# Habituation or sensitization? Short-term adjustment of flight initiation distance in incubating common eiders

Bertille Mohring, Frédéric Angelier, Kim Jaatinen, Benjamin Steele, Markus Öst

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Bertille Mohring, Frédéric Angelier, Kim Jaatinen, Benjamin Steele, Markus Öst. Habituation or sensitization? Short-term adjustment of flight initiation distance in incubating common eiders. Animal Behaviour, 2025, 219, pp.123030. 10.1016/j.anbehav.2024.11.008. hal-05019452

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# Habituation or sensitization? Short-term adjustment of flight initiation distance in incubating common eiders

Bertille Mohring<sup>a, b, c, *</sup>, Frédéric Angelier<sup>c</sup>, Kim Jaatinen<sup>d, e</sup>, Benjamin Steele<sup>f, g</sup>,
Markus Öst<sup>a, g, i</sup>

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ARTICLE INFO

Article history:

Received 25 October 2023

Initial acceptance 14 February 2024

Final acceptance 6 September 2024

Available online xxx

MS number: 23-00560R

Keywords:

escape decision

incubation

reproductive investment

risk assessment

risk-taking behaviour

Somateria mollissima

Optimal escape theory predicts that prey balance the costs and benefits of fleeing. However, the use of prior experience in prey escape decisions has been overlooked. Indeed, when repeatedly exposed to a nonlethal yet threatening stimulus, prey may adjust their escape response by habituating, delaying escape when repeatedly exposed to that stimulus, or by becoming sensitized, escaping sooner. Here, we investigated intrinsic and extrinsic drivers of short-term variation in flight initiation distance (FID, distance at initiating escape) in response to repeated human approaches in incubating female common eiders, *Somateria mollissima*. We predicted habituation to repeated exposure to threatening but nonlethal stimuli, reflected in decreased FID. We also expected increased habituation in females with a high current reproductive value (older breeders, females in good condition or laying large clutches) or brood value (early breeders). Last, we investigated whether changes in FID varied with threat level (i.e. different adult or nest predation risk or breeding habitat). The magnitude of change in FID was small and varied among individuals, depending on individual traits (body condition, clutch size and phenology) and perceived threat (island-specific adult predation risk). Accordingly, females in poorer body condition, that laid larger clutches or laid earlier in the season were more prone to become sensitized over repeated approaches. Consistent with the risk allocation hypothesis predicting a dampening of antipredator responses under chronic risk exposure, females were less prone to become sensitized when nesting on islands with high adult predation risk. These results underline the ability of animals to make plastic state- and context-dependent adjustments of antipredator behaviour when repeatedly confronted with nonlethal stimuli. Nevertheless, given the limited magnitude of within-individual, short-term adjustment of FID compared to the among-individual variation in FID, questions remain concerning the efficacy of these antipredator responses in a context of rapidly changing predation regimes.

© 2024 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license ([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)).

Optimal escape theory depicts prey escape decisions as a trade-off between the fitness costs and benefits of fleeing when approached by a potential predator (Cooper & Frederick, 2007; Ydenberg & Dill, 1986). This trade-off is exacerbated during reproduction because fleeing may reduce current reproductive success by disrupting incubation or giving predators cues of

offspring location (Broom & Ruxton, 2005), while delaying escape may jeopardise parent survival and chances of investing in future reproduction (Albrecht & Klvaňa, 2004). According to optimality models, prey that are able to flexibly adjust their escape strategy to the prevailing threat would gain a fitness advantage over less flexible individuals (e.g. Dowling & Bonier, 2018). Such behavioural flexibility could, ultimately, enable them to cope better with environmental challenges and therefore to persist in disrupted ecosystems (Eisenstein et al., 2012; Piersma & Drent, 2003).

<footnote><sup>a</sup> Environmental and Marine Biology, Åbo Akademi University, Turku, Finland</footnote>

<footnote><sup>b</sup> Centre d'Etudes Biologiques de Chizé (CEBC), UMR 7372 CNRS – Université de La Rochelle, Villiers-en-Bois, France</footnote>

<footnote><sup>c</sup> School of Environmental Sciences, University of Liverpool, Liverpool, U.K.</footnote>

<footnote><sup>d</sup> Finnish Environment Institute, Helsinki, Finland</footnote>

<footnote><sup>e</sup> Tvärminne Zoological Station, University of Helsinki, Hanko, Finland</footnote>

<footnote><sup>f</sup> School of Arts and Sciences, Colby-Sawyer College, New London, NH, U.S.A.</footnote>

<footnote><sup>g</sup> Novia University of Applied Sciences, Ekenäs, Finland</footnote>

<footnote>* Corresponding author.
E-mail address: bmohring@liverpool.ac.uk (B. Mohring).</footnote>
Nevertheless, the use of prior experience in guiding prey escape decisions is still poorly understood (Sutton & O'Dwyer, 2018). Indeed, a growing literature suggests that prior experience with predators can influence antipredator behaviour (Ferrari et al., 2010; Fraker, 2009; Sutton & O'Dwyer, 2018; Vilhunen, 2006). Consequently, when repeatedly exposed to a nonlethal yet threatening stimulus, prey animals may adjust their escape response by habituating, delaying escape when repeatedly exposed to that stimulus, or by becoming sensitized, advancing their escape (Blumstein, 2016; Hemmi & Merkle, 2009; Rankin et al., 2009). Interestingly, under high predation pressure, optimality models suggest that prey could benefit from either becoming sensitized or habituating to repeated exposure to nonlethal disturbance (Sirot, 2010), and both hypotheses have received empirical support. First, increased probability of attack by one predator may favour increased responsiveness to alternative predators through stimulus generalization and thus lead to sensitization to repeated exposure to a nonlethal stimulus (Brilot et al., 2012; Frid & Dill, 2002), especially because recognition errors may be fatal (Brown et al., 2015). Hence, generalized sensitization to all potential predators and heightened perception of risk are expected to maximize prey survivorship from predation attempts and, thus, the probability of reproducing in the future (Oteyza et al., 2021). Second, paradoxically, the risk allocation hypothesis predicts that chronic exposure to high predation risk will favour the dampening of antipredator responses, thereby facilitating habituation to nonlethal stimuli (Ferrari et al., 2009, 2010; Lima & Bednekoff, 1999). This is because maintaining high levels of antipredator behaviour is costly and may interfere with essential activities such as resource acquisition or reproduction (Lima & Dill, 1990; Wingfield & Sapolsky, 2003). By habituating to repeated exposure to a harmless stimulus such as a human approach, prey may maintain foraging activity and parental care, therefore leading to higher fitness.

