect by driving a car
at slow speed (30 km h\textsuperscript{-1}). Nightjars found on roads
were captured using a torch and a butterfly net (Jackson, 1984) and
marked with numbered metal rings (if not ringed already) for individual
recognition. Birds were sexed and aged as first-year (juvenile),
second-year (approx. 1 year old), or older using plumage characteristics
(Camacho, 2013a; Forero et al., 1995), and measured according to the
standard protocol for long-term monitoring of nightjars (see details in
Hidalgo-Rodríguez et al., 2021). Many individuals' ages are known
because they were first captured as juveniles (Camacho et al., 2022).

To estimate the frequency of damage on limbs, we carefully examined the
feet and toes of all captured individuals, noted anomalies (e.g. breaks
in skin, and full or partial amputations; Figure 1a) and checked for
remains of the attacker (e.g. mandibles or full head; Figure 1b-d). The
remains found during the three years of systematic record were removed
from the bird and preserved in 96º ethanol for taxonomic identification
in the laboratory. The remains, specifically a left mandible found in
2021 and a full head found in 2023, were closely examined under a 80x
stereomicroscope to identify the attacker to the genus or species level
(Borowiec, 2016; Fisher \& Bolton, 2016).

We calculated the proportion of juveniles and adults missing feet or
toes (or in the process to lose them, as determined from the severity of
damage) as a measure of the incidence of damage, and used the Fisher's
exact test as implemented in R (\emph{https://cran.r-project.org/}) to
test the null hypothesis that the incidence of damage does not differ
between age classes. Non-systematic data collected from birds trapped
during ringing operations in previous years (2009-2020) provided
additional useful information about the nature of damage.

\textbf{RESULTS}

Our dataset consisted of a total of 855 captures from 588 individual
nightjars that had been examined for limb damage and loss at least once
during 97 nocturnal surveys conducted in 2021-2023 (Table 1). Sampling
effort, measured as either the total number of captures (range: 282-287)
or as the number of different individuals examined (range: 207-225), was
uniform across the three years of systematic study (Table 1). No
evidence of severe damage (i.e., toe amputation) was observed in any of
the 219 juveniles examined, although one of the juveniles caught in 2023
had a full (ant) head attached to the claw of the central toe (Figure
1c). Of the 369 different adults captured during 2021-2023, only three
missed a toe: a 1-year-old male captured for the first time in 2021 (toe
still attached to the foot by a thin piece of skin; Figure 1b), a
5-year-old male captured in both 2021 and 2022 (first caught as a
second-year bird in 2017), and a 1-year-old female captured in 2023
(first caught as a non-injured juvenile the year before). The estimated
frequency of damage in adults across the three years of study was 1:123
(0.81\%), although annual estimates differed as much as 2-fold (Table
1). Four additional adults missing one or more phalanges in one or more
toes were recorded during non-systematic checks in previous years (Table
1). Despite the lack of records of toe amputations in juvenile
nightjars, there is insufficient statistical support to conclude that
there is a greater incidence of damage in adults (Fisher's exact test, P
= 0.299).

Damage generally consisted of total amputation (80\% of cases) or
incomplete excision (20\%) of one or two toes. The most severe damage
was recorded for an adult female captured in 2015, which missed three
toes of the right foot and one lateral toe and the claw of the medial
toe on the left foot (Figure 1a).

The encountered left mandible (year 2021) measured 2.3 mm in length by a
maximum width of 0.8 mm, was dark brown in colour and ended in a
bifurcation, with a robust central tooth. It was identified as belonging
to a worker ant of the army ant genus \emph{Dorylus} (Fabricius, 1793).
Some 20 species of \emph{Dorylus} ants have been listed for Mali and
Guinea (antsmap.org), where the captured nightjars spend the winter. It
is currently impossible to identify the attacker at the species level,
even if we had retrieved the entire individual, and the entire genus is
currently under taxonomic revision (Kiko Gómez, pers. comm.).

