Hares in the long grass: increased aircraft related mortality of the Irish hare (Lepus timidus hibernicus) over a 30‑year period at Ireland’s largest civil airport

Abstract

Collisions between wildlife and aircraft are a serious and growing threat to aviation safety. Understanding the frequency of these collisions, the identity of species involved, and the potential damage that can be inflicted on to aircraft aid mitigation efforts by airfield managers. A record of all animal carcasses recovered from Dublin International Airport, Ireland’s largest civil aviation airport, has been maintained since 1990 where strikes with the endemic Irish hare (Lepus timidus hibernicus), a protected subspecies of mountain hare, are of particular concern despite substantial management efforts from the airfield authority. The first strike event with a hare was recorded in 1997, and strike events have substantially increased since then, with a sharp increase recorded in 2011. Over a 30-year period, a total of 320 strike events with the Irish hare have been recorded at the airfield. To date, no strike event with a hare has resulted in damage to an aircraft. However, carcasses can present as a major attraction to avian scavenger species in addition to posing as a risk of causing foreign object damage in the event of an undetected carcass. Hare strikes are discussed in the context of the rate of civil aircraft movements, possible direct and indirect damage to aircraft, and airfield wildlife hazard management. Here, we demonstrate that not only are strike events increasing by 14% on an annual basis, but that the kinetic energy of such an event has the potential to cause significant damage to an aircraft.

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European Journal of Wildlife Research (2021) 67:80
https://doi.org/10.1007/s10344-021-01517-y
ORIGINAL ARTICLE
Hares inthelong grass: increased aircraft related mortality oftheIrish
hare (Lepus timidus hibernicus) overa30‑year period atIreland’s
largest civil airport
SamanthaBall1,2 · FidelmaButler1· AnthonyCaravaggi3· NeilE.Coughlan1,4· GerryKeogh2·
MichaelJ.A.O’Callaghan5· RickyWhelan6· ThomasC.Kelly1,2
Received: 10 July 2020 / Revised: 8 July 2021 / Accepted: 22 July 2021 / Published online: 10 August 2021
© The Author(s) 2021
Abstract
Collisions between wildlife and aircraft are a serious and growing threat to aviation safety. Understanding the frequency of
these collisions, the identity of species involved, and the potential damage that can be inflicted on to aircraft aid mitigation
efforts by airfield managers. A record of all animal carcasses recovered from Dublin International Airport, Ireland’s largest
civil aviation airport, has been maintained since 1990 where strikes with the endemic Irish hare (Lepus timidus hibernicus),
a protected subspecies of mountain hare, are of particular concern despite substantial management efforts from the airfield
authority. The first strike event with a hare was recorded in 1997, and strike events have substantially increased since then,
with a sharp increase recorded in 2011. Over a 30-year period, a total of 320 strike events with the Irish hare have been
recorded at the airfield. To date, no strike event with a hare has resulted in damage to an aircraft. However, carcasses can
present as a major attraction to avian scavenger species in addition to posing as a risk of causing foreign object damage in
the event of an undetected carcass. Hare strikes are discussed in the context of the rate of civil aircraft movements, possible
direct and indirect damage to aircraft, and airfield wildlife hazard management. Here, we demonstrate that not only are
strike events increasing by 14% on an annual basis, but that the kinetic energy of such an event has the potential to cause
significant damage to an aircraft.
Keywords Airfield management· Air safety· Human-wildlife conflict· Foreign Object Damage (FOD)· Scavenger guild·
Wildlife strike· Wildlife hazard management
Introduction
Air travel is vital to our global economy. In 2018 alone,
over 4.3 billion passengers globally were carried by aircraft
and revenues for the airline industry exceeded US$812 bil-
lion (IATA 2019). There are over 40,000 airports globally
according to the International Civil Aviation Organisation
(ICAO), many with expanses of semi-natural grassland often
providing unique and productive habitat (Hauptfleisch and
Avenant 2015) that is attractive to a range of wildlife taxa.