Establishing whether habituation or sensitization prevails in the face of high perceived risk requires that individual prey are subjected to controlled repetition of the same stimulus over time and under varying predation risk (Blumstein, 2016, 2019; Nisbet, 2000), but such studies are rare, especially when considering wild individuals. Most studies address habituation or sensitization using single measurements of antipredator behaviour in individuals belonging to species or populations evolving under very different environmental conditions. Such a comparative approach may obscure underlying individual adjustments of antipredator responses in relation to prior experience of threat (Bejder et al., 2009; Blumstein, 2016; Williams et al., 2020). Significant knowledge gaps thus remain on the specificity of antipredator habituation or sensitization (e.g. Geffroy et al., 2015) and the relative importance of current reproductive investment and perceived predation risk in shaping individual adjustment of antipredator behaviour (Samia et al., 2015).

To shed light on these knowledge gaps, we relied on individual-based data from a population of common eiders, *Somateria molissima* (hereafter, eiders), breeding in the Baltic Sea and subject to a dramatic increase in predation pressure (Öst et al., 2022). Predation is the main cause of nest failure in birds (Martin, 1993; Ricklefs, 1969), including eiders (Mohring et al., 2023), and many ground-nesting birds, such as eiders, rely on crypsis to reduce nest detectability (Wilson-Aggarwal et al., 2016). Long-lived female eiders are an ideal species for investigating the use of prior experience in the adjustment of escape decisions during a reproductive event, notably because the distance at which a cryptic incubating female eider initiates escape when approached by a potential predator (flight initiation distance, FID; Ydenberg & Dill, 1986) is thought to reflect how individuals optimize the trade-off between investment in the current breeding event and future reproduction

and survival (Cooper & Frederick, 2007; Ydenberg & Dill, 1986). Evolutionary pressures associated with prey encounters with predators in both the short and long term are thought to shape the ability of prey to assess the risk posed by an approaching predator and adjust their escape behaviour accordingly (Barnas et al., 2022; Carthey & Blumstein, 2018; Geldart et al., 2023a, 2023b).

During a given breeding event, we repeatedly measured FID using a standardized human approach protocol, given that humans are perceived as predators (Frid & Dill, 2002), and we focused on the individual changes in FID over repeated trials. Assuming that females progressively identify humans as nonlife-threatening (Sirot, 2010) or display positive reinforcement after successfully escaping a predation attempt (Knight & Temple, 1986), we predicted that incubating eiders would habituate to repeated human approaches carried out during a single breeding event (i.e. reduce FID over repeated trials; prediction 1). We then tested whether individuals that placed a greater emphasis on current reproduction, that is, experienced females, those in good body condition and those laying large clutches, showed pronounced habituation (prediction 2), because the larger the female's current reproductive asset, the more critical it becomes to protect it, even at the expense of the female's survival (Clark, 1994). Because early broods with a high relative brood value should be defended more intensively (Montgomerie & Weatherhead, 1988) given that the subsequent recruitment of offspring tends to decline with advancing hatch date (Shutler et al., 2006), we also expected increased habituation in early breeders (prediction 3). Finally, we investigated whether exposure to increased danger, that is, islands characterized by high adult and nest predation risk or offering little nest concealment, or years of high predator abundance, modulates escape responses to repeated human disturbance. We made two opposite predictions (predictions 4A and 4B). First, through stimulus generalization (Brilot et al., 2012; Frid & Dill, 2002), we expected females nesting in riskier environments to be more prone to become sensitized to repeated human approaches (prediction 4A). Alternatively, by extending the risk allocation hypothesis to encompass trade-offs between reproductive investment and predation risk, we predicted that exposure to intense danger in riskier environments should lead females to habituate to nonlethal threats to minimize the risk of reproductive failure due to excessive flightiness (Sirot, 2010; prediction 4B).

## METHODS

### *Study Area and Study Species*

Fieldwork for this study was carried out in 2009–2023 in Tvärminne, western Gulf of Finland (59°50′N, 23°15′E), on 22 islands on which female eider reproduction has been annually investigated since 1990. Islands were split into two categories: 10 forested islands, mainly covered by pine-dominated forest (forest cover estimated based on Google Earth satellite maps version 7.3.3; mean ± SD = 40.9 ± 10.08%; range 19.6–53.0%; Hermansson et al., 2023) and 12 open islands covered by bare rocks, patches of herbs, juniper, *Juniperus communis*, shrubs, or sparse trees (forest cover, mean ± SD = 3.8 ± 5.8%; range 0.0–15.4%).

Eiders are long-lived birds (mean life expectancy [95% confidence interval]: 11.8 years [5.4–25.2]; Wood et al., 2020) usually initiating their first breeding attempt at 3 years old (Hario & Rintala, 2009). Females are philopatric to their breeding area (Ekroos et al., 2012; Öst et al., 2011), potentially facilitated by mechanisms such as kin recognition (Hervey et al., 2019). They rely on a mixed capital breeding strategy, utilizing both endogenous reserves stored at the wintering grounds and locally ingested nutrients for egg production (Jaatinen et al., 2016). As a consequence
of displaying exclusive maternal care, female eiders fast throughout the incubation period and steadily lose mass while incubating a clutch of usually four to six eggs (Criscuolo et al., 2000). High female nest attendance during incubation also acts as a protection against nest predators (Andersson & Waldeck, 2006). Breeding female eiders and their nests face variable predation threat from avian and mammalian predators. In our study area, adults are primarily predated by white-tailed eagles, *Haliaeetus albicilla* (hereafter, eagles), American minks, *Neogale vison*, and raccoon dogs, *Nyctereutes procyonoides* (Öst et al., 2018), while nests are primarily predated by hooded crows, *Corvus cornix*, and gulls *Larus* spp., in particular when the female is not incubating the clutch (Hermansson et al., 2023; Jaatinen et al., 2014).

Every year between mid-May and early June, the study islands were surveyed in search of active or predated eider nests and carcasses of dead females. The researchers walked 2–5 m away from each other, carrying hand nets, and aimed to catch all breeding females. In the study area, females have been trapped every year since 1990 (with a constant trapping effort) and ringed with a unique metal and colour ring combination. Females are philopatric to their breeding area (Öst et al., 2011) and trapping success is high (Jaatinen & Öst, 2011; Öst et al., 2018). The number of years since the female had first been caught and ringed was thus used as a proxy of the minimum number of years of breeding experience. Trapped females were weighed (Pesola spring balance, to the nearest 10 g) and measured (radius-ulna length measured with a wing ruler to the nearest 1 mm). Nest position was recorded using a handheld GPS device (Garmin GPSMAP 64s) and nest location was marked with a coloured tape in the field. Clutch size was recorded during bird handling as a proxy of initial energetic investment in the current breeding attempt (Hanssen et al., 2003; Jaatinen et al., 2013), and hatching date was estimated using an egg flotation test (Kilpi & Lindström, 1997). We verified that all females had completed their clutches (incubation stage ≥ 5 days for all females according to the egg flotation test) to ensure the accuracy of the use of clutch size as a proxy of initial energetic investment in the current breeding attempt.

