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\begin{document}

\title{Lockdown effects on fear: direct and indirect effects mediated by
release of urban predators}



\author[1]{Mario Díaz}%
\author[2]{Anders Moller}%
\affil[1]{CSIC}%
\affil[2]{CNRS}%


\vspace{-1em}



  
  \date{May 18, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
The Covid-19 lockdown reduced drastically human presence outdoors,
providing an uncontrolled experiment for disentangling direct and
indirect effects of human presence on animal fearfulness. We measured
18,494 flight initiation distances (FIDs, the distance at which
individual animals fly away when approached by a human) from 1,333
populations of 202 bird species taken in four European cities both
before, during and after the lockdown. Differential responses to
lockdown among urban and rural habitats and between signing and
non-singing birds showed that lockdown relaxed direct disturbance
effects on birds in rural habitats, but increased indirect fear effects
mediated by predator release (mainly feral cats) in cities.%
\end{abstract}%



\sloppy


\textbf{Lockdown effects on fear: direct and indirect effects mediated
by release of urban predators}

Mario D\selectlanguage{ngerman}íaz\textsuperscript{1}, Anders Pape Møller\textsuperscript{2}

\textsuperscript{1}Department of Biogeography and Global Change
(BGC--MNCN), Museo Nacional de Ciencias Naturales, CSIC, C/Serrano 115
bis, E-28006 Madrid, Spain. ORCID: 0000-0002-6384-6674.
Email:\emph{Mario.Diaz@ccma.csic.es}

\textsuperscript{2}Laboratoire d'Ecologie, Systématique et Evolution,
CNRS UMR 8079, Université Paris-Sud, Bâtiment 362, F-91405 Orsay Cedex,
France. ORCID: 0000-0003-3739-4675.
Email:\emph{anders.moller@universite-paris-saclay.fr}

Running headline: Lockdown effects on bird fear

Keywords: birds, Covid-19 lockdown, Flight Initiation Distance,
horizontal distance, perch height, post height, rural habitat, urban
habitat

Type of article: Letter

Word count: abstract (104), main text (2155), 33 references, 2 figures,
2 tables, 1 appendix; 3 Supplementary tables, 1 Supplementary Figure

Corresponding author: Mario Díaz, Department of Biogeography and Global
Change (BGC--MNCN), Museo Nacional de Ciencias Naturales, CSIC,
C/Serrano 115 bis, E-28006 Madrid, Spain.
Email:\emph{Mario.Diaz@ccma.csic.es}Tel: +34 91 745 25 00 ext. 988110,
Fax +34 91 564 08 00

\textbf{Author contributions:}Investigation, Data analyses, Writing --
original draft: MD Conceptualization, Methodology, Writing -- review \&
editing: MD, APM.

\textbf{D} \textbf{ata and materials availability:} All data are
available in the main text or the supplementary materials. Should the
paper be accepted for publication, the data and code must be made
available in a public data repository (DRYAD, Zenodo, Figshare).

\textbf{Abstract:} The Covid-19 lockdown reduced drastically human
presence outdoors, providing an uncontrolled experiment for
disentangling direct and indirect effects of human presence on animal
fearfulness. We measured 18,494 flight initiation distances (FIDs, the
distance at which individual animals fly away when approached by a
human) from 1,333 populations of 202 bird species taken in four European
cities both before, during and after the lockdown. Differential
responses to lockdown among urban and rural habitats and between signing
and non-singing birds showed that lockdown relaxed direct disturbance
effects on birds in rural habitats, but increased indirect fear effects
mediated by predator release (mainly feral cats) in cities.