The presence of wildlife on airfields can be hazardous to
aviation, resulting in collisions between wildlife and aircraft,
henceforth referred to as ‘strikes’ e.g. (McKee etal. 2016). A
sustained increase in the rate of collisions between wildlife
and aircraft is considered a serious and growing threat to
aviation safety, globally (e.g. Metz etal. 2020; Ball etal.
2021).
This article is part of the Topical Collection on Road Ecology
Guest Editor: Marcello D’Amico
* Samantha Ball
Samantha.ball@ucc.ie
1 School ofBiological, Earth andEnvironmental Science,
Distillery Fields, University College Cork, CorkT23TK30,
Ireland
2 Dublin Airport Authority, Airport Fire & Rescue Service,
DublinK67CX65, Ireland
3 School ofApplied Sciences, University ofSouth Wales, 9
Graig Fach, Glyntaff,PontypriddCF374BB, UK
4 School ofBiological Sciences, Queen’s University Belfast,
19 Chlorine Gardens, BelfastBT95DL, UK
5 Department ofApplied Mathematics, School
ofMathematical Sciences, University College Cork,
CorkT12XF62, Ireland
6 BirdWatch Ireland, Bullford Business Campus,
KilcooleA63RW83, Greystones,Wicklow, Ireland
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European Journal of Wildlife Research (2021) 67:80
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Whilst not all wildlife strikes result in damage, some
wildlife strikes have led to human fatalities and substantial
economic losses for the aviation industry (see https:// avisu r e.
com/ incid ent- datab ase/). For example, bird strikes are esti-
mated to cost upwards of US$1.2 billion to the global civil
aviation industry, annually (Allan 2002). However, whilst the
majority of wildlife strikes involve bird species (e.g. 94% of
strikes in the USA;Dolbeer and Begier 2021), many volant
(bats) and terrestrial mammal species are also struck (e.g.
Crain etal. 2015; Kelly etal. 2017; Ball etal. 2021), as well
as reptiles, amphibians and even insects (e.g. Noaves etal.
2016; House etal. 2020). Strikes involving mammals have
been estimated to cost over US$103 million in the USA alone
(Ball etal. 2021) with some mammal groups (e.g. deer, car-
nivores) being particularly responsible for costly damages
(Biondi etal. 2011; Crain etal. 2015). Additionally, smaller
mammals can attract scavenger and predatory species,
introducing secondary strike risk (Pennell etal. 2016) and
additional ecological complexity to management decisions
(Washburn and Seamans 2013; Hauptfleisch and Avenant
2015). Hence, here we present a case study of mammal
strikes, focusing on the Irish hare (Lepus timidus hibernicus,
Bell 1837) at Ireland’s largest civil airport—Dublin Interna-
tional Airport.
The endemic Irish hare (Lepus timidus hibernicus), a
subspecies of the mountain hare (Lepus timidus, Linnaeus
1758), resides in and around the grasslands at the airfield
at Dublin. This subspecies is considered to be a priority
species for conservation action in Ireland (Reid etal. 2010;
Caravaggi etal. 2017). However, since 1997, strike events
with the Irish hare have been recorded annually. Indeed,
despite foxes (Vulpes vulpes), hedgehogs (Erinaceus euro-
paeus), rabbits (Oryctolagus cuniculus) and rats (Rattus
norvegicus) also being recorded at the airfield, there has
only been the occasional recorded incident with these spe-
cies (Bolger and Kelly 2008), as well as the occasional bat
strike (Kelly etal. 2017). Wildlife hazard management
actions for mitigation of hare strikes at Dublin Airport
include deployment of scaring techniques (e.g. pistol),
licensed trapping and translocation, and, subject to safety
considerations, a licenced cull. Notably, since 2014, more
than 650 hares have been removed from Dublin under
licence, whilst a minimum of an additional 191 were killed
by aircraft. Given the conservation status of this subspe-
cies, the Dublin Airport Authority is actively exploring
new, non-lethal technological solutions for hare population
management. Hare population densities are substantially
higher within the airfield environment for both Dublin and
Belfast International airports, at up to 30 hares/km2,than
elsewhere in Ireland (up to 3.19 hares/km2; McGowan
etal. 2019). Hence, the strike events are highly unlikely
to have population-level impacts and are of little concern
with regards to the conservation of the species.