To compare female body reserves without bias from the variation in mass induced by different incubation stages, we calculated an index of body condition at hatching for each trapped female. Following the method detailed and validated by Öst and Steele (2010), this index was obtained by standardizing the residuals of a linear regression of log-transformed estimated mass at hatching on log-transformed radius-ulna length (Öst et al., 2008). Mass at hatching was estimated by subtracting the mass the female was expected to lose during the remaining incubation period (number of days left before hatching*average daily mass loss) from her mass at trapping. Average daily mass loss was obtained by extracting, for each year, the slope of the linear regression of log-transformed body mass on log-transformed incubation time (Öst & Steele, 2010). The latter linear regression also included hatching date as a covariate to account for seasonal variation in average weight loss (Öst et al., 2008).

## Repeated Measurement of FID

Nests of trapped females were visited several times during a given breeding event to measure female FID. We relied on a standardized protocol to measure FID, based on a nest approach performed by a single observer (Mohring et al., 2022; Seltmann et al., 2012). The observer relied on nest GPS location and a colour flag marking to find the nest. Before initiating the approach, the observer ensured that the focal female could detect them. To do so, the observer left a free line of sight between themselves and the eider to ensure that the bird could see them. When approaching,

the observer aimed straight at the nest, walked with a constant slow velocity (ca 1 m/s), and measured FID to the nearest 10 cm with a tape measure as the distance at which the focal bird flew or walked away from the nest. In instances where the female did not leave the nest, FID was measured as 0 cm. The observer ensured that repeated approaches towards a given female were identical by following the same path and initiating the approach from the same location. Starting distances of the approaching observer were thus identical over repeated approaches towards a given nest but varied between nests due to island topography and vegetation cover. Although the starting distance of the approaching observer may affect FID in some species (Blumstein, 2003), a pilot study showed this not to be the case in this study population (Mohring et al., 2022). After the female escaped, the nest was carefully covered by the observer to minimize the risk of predation by egg predators such as hooded crows or gulls. Escape behaviour is demonstrably affected by incubation stage, notably because of the increased reproductive value of the clutch near hatching (Albrecht & Klvaňa, 2004). To prevent any bias linked to variation in incubation stage, we measured FID daily from 1 day before the hatching date, derived from the laying date predicted by egg flotation (Kilpi & Lindström, 1997), until the female left the nest with ducklings, which typically occurs within a day after synchronous hatching of the eggs (Öst & Bäck, 2003). In total, FID was recorded at least twice (mean number of approaches ± SD = 2.7 ± 0.8, range 2–6) at 735 nests (belonging to 439 unique females) between 2009 and 2023 ($N_{FID}$ = 2002 FID measurements). Females were measured more than twice when eggs had not hatched at the expected hatching date, either because of prolonged incubation duration or because of slight inaccuracy of egg flotation tests (Ackerman & Eagles-Smith, 2010; Maliguine et al., 2023).

## Predation Risk

For each island and year, we calculated indices of adult and nest predation risk. The island-specific adult predation risk index corresponded to the proportion of predated nesting females in a given year. For each island and year, it was obtained by dividing the number of predated females by the number of females attempting to breed. Visual inspection of female eider carcasses and remains allowed us to assess whether the female had died from a predator attack (Öst et al., 2018), and the rare instances of females found dead for unknown reasons ($N$ = 13 of 701 females found dead over the study period) were excluded from the calculation of this index. Breeding attempts corresponded to the sum of active nests, nests that had already been predated, nests from failed breeding attempts and nests from which ducklings had already hatched. The island-specific nest predation risk index corresponded to the proportion of failed breeding attempts, obtained for each island and year by dividing the number of failed breeding attempts by the number of nests whose fate (success or failure) was known (3807 of 4113 nest fates known, 92.56%). Nest fate was assessed by revisiting nests at the end of the breeding period. Successful breeding attempts were identified by the presence of either ducklings or hatched eggshells (recognizable by their leathery membrane) as eider ducklings leave the nest within a day after synchronous hatching (Öst & Bäck, 2003). Failed breeding attempts mainly corresponded to predated nests (1785 predated nests of 1990 failed breeding attempts, 89.70%) characterized by the presence of shells broken into pieces with a thin bloody membrane or by the absence of eggshells (Öst & Steele, 2010). However, a small proportion of failed breeding attempts were due to abandonment or, occasionally, females incubating unviable eggs (205 nonpredated but failed breeding attempts out of 1990 failed breeding attempts, 10.30%). The few unknown nest fates corresponded to females still
incubating at the end of the monitoring period or inability to assess whether eggs hatched or were predated. Seldomly, unknown nest fates corresponded to nests not being revisited.

In addition to island-specific predation risk indices, we relied on a yearly index of white-tailed eagle abundance during the eider breeding season (hereafter, eagle index) calculated at the scale of the whole study area. This yearly index corresponded to the mean daily number of resident white-tailed eagles counted at Hanko Bird Observatory (HALIAS, 59°49'N, 22°54'E, ca 20 km away from the study area) between 1 April and 15 June each year (Öst et al., 2022).

## Ethical Note

The study was conducted on 439 wild female eiders. To minimize the risk of nest abandonment (Bolduc & Guillemette, 2003), trapping was conducted during the later phase of incubation and we revisited the nests to measure FID during the last days of incubation. We held the time of bird handling to a minimum. Researchers' presence on an island during the trapping phase of data collection was limited to bouts of a maximum of 90 min to limit human disturbance. During FID measurement, the researcher relied on nest GPS locations as well as coloured tape marking to walk to the targeted nests and thereby avoided disturbing surrounding incubating birds. We placed the coloured tape a few metres away from the nests and, if possible, under foliage so that it would be visible for a walking human but not for aerial predators. Additionally, we used different tape colours every year to limit the risk of corvids associating tape marking with nest presence. The research was reviewed and approved by the Finnish Project Authorization Board of the Regional Administrative Agency for Southern Finland (permit numbers ESAVI/1697/04.10.03/2012; ESAVI/2831/04.10.07/2015; ESAVI/4053/2018; ESAVI/10022/2021) and complied with laws of Finland and the specific regulations of Tvärminne Zoological Station.