\textbf{Introduction}

Animal fitness depends on a continuous balance between foraging,
reproductive activities, and the associated risk of being depredated
(Stephens \& Krebs 1986). Human presence and activity produce direct and
indirect effects on both sides of this balance. Wild predators are less
abundant in human impacted habitats due to fear effects or to direct
persecution (Díaz et al. 2013; Torre et al. 2022), whereas access to
food and mates is usually higher for the individuals and species that
best tolerate human presence (Ydenberg \& Dill 1986). Heat island effect
resulting in elevated temperatures in urban habitats, light pollution,
or both, have additional positive effects on reproduction by extending
the duration of reproductive periods in cities (Møller et al. 2015).

Behavior is a prime short-term response to changes in environmental
conditions, which translate ecological effects into resource
acquisition, interactions with hetero- and conspecifics, and use of
resources for survival and reproduction (Huey et al. 2003). The
so-called flight initiation distance (FID), the distance at which an
animal flees when approached by a human observer under standardized
conditions (Blumstein \emph{2006)} , is a robust estimate of predator
avoidance behavior, that involves a trade-off between avoidance of
danger and acquisition of basic needs (Cooper \& Blumstein 2015). FID
increases under favorable climatic conditions that decrease caloric
needs (Díaz et al. 2021) and with increasing abundance of natural and
feral predators (Hediger 1934; St. Clair et al. 2010). Responses to
human presence are however variable. FIDs are longer in urban than in
nearby rural habitats, which implies negative relationships between
human density and FID (Sol et al. 2013; Samia et al. 2015), but both
positive and negative relationships within cities have been documented
(Mikula 2014; Morelli et al. 2018; Carlen et al. 2021). Shorter FID in
urban habitats is attributed to either habituation to human presence or
to natural selection for fearless individuals under conditions of low
predation risk (Samia et al. 2015; van Dongen et al. 2015; Johnson \&
Munshi-South 2017; Carrete et al. 2018), emphasizing the importance of
direct effects of human disturbance on FIDs. However, indirect effects
mediated by lower predation pressure in cities is also involved (Díaz et
al. 2013; Samia et al. 2015). The 2020 lockdown established to control
the Covid-19 pandemic was an excellent uncontrolled experiment for
disentangling these direct and indirect effects (Rutz et al. 2020).

Flying animals have the opportunity to take advantage of `escape in the
third dimension' by adjusting fleeing behavior and post or perch height
according to predation risk and display needs (Møller 2010). FIDs have
in fact two components: the horizontal distance at which animals flee,
and the height above ground at which they were located when flushed
(Møller 2010). Different activities may modulate both predation risk and
height selection. Singing birds tend to select higher posts to amplify
sound transmission, exposing them more to aerial predators, which are
more abundant in rural habitats, but less to ground predators, more
abundant in cities (Møller 2011). Escape from the ground should be
costlier than escape from higher perches (Hedenström \& Alerstam 1992).
Comparative analyses provided evidence for natural selection on song
post height and adjustments in the vertical and horizontal components of
escape behavior in relation to variation in predation risk associated
with human presence. Birds having higher song posts and fleeing higher
in the vegetation tend to have higher survival rates and lower
senescence rates, a trend that is facilitated in urban habitats due to
human effects on both aerial and ground-dwelling predators (Møller
2010).

Differential responses to lockdown are thus expected according to
habitat, height above ground and singing behavior if indirect effects of
humans on predators mediate effects of human presence on FIDs. These
differences are not expected in a scenario of direct effects of human
disturbance. Specifically, we expected a) shorter FIDs during lockdown
than before and after, if responses to human presence were due to
disturbance, and longer FIDs if due to effects of indirect predator
release effects (Díaz et al. 2013; Samia et al. 2015); b) lack of
changes in height above ground in the former case, and increased height
in the latter (Møller 2011; Díaz et al. 2022); c) differential lockdown
effects in urban as compared to nearby rural habitats, with stronger
changes in the latter (Díaz et al. 2021); d) differential changes for
singing compared to non-singing birds, with weaker changes in the latter
(Møller 2011); e) differential adjustments of vertical and horizontal
components of FID, by increasing the latter according to the former
(i.e. longer horizontal FIDs when individuals perch higher), according
to lockdown, habitat and singing behavior (Møller 2010; Díaz et al.
2021); and f) delayed effects of lockdown on FIDs to the year after
lockdown, as compared with pre-lockdown conditions, if fear responses to
lockdown influenced individual fitness (Johnson \& Munshi-South 2017).