The airfield at Dublin Airport contains approximately
680ha of grassland. Typical of many airfields located
within the temperate zone of the Northern Hemisphere,
these grasslands have been maintained on the principles of a
‘long grass’ management policy since the 1980s (Bolger and
Kelly 2008; UKCAA Safety Regulation Group CAP 2008)
and consist of a blend of tall fescue (Festuca arundinacea)
and Italian ryegrass (Lolium multiflorum). The presence of
long grass at the airfield successfully deters avian species.
As hares are present both airside and landside at Dublin, it
is unknown if the presence of long grass impacts on the hare
population or indeed acts as an attractant. However, airfields
elsewhere in Europe have consequently reported long grass
(< 20cm) as a wildlife attractant for mammal species, such
as hares, by providing suitable habitat and shelter (Ball etal.
Unpublished data), demonstrating the complexity of wildlife
management in sensitive airfield environments.
Here, we report on the number of hare strikes recorded
at Dublin Airport over the 30-year period from 1990–2019.
Hare strikes are of great concern to air safety at Dublin Air-
port, not only in their own right, but also due to the second-
ary risks associated with a strike event. These include the
luring of scavenger species to the site of a strike and the
ingestion of an undetected carcass into an engine as for-
eign object damage (FOD). Specifically, airfield operators
have a legal responsibility to reduce wildlife hazard at air-
fields (Mendonca etal. 2017) and are liable for any damages
inflicted. For example, in 2005, Air France was awarded €4
million after a bird strike incident induced by gulls scaveng-
ing from an undetected hedgehog carcass resulted in engine
damage (Dale 2009). Whilst, in the present study, we report
only on instances from one airfield, it is important to note
that strikes with the Lagomorpha are not unique to Dublin
or indeed to Ireland. Other European countries frequently
report strike events with species of this taxon (e.g. Kitowski
2016), as do Australia (ATSB 2019) and North America
(Canada and the USA, e.g. Dolbeer and Begier 2021), with
strikes also reported in countries in Africa and Central
America (Ball etal. 2021).
Materials andmethods
Carcass collection
Wildlife strike events and recovered carcasses from Dub-
lin Airport (Airport Reference Point (ARP): 532517N
0061612W) have been recorded since 1990. Carcasses
were collected from runways following reported strikes or
during mandatory routine inspections (i.e. the ‘recorded
check’), to prevent against foreign object damage (FOD) to
aircraft by debris. Any debris, including that composed of
organic material, is removed from the manoeuvring areas
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European Journal of Wildlife Research (2021) 67:80
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and the conditions surrounding the strike incident recorded
(e.g. date, location). Some strike incidents involved multiple
individuals but were recorded as a single strike event. Car-
casses resulting from collisions with service vehicles were
appropriately labelled and subsequently excluded from anal-
ysis. Post-collection, all carcasses were immediately placed
in cold storage (−20°C) until examination at a later date.
Morphological indicators were used to identify specimens to
species level via consultation of Harris and Yalden (2008).
A total of 54 carcasses had been retained by the airport and,
where possible, specimens were aged, sexed and weighed
(e.g. Ball etal. 2020).