The full head attached to the juvenile (year 2023) caused no apparent
damage to the bird, apart from nicking one of the teeth of the
characteristic pecten (comb) of the medial toe of nightjars (Figure 1c).
The full head was identified as belonging to a worker of \emph{Messor
barbarus} , an easily identifiable species that can be distinguished
from other species of the genus in the Iberian Peninsula by the unique
reddish colouration on the head (Lebas et al., 2017).



\textbf{DISCUSSION}

We provided quantitative estimates of the frequency of toe amputations
in juvenile and adult red-necked nightjars, evaluated the severity of
damage to the birds, and identified the body parts of the ants that
remained attached to nightjar toes. Less than \textless{}1\% individuals
missed one or more toes. Toe amputations were only observed in adults
that had performed at least one round-trip migration from Europe to
Africa and, although ant-nightjar encounters also occurred in juveniles,
these resulted in minor injuries. The taxonomic analysis of mandible
remains identified army-ant bites as the most likely cause of toe
amputations in adult nightjars.

Our estimate of the incidence of ant bites is based on the proportion of
toe-mutilated birds, but the cause of damage could only be confirmed for
those individuals that had remains of ant mandibles or the full head on
their toes. None of the nightjars examined in this population
(\textgreater{}4,500 captures) exhibited typical poxvirus-like lesions
on the eyes, feet or toes, suggesting that poxvirus infection does not
play a major role in the observed toe amputations. Besides amputations,
over the 15 years of study we have observed structural injuries to the
sternum (n=7), tarsus (n=1), mandibles (n=4), and toes (n=5) of
nightjars, affecting \textless{}1\% of all trapped individuals. These
injuries are hypothesized to reflect healed fractures based on the
presence of bone callus formation and/or angular deformity. Damaged toes
due to traumatic fracture differ from confirmed ant-bitten toes in that
fractured toes often become distorted in position, pointing obliquely
across the other toes, but the skin remains intact. By contrast, army
ant bites result in a break in the skin of the affected phalanx and,
even though the compressive force applied to the toe is not enough to
fracture the bone, prolonged compression might cause ischemia and
subsequent necrosis of the bone tissue, ultimately resulting in the
excision and loss of one or more of the phalanges or the entire toe. The
anatomical similarities of mutilations among birds missing toes and the
confirmed ant-bite injuries in birds in the process of loosing a toe
suggest that ant bites, rather than laceration from trauma (e.g. road
traffic or building collisions) or predator attacks (e.g. carnivores),
are the most likely cause of the observed toe amputations in adult
nightjars.

No trace of severe damage from ant bites was seen in juveniles despite
their sample size being even larger than that of adults (1,145
juveniles\emph{vs.} 1,104 adults considering all years of the long-term
study). Nevertheless, the total number of injured birds is too small to
support the statement that injuries from ant bites are restricted to
adults. The observation of the full head of a \emph{M. barbarus}
attached to the pecten of the medial toe of a juvenile demonstrates that
they are not entirely free from ant attacks, although ant species
occurring in south Spain are likely too small for their mandibles to
grasp on the relatively thick toes of a nightjar. \emph{M. barbarus} is
very common on the Mediterranean coast (Lebas et al., 2017), but it may
not pose as much a threat to nightjars as army ants because \emph{M.
barbarus} feed mainly on seeds and their mandibles are relatively small
in comparison to those of army ants.

The genus \emph{Dorylus} ranges from sub-Saharan Africa across North
Africa and Asia Minor to Borneo in Southeast Asia. The Afrotropics
harbour the largest number of species (116 so far described), including
subterranean-, leaf litter- and surface-dwelling species. Of these,
surface-dwelling species have, among other characteristics, the greatest
mandible length and mandibular aperture (Schöning et al.,
2005).\emph{Dorylus} species are generalist predators that take any type
of prey, from immature individuals of other insects to vertebrate
carrion (Schöning \& Moffett, 2007). Unlike leaf litter-dwelling and
subterranean species, \emph{Dorylus} ants spend much of their time
foraging on the soil surface, and given their mobile, predatory
lifestyle and aggressive behaviour, attacks on the birds they encounter
on the ground should be common.