\textbf{Materials and Methods}

Fieldwork was carried out in and around the cities of Brønderslev
(Denmark), Paris (France), Madrid and Toledo (Spain; Supplementary file
S1). We estimated flight initiation distance (FID, the distance at which
individual birds flee when approached by a human under standardized
conditions; Blumstein 2006) in paired urban and nearby rural sites
located 1-20 km away (Díaz et al. 2013). Briefly, trained observers (APM
in Denmark and France and MD in Spain) approached individual birds
previously detected at least 20 m away by eye or with binoculars during
random walks across study areas during the breeding seasons of 2018,
2020 and 2021. We recorded species, age, sex when possible, and whether
the birds were singing or not when detected. We avoided sampling young
birds or individuals engaged in nest building or offspring provisioning.
Only one individual per species and sex was recorded at each specific
site to avoid sampling the same individual more than once. We estimated
horizontal distance at fleeing by pacing or by means of a Nikon Forestry
550 hypsometer. We estimated height above the ground when the bird was
detected by eye or measured it with the hypsometer to the nearest 1 m.
FIDs were calculated as the square root of the sum of the squared
horizontal distance and the squared height above ground (Samia et al.
2015). Permission by the local authorities during the 2020 lockdown
allowed sampling even at restricted sites (i.e. urban parks). We were
thus able to sample during (2020) and after (2021) the Covid-19 lockdown
in the same areas that we sampled before lockdown (2018) in previous
studies (Díaz et al. 2013; 2021; 2022).

Effects of lockdown, habitat, singing behavior and their interactions on
FIDs and their horizontal and vertical (perch or song post height)
components were tested using generalized least square regression (PGLS)
models on log10-transformed means for all individuals of the same
species sampled in each habitat of each city (Díaz et al. 2013). We
estimated the phylogenetic scaling parameter lambda (\selectlanguage{greek}λ\selectlanguage{english}), that varies
between 0 (phylogenetic independence) and 1 (complete phylogenetic trait
conservatism or dependence (Freckleton et al. 2002), to calculate the
phylogenetically corrected model estimates. We obtained a consensus
phylogeny using Mesquite software (Maddison \& Maddison 2011) on 100
trees extracted from the phylogeny published by Jetz et al. (2012;
birdtree.org). Site effects were controlled for by including the
different populations of the same species as polytomies with a constant
small genetic distance of 0.0001 (D\selectlanguage{ngerman}íaz et al. 2013) (Supplementary file
S2). The inverse of sample sizes was used to correct for sampling effort
(Garamszegi \& Møller 2010). We used the R libraries ape, MASS and
mvtnorm and the function pglm3.3.r (Supplementary file S3). We tested
for adjustments between vertical and horizontal components of FID
following the same modelling approaches, but using the log-transformed
horizontal distance as the dependent variable, log-transformed height
above ground as a covariate, and lockdown, habitat, singing behavior and
their interactions with height as factors (Díaz et al. 2022). Effect
sizes were computed as Pearson's product--moment correlation
coefficients from t values (Lipsey \& Wilson 2001) and were judged small
(r \textless{} 0.10), intermediate (r = 0.11--0.49) or large (r
\textgreater{} 0.50) following (Cohen 1988).