Estimation oftherate ofhare strikes andpotential
impact damage
Incident rates of the Irish hare were calculated as the annual
number of strikes per 10,000 aircraft movements (one
movement = one landing or one take-off) between 1990 and
2019. Bespoke annual aircraft movement data were obtained
from the Irish Aviation Authority (IAA) and Dublin Airport
Authority (DAA) records. There have been no damaging
strike events with the Irish hare to an aircraft’s fuselage,
engines, undercarriage or otherwise. The Boeing 737–800
and the Airbus A320 have the greatest number of move-
ments at Dublin Airport (~ 60% of all aircraft movements;
Dublin Airport Authority, Unpublished Data). The potential
impact damage for these two aircraft was calculated in terms
of kinetic energy (KE; joules) and converted to the impact
descriptor of foot–pounds (ft–lbs), where body of mass (m),
in kilograms, moving with speed (v), in meters per second,
is expressed as KE = ½ mv2 (see Kelly etal. 2016, 2017).
We assume maximum take-off or landing speeds for aircraft
(A320 at 268.6 kph; B737 at 260.1 kph, from Kelly etal.
2017) and take the maximum mass for an adult Irish hare
from previous strike events (Online Resource 1), at 3.8kg
(Table1), as an estimate of the maximum potential damage
that a hare strike could possibly inflict on an aircraft during
a typical manoeuvre at Dublin Airport. The kinetic energy of
a strike event with a hare of low (juvenile, 1.3kg), medium
(2.2kg) and high (adult female, 3.8kg) mass for speeds
ranging from 1m/s to the take-off speed of the faster aircraft
(A320) at 75m/s was calculated. Respective masses were
used from hares involved in strike events at the airfield, from
intact carcasses (n = 14, Online Resource 1). The mean of
these weights was used for the medium weight. Under the
same conditions, we also calculate the kinetic energy of a
strike event with two species which scavenge and predate on
hares and are also present at the airfield, namely, the red fox
and the common buzzard (Buteo buteo). Equally, in obtain-
ing these estimates, we have ignored the secondary effects of
the speed of the fauna as they are negligible when compared
to that of an aircraft (Metz etal. 2020).
Statistical analysis
All statistical analysis was carried out in programme R v
4.0.4. As data spanned over a 30-year period, we divided
study periods into three, 10-year intervals (1990–1999,
2000–2009 and 2010–2019). To determine if there was
a significant difference in the median number of strikes
between study intervals, we used the Kruskal–Wallis test
for non-parametric data and used the ‘Dunn’s test’ for
further post hoc analysis between intervals with a ‘Ben-
jamini–Hochberg’ p-value correction (Benjamini and
Hochberg 1995) to allow for multiple comparisons. To
evaluate the trend (% increase) in the number of strike
events over time (years) with hares at Dublin Airport,
we used general linear modelling (GLM), with a ‘quasi-
Poisson’ error structure, implemented within the ‘lme4’
package (Bates etal. 2015). To investigate the relation-
ship between the number of strike incidents and aircraft
movements over time, we used a generalised linear mixed
model, with year fitted as a random effect to account
for temporal variation, with a ‘Poisson’ error structure.