## Statistical Analyses

All statistical analyses were conducted using R 4.0.4 (R Core Team, 2023). We visually verified that normality assumptions and homoscedasticity of residuals were met in selected models (Zuur, 2009; Zuur et al., 2010). FIDs, measured to the closest 10 cm, were converted to dm to meet model assumptions. We constructed two sets of generalized linear mixed models (GLMMs; R function 'glmmTMB', glmmTMB package; Brooks et al., 2017) with a Poisson error distribution to assess (1) individual responses to repeated human approaches and (2) intrinsic and extrinsic drivers of individual variation in FID in response to repeated human approaches. All models included FID as the response variable. Because of significant overdispersion when fitting a Poisson distribution (dispersion ratio = 2.00, *P* < 0.001; 'testZeroInflation' function, DHARMa package; Hartig, 2022), we added a zero-inflation parameter (ziformula = ~1) to account for females not leaving the nest during a FID measurement (test after inclusion of the zero-inflation parameter: dispersion ratio = 1.06, *P* = 0.38). In addition, breeding attempt identity ('Breeding attempt', the unique combination of individual and year), individual identity ('Individual') and year ('Year') were included as random effects to account for repeated measurements of a given female within and across years and for the nonindependence of females tested in a given year. All numerical explanatory variables were centred and scaled. Fixed effect parameter estimates and their 95% credible intervals (CI) were estimated from posterior distributions of 10 000 simulations ('sim' function, arm package; Gelman & Su, 2024). We verified that we could not detect multicollinearity among the variables included in final selected models (all variance inflation

factors, VIFs < 2.28, i.e. below the conservative threshold of 2.5; Allison, 2012). Variance inflation factors were calculated using the 'check_collinearity' function (performance package; Lüdecke et al., 2021).

In the first set of GLMMs, we compared the Akaike's information criterion corrected for small sample size (AICc; Barton, 2020) of a null model ($M_0$, testing for a lack of adjustment of FID to repeated human approaches), a model including trial number (numerical explanatory variable) as a fixed effect ($M_1$, testing for a uniform population level adjustment of FID to repeated human approaches) and a model including trial number as a fixed effect and as a random slope effect ($M_2$, testing for interindividual variation in adjustment of FID to repeated human approaches). Models were fitted using restricted maximum likelihood (REML) to allow comparison of random effect structures. In the second set of GLMMs, we assessed whether interindividual variation in FID over repeated trials could be explained by variation in female investment in current reproduction or predation risk. To this aim, we tested the effect of two-way interactions between trial number and minimum breeding experience, body condition, laying date, clutch size, island type, island-specific adult or nest predation risk and eagle index on FID. All explanatory variables except island type were numerical variables. All possible combinations of the aforementioned explanatory variables and two-way interactions were generated using the 'dredge' function (MuMln package; Barton, 2020) under the condition that variables were not collinear (Pearson absolute correlation coefficients, $r \le 0.40$). Accordingly, island-specific adult and nest predation risk ($r = 0.43$; Appendix Fig. A1) were not included in the same model. The two full models therefore included female FID as the response variable and trial number, female minimum breeding experience, body condition, clutch size, laying date, island type, either island-specific adult or nest predation risk (see above), eagle index and the two-way interactions between trial number and female minimum breeding experience, body condition, clutch size, laying date, island type, island-specific adult or nest predation risk and eagle index as explanatory variables. Model selection was based on AICc and models were computed using maximum likelihood estimate (ML). A single top-ranked model was identified ($\Delta$AICc to the second-best model = 4.19; Appendix Table A1). The final model was refitted using REML to provide reliable estimates for the parameters, and parameter estimates and their 95% CI were calculated using 'simulate_parameters' function (parameters package; Lüdecke et al., 2020). In addition, we calculated marginal $R^2$ ($R^2_m$, the variance explained by the fixed effects) and conditional $R^2$ ($R^2_c$, the variance explained by both the fixed and random effects) ('r2' function, performance package; Lüdecke et al., 2021). To quantify the variance in FID explained by short-term adjustments of FID over repeated trials, we compared the $R^2_m$ of two models identical to the final model, except for either including or excluding the variables related to individual responses to human approaches, i.e. trial number and the two-way interactions between trial number and variables related to female investment in reproduction or predation risk. These two models are hereafter called $R^2_m$ including and $R^2_m$ excluding, respectively (see Appendix Table A2 for details on model structure).

## RESULTS

The model with a random slope for trial number ($M_2$) had a smaller AICc value than that without the random slope ($M_1$, $\Delta$AICc = 3060.58; Table 1) or the null model ($M_0$, $\Delta$AICc = 3109.46; Table 1). Combined with the fact that the fixed effect of trial number was not different from zero (estimate = 0.03, 95% CI = [-0.01, 0.08]), this result indicates that females varied in their
<caption>**Table 1**</caption>

Akaike's information criterion corrected for small sample size (AICc) for comparison of alternative formulations of zero-inflated Poisson generalized linear mixed models explaining female common eider flight initiation distance (FID) as a function of trial number (explanatory fixed effect) or random slope effects across individual breeding attempts

<table><thead><tr><th>Model</th><th>Fixed-effect structure</th><th>Random effect structure</th><th>AICc</th></tr></thead><tbody><tr><td>M<sub>0</sub></td><td>1</td><td>(1 | Breeding attempt) + (1 | Individual) + (1 | Year)</td><td>20524.65</td></tr><tr><td>M<sub>1</sub></td><td>Trial number</td><td>(1 | Breeding attempt) + (1 | Individual) + (1 | Year)</td><td>20475.77</td></tr><tr><td>M<sub>2</sub></td><td><b>Trial number</b></td><td>(1 + Trial number | Breeding attempt) + (1 | Individual) + (1 | Year)</td><td><b>17415.19</b></td></tr></tbody></table>

Breeding attempt identity, individual identity and year were included as random intercept effects in all models. The model with the lowest AICc is presented in bold.

adjustment of FID over repeated trials (Fig. 1), with no overall population level habituation or sensitization pattern.