Nineteenth-century naturalists assumed that birds eat army ants but, as
evidence accumulated about the foraging patterns of birds, it became
clear that birds peck only at ants attacking their feet or plumage
(Willis \& Oniki, 1978). No mention of army ants is made in the
published studies of diet of Afrotropical nightjars (Jackson, 2000). In
addition, although army ants are major predators of vertebrates (e.g.
birds, sea turtles), most documented cases of predation on birds took
place during the nesting stage (Ikaran et al., 2020; Robinson \&
Robinson, 2001). None of these organisms therefore appear to serve as a
trophic resource for the other, suggesting that agonistic ant-nightjars
encounters are coincidental and detrimental to both parties.

Missing and damaged toes have also been observed in other migratory and
sedentary bird species and, as in the red-necked nightjar, these
injuries were hypothesized to result from ant attacks (e.g. Ibáñez et
al., 2007; Sugg, 1974). Sugg (1974) reported on the specific bite
effects by \emph{Dorylus} sp. on 10 out of 343 (2.9\%) pied kingfishers
(\emph{Ceryle rudis} ) from a breeding colony on the Kenyan shore of
Lake Victoria. Toe amputations and damage from ant bites appear to be
more common in pied kingfishers (3\%) than in red-necked nightjars
(\textless{}1\%) despite the latter spending more time on the ground.
The estimated proportion of toe amputations in another population of
red-necked nightjars monitored for six years in E Spain is 0.4\% (3
cases out of 751 different adults examined in 2017-2022; J.M. Zamora,
pers. comm.). The field protocols in this study, as in the first years
of our long-term study, did not include routine screening for toe or
foot anomalies, and so estimates are based on opportunistic observations
made during ringing sessions. The proportions of toe amputations in both
populations, as estimated from opportunistic observations, are identical
(0.4\%) and half the magnitude of that based on systematic examination
of \emph{both} feet (0.8\%), suggesting that a routine screening for
anomalies is needed to obtain unbiased estimates. Caution is therefore
required in extrapolating our results to other populations, species and
geographic regions. Different estimates could be obtained if we study
nightjars in their wintering area (where ants occur). The frequency of
ant-bite damage in breeding nightjars appears almost negligible.
However, this might be an underestimate if only birds that manage to
survive their injuries migrate successfully from their non-breeding
grounds to our study area.

Most injuries observed on nightjars, as in the pied kingfishers,
consisted of single-toe amputations that should not necessarily impair
the birds' functional capacities and compromise performance in tasks
such as predator avoidance or food acquisition. One of the injured
nightjars actually survived from one breeding season to the next.
Nonetheless, the loss of the medial toe and, therefore, of the pecten
could be detrimental to nightjars because, even though the exact purpose
of the pecten remains unclear, it has been suggested that it might be
used as a comb for removing parasites during preening, or to straighten
out the rictal bristles (Cleere, 2010). Moreover, as evidenced from at
least one of the injured nightjars, multiple ant bites on the same bird
might cause multiple amputations, potentially compromising locomotion on
a chronic basis and, ultimately, long-term survival. Breaks in the skin
from ant bites, even if affecting only one toe, might increase the risk
of fatal complications, such as severe bleeding and sepsis from
opportunistic infections, so it is possible that some individuals die
before migrating and being captured on their breeding grounds. That is,
only the fraction of birds that are able to survive ant attacks and
subsequent damage might be recorded by us. Hence, the true incidence of
ant bites on birds could be much higher than it appears to be.