Temporal strike data (season, month, time) were avail-
able for 2012–2019. To investigate when strikes occurred
throughout the year, we used a second generalised lin-
ear mixed model, using both season and month as the
response variables and year fitted as a random effect,
and another for time of day. For time of day, light con-
ditions were summarised as ‘dawn’, ‘day’, ‘dusk’ and
‘night’ whereby a strike which occurred within 60min-
utes either side of sunrise or sunset was categorised as
either dawn or dusk respectively. Strikes which occurred
more than 60minutes after sunrise were classified as
day and strikes which occurred more than 60minutes
after sunset were classified as night. Model assumptions
were checked using the ‘DHARMa’ package (Hartig
Table 1 Kinetic Energies of Irish hare (Lepus timidus hibernicus),
red fox (Vulpes vulpes) and common buzzard (Buteo buteo) collisions
with aircraft. Kinetic energies are calculated using the maximum
mass of an adult Irish hare and the maximum speeds observed during
take-off or landing of aircraft
1 Harris and Yalden (2008)
2 Demongin (2016)
Species Aircraft Maximum
mass
(kg)
Max speed
m s−1 (kph)
Kinetic energy
Joules (ft–lbs)
Irish hare A320 3.8 74.61 (268.6) 10,576 (7,800)
B737-800 3.8 72.02 (260.1) 9,855 (7,269)
Red Fox A320 9.3174.61 (268.6) 25,885 (19,092)
B737-800 9.3 72.02 (260.1) 24,119 (17,789)
Buzzard A320 1.4274.61 (268.6) 3,897 (2,874)
B737-800 1.4 72.02 (260.1) 3,631 (2,677)
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European Journal of Wildlife Research (2021) 67:80
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2019) according to Nakagawa & Schielzeth (2013). The
strength of association between strikes and year of inci-
dence was tested with a Spearman’s Rho (ρ).
Results
Strike numbers
A total of 320 strike events with hares occurred at Dub-
lin Airport between 1990 and 2019. From carcasses where
sex could be identified (n = 48% of carcasses), 62% were
male, and 73% of hares struck were adults. Average car-
cass weight was 2172g ± 664 (mean ± SD; n = 14; Online
Resource 1). Whilst some hare carcasses displayed signs of
scavenger interference, no carcass was considered the result
of a predator kill (from n = 54 carcasses). Since 1997, there
has been at least one hare strike per year, with a maximum
of 44 strike events (n = 46 hares) recorded in 2018. There
were 10.66 ± 2.35 (mean ± SE) hare strikes per year across
all years. Increased numbers of strikes between 2010 and
2019 resulted in a higher mean value of 26.5 ± 3.16 strikes
per year (Fig.1). A significant difference between decennial
sampling periods was identified (χ2(2, N = 30) = 23.8, p < 0.01).
Post hoc analyses showed a significant difference between
the median strike number for all three sampling periods,
with a median of zero strikes for 1990–1999, five strikes for
2000–2009 and 25.5 strikes for 2009–2019 (Fig.1). Both
dawn (GLMM y = 1.46, χ2 (1, 3), p < 0.01) and night (GLMM
y = 1.18, χ2 (1, 3), p < 0.01) light conditions had significantly
higher strikes than other times of day. Since the first strike
in 1997, there has been a 14% (CI 9–16%) annual increase in
strike events with the Irish hare (GLM y = 0.128, χ2 (23, 328),
p < 0.05). Lastly, strike events were recorded year-round;
whilst the highest proportion of strikes occurred in May
(12%), strike frequency fluctuated across the year without
any discernible pattern for month or season (Fig.2).
Number ofhare strikes inrelation toaircraft
movements
There were over 5.1 million aircraft movements recorded at
Dublin Airport during the study period. Overall, the number
of strike events has increased year-on-year at Dublin and
the number of aircraft movements has generally increased
(Fig.3). The mean number of hare strikes per 10,000 aircraft
movements over the 30-year study period was 0.55 ± 0.12,
with a peak in 2018 of 1.89 strikes per 10,000 movements
(Fig.3). In 2011, the number of strike events with hares
surpassed one strike per 10,000 aircraft movements and
has been sustained above this level for the remainder of the
study period, except for 2019, where 0.54 strikes per 10,000
Fig. 1 Left: Irish hare (Lepus timidus hibernicus) strikes at Dublin
Airport for each of the three, 10-year study periods. Right: Number
of aircraft movements recorded at Dublin Airport for the same three
time periods. The median number of events (strikes and aircraft
movements) are displayed for each decade. Dots show the spread of
the number of events recorded for each year within the decade
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aircraft movements were recorded. This dip corresponded
in a reduction in available grassland habitat by approxi-
mately 80ha due to the commencement of construction
works for the installation of an additional runway. There
was a significant positive association between the number of
aircraft movements and the number of recorded hare strike
events, across all years (GLMM y = 1.38, χ2 (1, 30), p < 0.01,
ρ = 0.66). However, the number of aircraft movements only
explained 57% of the model variance, indicating that there
was a strong temporal influence (year) on the number of
recorded strike events, as evidenced in Fig.3C.