The final model predicting variation in FID over repeated trials included trial number, female minimum breeding experience, body condition, clutch size, laying date, island type, island-specific adult predation risk, eagle index and the two-way interactions between trial number and female minimum breeding experience, body condition, clutch size, laying date, island type, island-specific adult predation risk and eagle index (Table 2). In this model, fixed effects explained 30.0% of the variance ($R^2_m = 0.300$) and both the fixed and random effects explained 87.4% of the variance ($R^2_c = 0.874$). To aid the visualization of the two-way interaction effects, we only present predicted response slopes (Fig. 2) while further description of these interactions (including the raw data and confidence intervals of predicted responses) can be found in Appendix Fig. A2. Females in poorer body condition displayed a more pronounced increase in FID, i.e. sensitization, in response to repeated human approaches, compared to females in better body condition (trial number*body condition effect; Table 2, Fig. 2a). Females laying a larger clutch and those laying earlier in the breeding season displayed stronger sensitization over trials than those laying smaller or later clutches (trial number*clutch size and trial number*laying date effects; Table 2, Fig. 2b and c). Females nesting under lower adult predation risk increased FID over trials, while those nesting under higher risk slightly reduced FID over trials (trial number*island-specific adult predation risk effect; Table 2, Fig. 2d). The two-way interactions between trial number and female minimum breeding experience, island type and eagle index were retained in the final selected model, but their estimates were close to zero and CIs overlapped zero, indicating a limited effect of these interactions in explaining variation in FID (Table 2, Fig. 2e, f and g). Overall, FID decreased with increasing breeding experience (female minimum breeding experience effect; Table 2) and females nesting on forested islands displayed shorter FIDs than those nesting on open islands (island type effect; Table 2). Including trial number and the two-way interactions between trial number and female minimum

breeding experience, body condition, clutch size, laying date, island type, island-specific adult predation risk and eagle index explained 0.4% of the variance in FID ($R^2_m$ including = 0.300; $R^2_m$ excluding = 0.296; Appendix Table A2).

## DISCUSSION

In this study, we repeatedly measured FID of incubating female eiders exposed to nonlife-threatening human approaches during a given breeding event, to shed light on whether and how individuals adjust antipredator behaviour to prior experience. Intriguingly, we did not detect the predicted population level waning of antipredator behaviour expected from a decrease in risk perception with repeated exposure to a nonlife-threatening stimulus (Sirot, 2010). Instead, our results provide support for interindividual variation in adjustment of FID to repeated human approaches (Fig. 1). Although the magnitude of the within-individual adjustment of FID over trials was very limited compared to among-individual variation in FID, our results show that this adjustment differed depending on individual variation in investment in current reproduction and risk assessment. A particularly interesting finding was that sensitization over repeated trials was stronger for females breeding under lower adult predation risk, in agreement with the risk allocation hypothesis (Ferrari et al., 2009).

### Individual Responses to Repeated Human Approaches

Contrary to prediction 1, female eiders did not consistently reduce FID, i.e. habituate, to repeated human approaches over a timescale of days. Instead, females displayed varied responses, by remaining consistent, increasing or decreasing FID over repeated trials (Fig. 1). Habituation to repeated human approaches can be theoretically expected (Sirot, 2010), and has been empirically observed (Cabrera et al., 2017; Runyan & Blumstein, 2004; van Polanen Petel et al., 2008; Vennesland, 2010; Walker et al., 2006). Nevertheless, this pattern is not universal (Magle et al., 2005;

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Histogram presenting the predicted interindividual variation in the magnitude of change in flight initiation distance (FID) of incubating female common eiders over repeated human approaches. Values above 0 indicate sensitization to repeated human approaches and values below 0 reflect habituation.</figcaption></figure>
<caption>**Table 2**</caption>

Final zero-inflated Poisson generalized linear mixed model explaining variation in incubating female common eider flight initiation distance (FID) in response to repeated human approaches

<table><thead><tr><th></th><th>Variance ± SD</th></tr></thead><tbody><tr><td><strong>Random effects</strong></td><td></td></tr><tr><td>Breeding attempt</td><td>0.52 ± 0.72</td></tr><tr><td>Individual</td><td>0.72 ± 0.85</td></tr><tr><td>Year</td><td>0.04 ± 0.20</td></tr><tr><td><strong>Fixed effect</strong></td><td><strong>Estimate</strong></td><td><strong>95% CI</strong></td></tr><tr><td><strong>Intercept</strong></td><td><strong>2.35</strong></td><td><strong>[2.19, 2.51]</strong></td></tr><tr><td><strong>Trial number</strong></td><td><strong>0.03</strong></td><td><strong>[0.01, 0.04]</strong></td></tr><tr><td><strong>Female minimum breeding experience</strong></td><td><strong>-0.56</strong></td><td><strong>[-0.65, -0.46]</strong></td></tr><tr><td><strong>Body condition</strong></td><td><strong>-0.21</strong></td><td><strong>[-0.30, -0.12]</strong></td></tr><tr><td>Clutch size</td><td>-0.04</td><td>[-0.11, 0.03]</td></tr><tr><td><strong>Laying date</strong></td><td><strong>-0.16</strong></td><td><strong>[-0.24, -0.06]</strong></td></tr><tr><td><strong>Island type (Open island)</strong></td><td><strong>0.90</strong></td><td><strong>[0.67, 1.16]</strong></td></tr><tr><td>Island-specific adult predation risk</td><td>0.04</td><td>[-0.04, 0.12]</td></tr><tr><td>Eagle index</td><td>-0.02</td><td>[-0.14, 0.11]</td></tr><tr><td>Trial number*Female minimum breeding experience</td><td>0.01</td><td>[-0.01, 0.02]</td></tr><tr><td><strong>Trial number*Body condition</strong></td><td><strong>-0.04</strong></td><td><strong>[-0.06, -0.03]</strong></td></tr><tr><td><strong>Trial number*Clutch size</strong></td><td><strong>0.01</strong></td><td><strong>[0.00, 0.02]</strong></td></tr><tr><td><strong>Trial number*Laying date</strong></td><td><strong>-0.05</strong></td><td><strong>[-0.06, -0.03]</strong></td></tr><tr><td>Trial number*Island type (Open island)</td><td>0.01</td><td>[-0.01, 0.03]</td></tr><tr><td><strong>Trial number*Island-specific adult predation risk</strong></td><td><strong>-0.03</strong></td><td><strong>[-0.05, -0.02]</strong></td></tr><tr><td>Trial number*Eagle index</td><td>-0.01</td><td>[-0.02, 0.01]</td></tr></tbody></table>

Fixed effects were trial number, female minimum breeding experience, body condition at hatching, initial clutch size, laying date, breeding habitat (open or forested island), island-specific adult predation risk and eagle index, as well as the two-way interactions between trial number and female minimum breeding experience, body condition at hatching, initial clutch size, laying date, breeding habitat (open or forested island; forested islands as the reference group) and island-specific adult predation risk and eagle index. Breeding attempt identity, individual identity and year were included as random intercept effects in all models. Parameter estimates are presented with 95% credible intervals (95% CI) in squared brackets and in bold when excluding zero.