Further research is required to understand the true incidence and
eco-evolutionary implications of antagonistic ant-bird interactions.
Being accidental, the factors promoting ant-nightjar encounters are
difficult to determine, and consequences different from injuries (e.g. a
change in behaviour, systemic symptoms) are difficult to assess and
require more detailed study. Fortunately, migratory nightjars are
becoming popular model organisms in movement ecology studies across the
globe, including capture programs conducted in tropical areas during the
non-breeding season (e.g. (Cockle et al., 2023; Evens et al., 2023), and
so it is possible that studies similar to ours could be attempted in
other populations, species, and geographic regions. More of these
studies should encompass areas of the distribution range of large,
tropical ants and incorporate routine examination and systematic
recording of injuries into their protocols in order to increase the
chances of encountering clear evidence of the agent of damage and to
obtain reliable estimates of the true frequency of injuries and their
origin (stochastic or not). Moreover, studies that consider aspects of
performance (e.g. mating success, foraging success) and components of
fitness (e.g. survival, reproductive success or lifespan) in injured
birds relative to uninjured ones are encouraged as a critical step to
understand the influence of injuries on natural selection and evolution
(Hendry et al., 2022).

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\textbf{DATA ACCESSIBILITY STATEMENT}

The data used in this study are presented in Table 1.

\textbf{CONFLICT OF INTEREST}

The authors declare that there are no conflicts of interest related to
this study.

\textbf{AUTHOR CONTRIBUTIONS}

Conceptualization: CC, PE. Data curation: CC, JR-G. Methodology: CC,
JMV-C, PS-G, JR-G. Investigation: CC, JMV-C, PS-G, PH-R, JR-G, CM, PE.
Visualization: CC, JMV-C. Funding acquisition: PE, CC. Project
administration: CC, PE. Supervision: CC, PE. Writing -- original draft:
CC. Writing -- review \& editing: CC, JMV-C, PS-G, PH-R, JR-G, CM, PE.

\textbf{ACKNOWLEDGMENTS}

Funding was provided by the European Regional Development Fund (FEDER)
and the specific objective 1.2.3 of the FEDER 2014-2020 operational
program of the Junta de Andalucía, reference no. UPO--1381547 (to PE).
Additional support came from the Ministry of Science and Innovation of
Spain/AEI/10.13039/501100011033 and the European Union
NextGenerationEU/PRTR through the contract no. RYC2021-033977-I (to CC).
We thank Carlos M. Herrera for helpful advice and suggestions. Sergio
Ibarra Mellado assisted us in preparing the final figure.

TABLE 1. Number of captures, number of individuals examined, and
proportion of injured birds recorded during the three-year period of
systematic study and the previous 12 years.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
& Sampling effort (captures) & No. Adults & No. juveniles & No. injured
adults (\%) & No. injured juveniles (\%)\tabularnewline
\midrule
\endhead
\emph{Systematic study} & & & & &\tabularnewline
2023 & 286 & 134 & 73 & 1 (0.75) & 0 (0)\tabularnewline
2022 & 287 & 160 & 65 & 1 (0.63) & 0 (0)\tabularnewline
2021 & 282 & 139 & 81 & 2 (1.44) & 0 (0)\tabularnewline
2021-2023 & 855 & 369 & 219 & 3 (0.81) & 0 (0)\tabularnewline
\emph{Non-systematic study} & & & & &\tabularnewline
2009-2020 & 3,554 & 1,099 & 926 & 4 (0.4) & 0 (0)\tabularnewline
\bottomrule
\end{longtable}

NOTE: Since bird-ant interactions can occur at any time, individuals are
included for as many years as they have been checked. Note that for this
reason the summation of annual numbers of recorded adults does not match
the grand total of different adults recorded in 2021-2023.

Figure legends

\textbf{Figure 1.} Observed ant-induced injuries on Nightjars (a, b, c),
and detail of \emph{Dorylus sp} . mandible (d) collected from one of
them. Note that image c does not correspond to a \emph{Dorylus} ant, but
to a \emph{Messor barbarus} found on a juvenile nightjar.

FIG. 1\selectlanguage{english}
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\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
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