Potential ofimpact damage
The potential impact damage caused by a hare strike, to
the two civil aircraft types with the greatest number of
movements at airfields in the Republic of Ireland is dem-
onstrated in Table1. The corresponding kinetic energy of
7800 ft–lbs for the Airbus A320 and 7269 ft–lbs for the
Boeing 737 would likely cause significant metal distortion
and/or mechanical damage in the event of an ingestion into
the moving parts of an aircraft jet engine (European Avia-
tion Safety Agency 2010; Metz etal. 2021). Whilst this is
Fig. 2 A Percentage of strike events with the Irish hare (Lepus timi-
dus hibernicus) recorded at Dublin Airport for each season between
2012 and 2019. B Cumulative percentage of all strike events with the
Irish hare recorded for each month at Dublin Airport, between 2012
and 2019 (n = 224). Spring = March–May; Summer = June–August;
Autumn = September–November; Winter = December–February
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unlikely, it remains a cause of concern at the airfield, with
an isolated incident resulting in the turning around of an
aircraft over concerns of an ingestion event. The kinetic
energy values obtained for both aircraft types also have the
potential to cause significant damage to the landing gear
(~ 10,000J) of aircraft that strike hares. Damaged land-
ing gear could not only require airside emergency services
to remain on standby at the aircrafts destination but could
potentially result in runway excursions—something which
could be particularly dangerous at high speeds. Addition-
ally, given the mass of a hare (≤ 3.8kg), the hitting of
FOD on the lower wings or underbelly of the aircraft could
cause substantial damage (European Union Aviation Safety
Agency Certification 2020), particularly in an event involv-
ing a large hare (Fig.4). Lastly, the kinetic energy values
of both the red fox (A320: 26,442J; B737: 24,638J) and
Fig. 3 A Number of recorded strike events with Irish hares (Lepus
timidus hibernicus) at Dublin Airport from 1990 to 2019. No strikes
were recorded between 1990 and 1996. B Number of aircraft move-
ments (defined as take-off and landing manoeuvres) recorded at Dub-
lin Airport between 1990 and 2019. C Number of strike events with
Irish hares per 10,000 aircraft movements (take-off and landings) at
Dublin Airport
Fig. 4 Kinetic energies (Joules
and ft–lbs) of collisions
between Irish hares (Lepus timi-
dus hibernicus) of low (1.3kg),
medium (2.2kg) and high
(3.8kg) mass with aircraft rang-
ing from 1 to 75m/s. Weights
were obtained from carcasses
previously involved in strike
events at Dublin Airport, with
the lowest weight recorded from
a juvenile and the highest from
an adult female
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the common buzzard (A320: 3,897J, B737: 2,874J) are
sufficient to cause significant damage to an aircraft (Dennis
etal. 2009).
Discussion
The number of aircraft movements per annum at Dublin
Airport has increased from 116,000 movements in 1990,
to almost 249,000 in 2019. The rate of increase has not
been constant, with several peaks and troughs over time
coinciding with socio-economic and global events, such as
the 2008 economic crash, which caused air travel demand
to plummet (Franke and John 2011). Further, whilst air
traffic is at least partially responsible for strike events,
some of the variance observed in annual strike rates could
be attributed to fluctuations in the size of the hare popula-
tion in and around the airfield. We suggest, therefore, that
management plans must consider the population ecology
of local species involved in strike events if they are to be
effective.
Strike events with hares at Dublin Airport have the
potential to cause significant damage to an aircraft’s engine
if ingested or cause severe damage to the landing gear.