Osiejuk & Kuczyński, 2007; Wam et al., 2014; Williams et al., 2020) and interindividual variation in responses is sometimes observed (Carrete & Tella, 2010; Ellenberg et al., 2009; Uchida & Blumstein, 2021), as also supported by this study. Displaying habituation is relevant if delaying escape when facing a harmless stimulus provides fitness gains (e.g. higher food intake or lower risk of nest or offspring predation; Cooper & Frederick, 2007). However, such potential benefits of habituation may be thwarted if survival, and hence lifetime fitness, is jeopardized (Oteyza et al., 2021; Vilhunen, 2006). Overall, the diversity of observed adjustment of FID over repeated human approaches may reflect different individual responses depending on individual characteristics and predation risk, as discussed below.

While the results of this study support the hypothesis that individuals are able to plastically adjust their antipredator behaviour to prior experience with a threat, the magnitude of this adjustment was found to be very low and of little biological significance (average absolute change in FID over trials = 3 cm/trial, range 0–16; Fig. 1). Indeed, out of 30.0% of the variance in FID explained by fixed effects in the final model ($R^2_m$ = 0.300), short-term behavioural adjustments over a breeding event (i.e. trial number effect and the two-way interactions between trial number and individual characteristics or predation risk) only accounted for 0.4% of the variance. In contrast, the random effects explained 57.4% of the variance in FID ($R^2_c$ = 0.874), indicating greater variation in FID across breeding events, individual females or years than within a given breeding event. In part, the limited change in FID over repeated approaches could be due to an underestimation of such effects because FIDs were measured towards the end of incubation to control for the effect of variable incubation stages on FID

dynamics. Indeed, investment in the current breeding attempt is expected to be the highest around hatching in precocial species (Albrecht & Klvaňa, 2004; Montgomerie & Weatherhead, 1988). While effectively controlling for this source of confounding, our sampling design may have inadvertently biased the pool of sampled females towards more successful, presumably higher-quality, females that managed to protect their clutch from predation attempts until FID measurements (Mohring et al., 2021, 2022; Öst et al., 2022), hence reducing the interindividual variance in quality.

## Adjustment of Escape Decisions to Investment in Current Reproduction

Adjustment of escape decisions over repeated human approaches was linked to body condition in a manner consistent with our prediction. Females in poor condition displayed longer FIDs (Mohring et al., 2022; this study) and increased FID over repeated approaches, while the reverse was true for females in good condition. Long-lived individuals in poor condition are likely to provide lower-quality parental care to maximize their own survivorship and the opportunities for future reproduction (Stearns, 1992, p. 249; Williams, 1966), for example through reduced incubation constancy (Garbus et al., 2018; Gatti, 1983), nest defence, brood guarding and provisioning (O'Dwyer et al., 2007; Öst et al., 2007). This lower investment in reproduction by parents in poor condition may translate into lower risk taking (Davidson et al., 2018; Mohring et al., 2022) and a higher probability of abandoning the clutch or offspring (Kilpi et al., 2001; Wiggins et al., 1994), promoting sensitization to repeated exposure to a stressful, yet nonlife-threatening, stimulus, as observed in this study. Consistent with the idea that laying a larger clutch is associated with higher energetic reproductive investment in capital breeders (Hanssen et al., 2003; Jaatinen et al., 2013), females laying larger clutches displayed shorter FIDs. However, despite this initial difference in FID, the responses were homogenized as the number of trials increased and females progressively increased their FID. The effect size of this response was small (estimate = 0.01, 95% CI = [0.00, 0.02]; Table 2), suggesting a limited role of clutch size in shaping adjustment of escape decisions to repeated human approaches.

Intriguingly, early breeders displayed longer FIDs and increased FID over repeated approaches while late breeders displayed shorter FIDs and were more prone to reduce FID. This finding is not in line with the prediction that the higher value of offspring hatching early in the season, associated with a higher parental investment (e.g. in nest defence; Montgomerie & Weatherhead, 1988), would translate into habituation to repeated human approaches (prediction 3). The observed increase in FID over repeated approaches may be linked to the low but still existing renesting opportunities for early breeders, which, however, rapidly diminish as the reproductive window is short in eiders (Hanssen & Erikstad, 2013). This would explain why early breeders would be more prone to becoming sensitized to repeated exposure to a nonlife-threatening yet stressful stimulus than late breeders.

While experienced females displayed shorter FIDs, a result in accordance with previous studies (Candolin, 1998; Clark, 1994; de Jong et al., 2021; Mohring et al., 2022), short-term adjustment of FID to repeated human approaches was not linked to female breeding experience. This result indicates that even though experienced individuals may make a greater investment in current reproduction than less experienced ones (Forslund & Pärt, 1995; Pitera et al., 2021), the ability of breeding adults to constantly assess risk and immediately adjust their antipredator behaviour accordingly may not be age dependent. A similar lack of age-dependent response to repeated human disturbance has previously been
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Relationship between incubating female common eider flight initiation distance over repeated human approaches (trial number) and (a) female body condition at hatching, (b) clutch size, (c) laying date, (d) island-specific adult predation risk, (e) female minimum breeding experience, (f) island type and (g) eagle index. Regression lines depict predicted effects of the two-way interaction between trial number and the explanatory variable of interest. For interactions between trial number and numerical explanatory variables, the solid lines correspond to females with larger values of the explanatory variable of interest (mean±1 SD), the dashed line females with intermediate values of the explanatory variable (mean) and the dotted line females with smaller values of the explanatory variable (mean-1 SD). For the interaction between trial number and island type, the solid and dashed lines depict responses on forested and open islands, respectively.</figcaption></figure>

highlighted in yellow-eyed penguins, *Megadyptes antipodes* (Ellenberg et al., 2009), perhaps reflecting a tendency for seabird stress responses to exhibit only weak age dependence (Angelier et al., 2007).