\textbf{Results}

We obtained 18,494 FID records from 1,333 populations of 202 bird
species both before the Covid-19 lockdown (2018, N=3261 records, 319
populations), during lockdown (2020, N=3688 and 345) and after (2021,
N=11,545 and 669; Appendix). Sample sizes were unbalanced because we
used all FIDs taken from other studies on bird fear that we regularly
conducted from 2006 onwards in the study sites (Díaz et al. 2013; 2021;
2022). Phylogenetic, site and sample size effects were significant in
all models (Tables 1 and 2). Lockdown, habitat and singing behavior had
large effect sizes on FIDs and their components after controlling for
these effects (Table 1). Lockdown also interacted with habitat and
singing behavior with large effect sizes, although there were no
three-way interactions (Table 1). FID and its horizontal component
decreased during lockdown, then after it increased in rural habitats,
especially for non-singing birds (Fig. 1). Singing birds, and
non-singing birds in urban habitats, showed the same trend although less
markedly so, and singing birds showed the opposite trend (increased FIDs
during lockdown) in urban habitats. Mean post height of singing birds
did not change in neither habitat and did not differ among habitats.
Non-singing birds perched higher during lockdown, and higher in rural
habitats; perch height remained higher before lockdown in urban habitats
and decreased in rural habitats (Fig. 1).

Overall, birds did not horizontally adjust flight distance significantly
according to the height above ground where they were located or perched.
Rather, such adjustments were modulated by lockdown, habitat and singing
behavior with moderate to large effect sizes. There were no significant
higher-order interactive effects (Table 2). Horizontal distances were
negatively related to height both before and after lockdown (i.e., birds
that perched higher allowed for closer horizontal approaches), but this
effect disappeared during lockdown (Fig. 2, above). Negative
relationships between components were steeper in rural than in urban
habitats, and birds allowed for closer approaches in urban habitats for
the same perch height (Fig. 2, middle). Finally, non-singing birds
modulated horizontal flight distances whereas singing birds did not
(Fig. 3, below).

\textbf{Discussion}

Lockdown, habitat and singing behavior had clear-cut direct and
interactive effects on FIDs and their horizontal and vertical
components. Responses varied between urban and nearby rural habitats,
and on whether birds were singing or engaged in other activities.
Overall, direct disturbance effects seemed to predominate in rural
habitats, where FIDs decreased by more than a half in non-singing birds
to mean distances as short as in urban habitats. Weaker decreases were
found for singing rural birds. Contrarily, no significant FID change was
found for non-singing birds in urban habitats, whereas singing birds
increased rather than decreased FIDs, as expected under increased risk
of predation from ground-dwelling predators (Møller 2011). Mean height
above ground of non-singing birds increased during lockdown in both
urban and nearby rural habitats, also fitting expectations from
increased risk of predation from ground-dwelling predators such as feral
cats (Díaz et al. 2022). Singing birds posted higher than non-singing
birds, and post height was not affected by either habitat or the
lockdown, suggesting strong stabilizing selection for song post height
selection and/or low behavioral flexibility for this trait (Møller
2011).

Birds that perched higher in the vegetation allowed for closer
horizontal approaches than species perched lower (Møller 2010; but see
Blumstein et al. 2004, Díaz et al. 2022\emph{)} , a behavioral
adjustment that tended to reduce the variability of FIDs towards a
species- and site-specific mean value, and is associated to increased
survival and reduced senescence (Møller 2011). Singing behavior and the
Covid-19 lockdown suppressed this adjustment, implying higher FID
variability during lockdown and more commonly singing birds. Adjustments
were stronger in rural habitats, as expected by the lower variance in
FID usually found in urban habitats, that has been interpreted as
resulting from stabilizing selection of individuals able to endure high
levels of human disturbance under conditions of low predation risk
(Samia et al. 2015). Suppression of this adjustment during lockdown and
in singing birds may have allowed individuals to better adjust
horizontal and height components of FID to varying levels of predation
risk coming from ground-dwelling predators released by human absence.