Equally, the mass of an undetected hare carcass from an
earlier strike has the potential to cause significant FOD
to an aircraft. Whilst there have been no damaging strike
events to date, strike incidents have resulted in the turning
around of aircraft. This is particularly concerning given the
14% annual increase in strike events and the simultaneous
increase in the number of strikes per 10,000 aircraft move-
ments from 0.07 in 1997, to a peak of 1.89 in 2018. The
noticeable dip in strikes per 10,000 aircraft movements
in 2019 is likely to be attributed to the commencement of
intensive airside construction works. The overall average
strike rate of 0.55 ± 0.12 per 10,000 aircraft movements
over the 30-year period is relatively low, compared to an
average strike rate of 2.89 ± 0.14 for birds (Kelly etal.
unpublisheddata). Indeed, Irish hare strikes make up an
average of 30% ± 2.71 (2010–2018) of all wildlife strike
events at the airfield and are the only mammal species to
regularly be involved in strike events at Dublin Airport. Fur-
thermore, the rejection of take-offs and/or landing windows
by pilots due to the presence of hares on a runway can incur
additional financial costs and flight delays, as can temporary
closure of a runway for decontamination (i.e. removal of the
debris field). For example, in the USA, terrestrial mammal
strikes are estimated to have caused upwards of 330,500h of
aircraft downtime (Dolbeer and Begier 2021). Strike events
with mammalian species are increasing, with terrestrial
mammals, including lagomorphs, having caused substan-
tial damage to aircraft, globally (Dolbeer etal. 2000; Biondi
etal. 2011; Crain etal. 2015). The increased hare strike
rate per 10,000 movements observed in the present study
suggests that strikes will become increasingly common if
effective mitigation measures are not implemented, thereby
increasing the likelihood of a damage-inflicting event.
Not only are mammals a primary strike threat in them-
selves, but their presence can introduce secondary strike risk
(Pennell etal. 2016) with predatory and scavenger species
(Hauptfleisch and Avenant 2015). This is of particular con-
cern at Dublin Airport, where experience has shown that
there is a scavenger guild of avian and mammalian taxa pre-
sent at the airfield. This includes the common buzzard, her-
ring gull (Larus argentatus), lesser black-backed gull (Larus
fuscus), greater black-backed gull (Larus marinus), the rook
(Corvus frugilegus), hooded crow (Corvus cornix) and the
raven (Corvus corax) all of which are > 500g in weight, in
addition to the red fox. These species—widespread through-
out Ireland (Cummins etal. 2019; Lewis etal. 2019)—are
frequently encountered at the airfield (Kelly, unpublished
data; Online Resource 2), and many have caused significant
damage to aircraft over the 1990–2019 interval. These scav-
engers may have been lured to the airfield by the presence
of a hare carcass, or the debris of edible tissues, resulting
from an earlier collision. Indeed, the debris from a single
strike event has been recorded to cover an area in access of
20 m2 (Bolger and Kelly 2008). Here, we have demonstrated
that a secondary strike event with a scavenger could inflict
significant damage to an aircraft.
Generally, strike frequency with wildlife is considered
to be influenced by local occurrence and abundance of spe-
cies (Schwarz etal. 2014). However, strike events have been
recorded to have occurred for every month of the year at
Dublin Airport, and although May had the highest recorded
monthly proportion of strikes, events were relatively con-
sistent across months with no distinct seasonal patterns.
Further, hares, including the Irish hare, are known to dis-
play inter-annual population fluctuations, which can vary
substantially within short periods of time (Reynolds etal.
2006; Reid etal. 2007; McGowan etal. 2019). This means
that strike events at Dublin cannot be affiliated with a sin-
gle life history stage or event, adding additional complexity
to mitigation measures, as all behaviours and life stages of
the species need to be incorporated into wildlife manage-
ment. For example, mammal strikes have been affiliated with
seasonal increases in food availability (e.g. bats; Parsons
etal. 2009) and the breeding cycle (e.g. canids; Crain etal.