## Adjustment of Escape Decisions to Perceived Level of Threat

Supporting prediction 4B (the risk allocation hypothesis; Ferrari et al., 2009), we found that females breeding on islands characterized by lower adult predation risk displayed a steeper increase in FID over repeated approaches (i.e. sensitization), compared to

females nesting on islands characterized by higher predation risk. The risk allocation hypothesis postulates that in risky environments where chronic stress is elevated, maintaining high sensitivity to predator cues is costly and can compromise investment in other essential activities (Lima & Dill, 1990; Wingfield & Sapolsky, 2003). Interestingly, empirical examinations of this hypothesis have mostly been limited to trade-offs between resource acquisition and safety (Ferrari et al., 2009; Sirot, 2010), and we here provide some of the first evidence of its extension to the trade-off between reproduction and survival. In support of this hypothesis, prey evolving in dangerous environments have been shown to display
attenuated sensitivity to predators (Fischer et al., 2014; Stankowich & Blumstein, 2005). Individuals exposed to high predation risk are also expected to benefit particularly from distinguishing between life-threatening and inconsequential stimuli, leading to a waning of antipredator responses when confronted with harmless stimuli (Sirot, 2010). Indeed, experimental findings show that cognitive functions and predator recognition are enhanced in riskier environments (Ferrari, 2014). A similar response was not observed when comparing the responses of females breeding under different island-specific nest predation risk. This result is in line with a previous study showing that female eiders plastically adjusted FID across breeding events to predation risk on themselves but not on their nest (Mohring et al., 2022). Differential prey responses may be triggered by predators of adults or offspring (Ghalambor & Martin, 2000; Hua et al., 2014; Malone et al., 2017), and plasticity towards predators of adults may reflect stronger eco-evolutionary pressures posed on traits increasing adult survivorship, and the resultant canalization of survival rates of long-lived species against environmental fluctuations (Gaillard & Yoccoz, 2003; Sæther & Bakke, 2000).

Despite eagles being primary predators of adult female eiders, we did not detect any association between individual adjustment of escape responses to repeated human approaches and eagle abundance, annually calculated at the scale of the entire study area. Two nonexclusive hypotheses could explain this finding. First, although white-tailed eagles are the main predators of incubating eiders in the study area, yearly eagle abundance does not encompass yearly abundance of other predators of adult female eiders (mainly raccoon dogs and American minks, Jaatinen et al., 2022). Accordingly, measuring yearly abundance of American minks and raccoon dogs may provide more insight into eider adjustment of escape responses to repeated approaches. Second, island-specific predation risk indices within the study area are known to fluctuate both temporally (i.e. over years; Mohring et al., 2022) and spatially (i.e. across islands; Mohring et al., 2022). Consequently, annual predation risk indices calculated at the scale of individual islands, rather than the entire study area, may better describe local threat or fine-scale cues of predator presence used by incubating females to assess risk. Similarly, although females displayed longer FIDs on open islands compared to forested islands (Mohring et al., 2022; this study), we did not detect a habitat-specific response to repeated human approaches.

## Conclusions and Perspectives

In this study, we investigated the role of prior experience in modulating prey escape decisions during a breeding event. Females exhibited varied responses when exposed to repeated human approaches over a short period of time, reflecting dynamic and complex decision making. Our study underlined that short-term adjustment of antipredator behaviour to prior experience was sensitive to the trade-off between current and future reproduction and influenced by perception of threat. Despite the evidence for short-term behavioural plasticity (here, over a timescale of days), the magnitude of adjustment of FID over repeated approaches was low and of little biological significance, especially when compared to interindividual variation in FID. This finding casts doubt on the adequacy of adjustment of antipredator responses when individuals are repeatedly exposed to predation threat. If such a limited flexibility in individual responses to repeated human approaches generalizes to other predators, this could increase the vulnerability of ground-nesting species to both alien and native predators (Geldart et al., 2023a, 2023b; Jónsson et al., 2023), especially when predation regimes are rapidly changing. Particularly concerning in light of the limited scope of short-term adjustment of escape responses is the fact that most prey animals

are thought to display similar antipredator responses to humans as to other terrestrial predators (Bar-Ziv et al., 2022; Brilot et al., 2012; Geffroy et al., 2015). Indeed, negative effects of alien mammalian predators have been documented not only on the nest success and survival of eiders, but also on their breeding propensity in the Baltic Sea (Jaatinen et al., 2022). There is, therefore, a pressing need to experimentally evaluate the ability of ground-nesting sea ducks, many of which like the eider are declining (Fox et al., 2015), to plastically adjust their behaviour and life history decisions when presented with taxonomically diverse predators (Barnas et al., 2022; Geldart et al., 2023a, 2023b), in order to better assess the vulnerability of these species to aerial and terrestrial predators alike.

## Author Contributions

**Benjamin Steele:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization. **Bertille Mohring:** Writing – review & editing, Writing – original draft, Visualization, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Frédéric Angelier:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Conceptualization. **Kim Jaatinen:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Markus Öst:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

## Data Availability

Data that support the findings of this study are available as Supplementary material and at https://github.com/BertilleMohring/Common-eider-Habituation-Sensitization.

## Declaration of Interest

The authors declare that there is no conflict of interest.

## Acknowledgments

We acknowledge all the fieldwork assistants who helped us monitor eiders over the study period, with special thanks to Heikki Eriksson, Niilo Aro and Ida Hermansson. We are grateful to Tvärminne Zoological Station for providing facilities. We thank HALIAS for providing the yearly index of eagle abundance and David Pinaud for helpful conversation and advice regarding data analysis. This work was supported by the Swedish Cultural Foundation in Finland (Grant Nos. 17/3317, 16/1476, 15/3296, 14/2657, 13/2654, 138139, 149014, 158026, 168333, 177733 and 188437 to M.Ö.), the Academy of Finland (Grant No. 266208 to K.J. and Grant No. 128039 to M.Ö.), Sophie von Julin Foundation (to K.J.), Finnish Ministry of Agriculture and Forestry (to K.J.).

## Supplementary Material

Supplementary material associated with this article is available, in the online version, at https://doi.org/10.1016/j.anbehav.2024.11.008.