Overall, our results point to differential mechanisms underlying
lockdown effects on bird fearfulness. Reduced direct disturbance seemed
to predominate in rural habitats, where non-singing birds would have
benefited from human absence. In urban habitats, however, human absence
seemed to have increased the risk of predation by ground-dwelling
predators, most likely the colonies of free-roaming feral cats
maintained by animal-loving citizens (Díaz et al. 2022). Lockdown
effects lagged to the following year, especially in urban habitats and
for singing birds, suggesting stronger selection effects in sedentary
urban and/or territorial populations than in more transient rural
populations (Møller et al. 2013). Selection by these predators on bird
scape behavior may explain time-lagged responses to lockdown in urban
habitats, that may compensate for effects of continued human presence on
loss of antipredator behaviors (Geffroy et al. 2020).

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\textbf{Acknowledgments} \textbf{:}S. Soria and C. Trivino provided the
permissions for sampling FIDs during lockdown in Madrid and Toledo,
respectively. No specific financial aid supported this study, although
it is a contribution by MD to the project URBILAND
(PID2019-107423GA-I00) financed by MCIN/AEI/10.13039/501100011033.

\textbf{Competing interests:}Authors declare that they have no competing
interests.

Supplementary Materials

Figure S1. Disease reveals the importance of urban predators. The
Covid-19 lockout released bird fear in four European cities.

Table S1. Location and human population size of the study cities, and
duration of lockdown in 2020 (taken from Wikipedia).

Table S2. Consensus phylogeny.

Table S3. R scripts.

Appendix. Mean, SE, minimum and maximum (m) Flight Initiation Distances
and their horizontal and vertical components according to bird species,
city, habitat (U: urban; R: rural surroundings), year (2018, Before the
Covid-19 lockdown; 2020, During lockdown, and 2021, After it), and
whether birds were singing or not when detected. N=sample size.

\textbf{Fig 1. Mean Flight Initiation Distance (m +- SE) and its
components before, during and after the 2020 lockdown associated with
the Covid-19 pandemic.} Filled symbols, lines: rural habitats; open
symbols, dashed lines: urban habitats. Right: singing birds; left:
non-singing birds

\textbf{Fig 2. Adjustment of horizontal distances according to height
above ground and their variation according to lockdown, habitat and
singing behavior.} Top graph (lockdown); grey dots, dashed line: After;
filled dots, continuous lines: During; and open dots, pointed line:
Before. Middle graph (habitat); filled squares, continuous line: rural;
open squares, pointed line: urban. Bottom graph (singing behavior);
filled diamonds continuous line: singing; open diamonds, pointed line:
not singing). Note log10 (+1) transformations of both components. Lines
are linear trends.

\textbf{Table 1. Results of phylogenetically corrected analyses on the
effects of lockdown, habitat, singing activity and their interactions on
Flight Initiation Distances (FID) and their horizontal and vertical
(height above ground) components.} Model statistics, adjusted R2, AICc,
the \selectlanguage{greek}λ \selectlanguage{english}parameter measuring degree of phylogenetic dependence and its
associated tests against the null hypothesis of independence, and the W
parameter controlling for sample size effects, are also shown. Boldface
indicates p\textless{}0.05. Effect sizes (r) were computed from F (F=[?]t)
values following \emph{(33)} .

\textbf{Table 2. Results of phylogenetically corrected analyses on
adjustments of the horizontal component of FID to the height above
ground where the birds were posted or perched, and the effects of
lockdown, habitat, singing activity and their interactions on these
adjustments.} Model statistics, adjusted R2, AICc, the \selectlanguage{greek}λ \selectlanguage{english}parameter
measuring degree of phylogenetic dependence and its associated tests
against the null hypothesis of independence, and the W parameter
controlling for sample size effects, are also shown. Boldface indicates
p\textless{}0.05. Effect sizes (r) were computed from F (F=[?]t) values
following \emph{(33)} .\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Fig. 1}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Fig. 2}