2015). Accordingly, we suggest that management plans must
consider the population ecology of local species involved
in strike events if they are to be effective. Indeed, as some
particularly fecund taxa, such as leporids (Caravaggi 2018),
can require near-continuous management inputs at airfields.
In order to develop targeted and effective strike mitigation,
the ecology and behaviours of mammal species using the
airfield environment need to be understood and incorporated
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European Journal of Wildlife Research (2021) 67:80
1 3
80 Page 8 of 9
into wildlife hazard management plans. Whilst the transloca-
tion of individuals of conservation concern (e.g., Irish hare)
to suitable, low-density areas could help to control airside
population size whilst aiding national conservation efforts,
removal programs on their own may not always be an effec-
tive long-term solution due to rapid recolinisation by a spe-
cies (e.g. Palmas etal. 2020). Therefore, the presence and
activity patterns of animals at airfields can be determined
through the use of modern, remote monitoring equipment
(e.g., camera traps and GPS collars) allowing for the collec-
tion of high-quality data in sensitive, airport environments
(e.g. Scheideman etal. 2017). Additionally, the maintenance
of comprehensive strike data allows for the identification of
increased risk both spatially and temporally, which can allow
for increased staffing and runway patrols to reduce strikes
(Crain etal. 2015). Lastly, a future research priority should
be the development of a suitable risk index (e.g. Soldatini
etal. 2011) for mammal species at airfields. Combined, such
techniques can allow for the implementation of targeted strike
mitigation at the individual airport level.
Conclusion
Strike events with mammal taxa have been recorded to be
increasing in several countries globally. Here, we demon-
strate that the number of annual strike events involving Irish
hares at Ireland’s busiest international airport has increased
since 1990, with a ten-fold increase between 1998 and 2018.
Despite this increase, strikes are of little concern regarding
the conservation status of the species. Whilst the number of
strike events declined in 2019 possibly due to commence-
ment of construction for an additional runway, it is likely that
the strike rate will increase when disturbance has ceased. Not
only are hare strike events disruptive and costly to ongoing
operations, but an undetected carcass can pose a significant
threat to an aircraft as debris or by attracting scavengers.
Strike events with lagomorphs are recorded globally; hence,
the threat that leporids pose to aircraft safety is not unique
to Ireland. Dublin Airport provides a model system to study
strike patterns, associated costs and implications, and miti-
gation measures with leporids. Robust, ecological survey
methods utilising modern technology (e.g., camera traps,
GPS tracking) need to be implemented at Dublin Airport to
better understand hare ecology at the airfield and to inform
the development of targeted strike mitigation measures.
Supplementary information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s10344- 021- 01517-y.
Acknowledgements We gratefully acknowledge the contribution of the
airfield staff and airport authorities at Dublin Airport for compiling the
database upon which this paper is based. This work was conducted as
part of a PhD studentship funded by the Irish Research Council (IRC)
and the Dublin Airport Authority in collaboration with University Col-
lege Cork (project EBPPG/2018/43).
Author contribution The lead author (SB) led writing and data analysis
for the article. Data were collected and collated for 28years by TCK
and for 2 by RW. All co-authors contributed to editing and approving
the manuscript and are listed alphabetically.
Funding Open Access funding provided by the IReL Consortium. This
research is funded by the Irish Research Council (IRC) and the Dublin
Airport Authority (project: EBPPG/2018/43).
Declarations
Ethics approval This research has received ethical approval from the
University College Cork Animal Experimentation Ethics Committee
(AEEC) (#2019–003).
Consent for publication All parties involved consent to the publication
of this work.
Conflict of interest This work was conducted as part of a collabora-
tive PhD studentship. The lead author (SB) is employed by the Dublin
Airport Authority for the purpose of conducting airside research, for
the duration of the studentship.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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