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# Appendix

<caption>**Table A1**</caption>

Table presenting the eight top-ranked zero-inflated Poisson generalized linear mixed models within ΔAICc ≤ 10 explaining variation in incubating female common eider flight initiation distance (FID) in response to repeated human approaches

<table><thead><tr><th></th><th>Fixed effect</th><th>df</th><th>log-lik</th><th>AICc</th><th>ΔAICc</th><th>w<sub>i</sub></th><th>R<sup>2</sup><sub>m</sub></th><th>R<sup>2</sup><sub>c</sub></th></tr></thead><tbody><tr><td>M<sub>1</sub></td><td>Trial nb + Breed exp + BC + Lay date + CS + Eagle + Ad pred + Island type + Breed exp*Trial nb + BC*Trial nb + Lay date*Trial nb + CS*Trial nb + Eagle*Trial nb + Ad pred*Trial nb + Island type*Trial nb</td><td>20</td><td>-9966.93</td><td>19974.28</td><td>0.00</td><td>0.74</td><td>0.30</td><td>0.87</td></tr><tr><td>M<sub>2</sub></td><td>Trial nb + Breed exp + BC + Lay date + CS + Ad pred + Island type + BC*Trial nb + Lay date*Trial nb + Ad pred*Trial nb</td><td>15</td><td>-9974.11</td><td>19978.47</td><td>4.19</td><td>0.09</td><td>0.30</td><td>0.87</td></tr><tr><td>M<sub>3</sub></td><td>Trial nb + Breed exp + BC + Lay date + Eagle + Ad pred + Island type + BC*Trial nb + Lay date*Trial nb + Ad pred*Trial nb</td><td>15</td><td>-9974.54</td><td>19979.32</td><td>5.03</td><td>0.06</td><td>0.30</td><td>0.87</td></tr><tr><td>M<sub>4</sub></td><td>Trial nb + Breed exp + BC + Lay date + CS + Eagle + Ad pred + Island type + BC*Trial nb + Lay date*Trial nb + CS*Trial nb + Eagle*Trial nb + Ad pred*Trial nb + Island type*Trial nb</td><td>19</td><td>-9971.12</td><td>19980.63</td><td>6.35</td><td>0.03</td><td>0.30</td><td>0.87</td></tr><tr><td>M<sub>5</sub></td><td>Trial nb + Breed exp + BC + Lay date + Ad pred + Island type + Breed exp*Trial nb + BC*Trial nb + Lay date*Trial nb + Ad pred*Trial nb + Island type*Trial nb</td><td>16</td><td>-9974.26</td><td>19980.79</td><td>6.51</td><td>0.03</td><td>0.30</td><td>0.87</td></tr><tr><td>M<sub>6</sub></td><td>Trial nb + Breed exp + BC + Lay date + Ad pred + Island type + BC*Trial nb + Lay date*Trial nb + Ad pred*Trial nb</td><td>14</td><td>-9976.41</td><td>19981.03</td><td>6.74</td><td>0.03</td><td>0.30</td><td>0.87</td></tr><tr><td>M<sub>7</sub></td><td>Trial nb + Breed exp + BC + Lay date + CS + Ad pred + Island type + BC*Trial nb + Lay date*Trial nb + CS*Trial nb + Ad pred*Trial nb</td><td>16</td><td>-9975.28</td><td>19982.83</td><td>8.54</td><td>0.01</td><td>0.30</td><td>0.86</td></tr><tr><td>M<sub>8</sub></td><td>Trial nb + Breed exp + BC + Lay date + CS + Ad pred + Island type + BC*Trial nb + Lay date*Trial nb + Ad pred*Trial nb + Island type*Trial nb</td><td>16</td><td>-9975.58</td><td>19983.43</td><td>9.15</td><td>0.01</td><td>0.30</td><td>0.87</td></tr></tbody></table>

Fixed effects were trial number, female minimum breeding experience, body condition at hatching, initial clutch size, laying date, breeding habitat (open or forested island), island-specific adult predation risk and eagle index, as well as the two-way interactions between trial number and female minimum breeding experience, body condition at hatching, initial clutch size, laying date, breeding habitat (open or forested island), island-specific adult or nest predation risk and eagle index. Breeding attempt identity, individual identity and year were included as random intercept effects in all models. 'Trial nb': trial number; 'Breed Exp': female minimum breeding experience; 'BC': female body condition; 'Lay date': laying date; 'CS': clutch size; 'Eagle': eagle index; 'Ad pred': island-specific adult predation risk; 'Nest pred': island-specific nest predation risk; log-lik: log-likelihood; wᵢ: Akaike weight; R²ₘ: marginal R², representing the variance explained by fixed factors; R²ᶜ: conditional R², representing the variance explained by both fixed and random effects.

<caption>**Table A2**</caption>

The variance explained by the fixed effects (marginal R², R²ₘ) of the model excluding or including trial number and the two-way interactions between trial number and female minimum breeding experience, body condition, clutch size, laying date, island type and island-specific adult predation risk

<table><thead><tr><th>Model</th><th>Fixed effect structure</th><th>R<sup>2</sup><sub>m</sub></th><th>R<sup>2</sup><sub>c</sub></th></tr></thead><tbody><tr><td>RM<sub>Excluding</sub></td><td>FID ~ Breed exp + BC + Lay date + CS + Eagle + Ad pred + Island type</td><td>0.296</td><td>0.851</td></tr><tr><td>M<sub>Including</sub></td><td>FID ~ Breed exp + BC + Lay date + CS + Eagle + Ad pred + Island type + Trial nb + Breed exp*Trial nb + BC*Trial nb + Lay date*Trial nb + CS*Trial nb + Eagle*Trial nb + Ad pred*Trial nb + Island type*Trial nb</td><td>0.300</td><td>0.874</td></tr></tbody></table>

'Trial nb': trial number; 'Breed Exp': female minimum breeding experience; 'BC': female body condition; 'Lay date': laying date; 'CS': clutch size; 'Eagle': eagle index; 'Ad pred': island-specific adult predation risk. Breeding attempt identity, individual identity and year were included as random intercept effects in all models. R²ₘ: marginal R², representing the variance explained by fixed factors; R²ᶜ: conditional R², representing the variance explained by both fixed and random effects.
<figure><img src="image_7.png" /><figcaption>**Figure A1.** Correlation plot of the continuous explanatory variables tested in the zero-inflated Poisson generalized linear mixed models. The variables include trial number, eagle index, clutch size, female body condition at hatching, island-specific adult predation risk, female minimum breeding experience, island-specific nest predation risk and laying date.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

**Figure A2.** Relationship between incubating female common eider flight initiation distance over repeated human approaches (trial number) and (a) female body condition at hatching, (b) clutch size, (c) laying date, (d) island-specific adult predation risk, (e) female minimum breeding experience, (f) island type and (g) eagle index. Dots show the raw data. Regression lines depict predicted effects of the two-way interaction between trial number and the explanatory variable of interest. For interactions between trial number and quantitative explanatory variable, the solid lines correspond to females with larger values of the explanatory variable of interest (mean+1 SD), the dashed line females with intermediate values of the explanatory variable (mean) and the dotted line females with smaller values of the explanatory variable (mean - 1 SD), and shaded areas are 95% confidence intervals. For the interaction between trial number and island type, the solid and dashed lines, with 95% confidence intervals, depict responses on forested and open islands, respectively.