\textbf{Table 1} .\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllll@{}}
\toprule
& & FID & FID & FID & Horizontal distance & Horizontal distance &
Horizontal distance & Height above ground & Height above ground & Height
above ground\tabularnewline
\midrule
\endhead
Source & df & F & p & effect size & F & p & effect size & F & p & effect
size\tabularnewline
Lockdown & 2, 1132 & 4.53 & \textbf{0.011} & \textbf{0.521} & 6.59 &
\textbf{0.001} & \textbf{0.790} & \textbf{16.19} & \textbf{0.000} &
\textbf{0.992}\tabularnewline
Habitat & 1, 1132 & 207.72 & \textbf{0.000} & \textbf{1.000} & 179.11 &
\textbf{0.000} & \textbf{1.000} & \textbf{31.27} & \textbf{0.000} &
\textbf{0.999}\tabularnewline
Singing & 1, 1132 & 3.67 & 0.056 & 0.372 & 15.25 & \textbf{0.000} &
\textbf{0.990} & \textbf{257.52} & \textbf{0.000} &
\textbf{1.000}\tabularnewline
Lockdown x habitat & 2, 1132 & 8.13 & \textbf{0.000} & \textbf{0.891} &
5.08 & \textbf{0.006} & \textbf{0.609} & \textbf{5.20} & \textbf{0.006}
& \textbf{0.626}\tabularnewline
Lockdown x singing & 2, 1132 & 9.18 & \textbf{0.000} & \textbf{0.929} &
9.23 & \textbf{0.000} & \textbf{0.930} & 1.11 & 0.329 &
0.037\tabularnewline
Lockdown x habitat x singing & 2, 1132 & 0.10 & 0.909 & 0.000 & 0.58 &
0.563 & 0.010 & 2.79 & 0.062 & 0.225\tabularnewline
Model & 12, 1144 & 23.96 & \textbf{0.000} & & 21.76 & \textbf{0.000} & &
31.3796 & \textbf{0.000} &\tabularnewline
Adjusted R\textsuperscript{2} & & 0.181 & & & 0.167 & & & 0.226 &
&\tabularnewline
AICc & & -329.56 & & & 251.54 & & & 377.33 & &\tabularnewline
\selectlanguage{greek}Λ & & 0.759 & \selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{12}= 314.91, p=0.000 &
\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{12}= 314.91, p=0.000 & 0.546 &
\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{12}= 199.62, p=0.000 &
\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{12}= 199.62, p=0.000 & 0.597 &
\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{12}= 356.43, p=0.000 &
\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{12}= 356.43, p=0.000\tabularnewline
W & & 100 & & & 300 & & & 100 & &\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 2} .\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& Horizontal distance & Horizontal distance & Horizontal distance &
Horizontal distance\tabularnewline
\midrule
\endhead
Source & df & F & P & effect size\tabularnewline
Log\textsubscript{10}(height above ground) & 1, 1126 & 3.15 & 0.076 &
0.284\tabularnewline
Height x lockdown & 2, 1126 & 4.26 & \textbf{0.014} &
\textbf{0.476}\tabularnewline
Height x habitat & 1, 1126 & 6.26 & \textbf{0.012} &
\textbf{0.760}\tabularnewline
Height x singing & 1, 1126 & 14.60 & \textbf{0.000} &
\textbf{0.988}\tabularnewline
Height x lockdown x habitat & 2, 1126 & 1.20 & 0.302 &
0.043\tabularnewline
Height x lockdown x singing & 2, 1126 & 2.13 & 0.120 &
0.134\tabularnewline
Model & 18, 1144 & 16.90 & \textbf{0.000} &\tabularnewline
Adjusted R\textsuperscript{2} & & 0.191 & &\tabularnewline
AICc & & 255.93 & &\tabularnewline
\selectlanguage{greek}Λ & & 0.534 & \selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{18}= 167.80, p=0.000 &
\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}\textsubscript{18}= 167.80, p=0.000\tabularnewline
W & & 500 & &\tabularnewline
\bottomrule
\end{longtable}

\selectlanguage{english}
\FloatBarrier
\end{document}

