The importance of chemical communication in different contexts of an organism's life has been extensively studied for a great number of taxa, including invertebrates (Dicke & Grostal 2001) and vertebrates such as reptiles or mammals (Kats & Dill 1998). However, in the case of birds it has been historically neglected (Kats & Dill 1998). Nowadays, it has been shown that olfaction may be a more important sense in birds than was traditionally believed (Steiger et al. 2008). Recent evidence suggests that birds can use odours in several ecological contexts and with different functions (Roper 1999, Hagelin 2007, Hagelin & Jones 2007, Rajchard 2007, 2008, Balthazart & Taziaux 2009, Caro & Balthazart 2010). At the intra-specific level, they are known to emit chemical compounds, which are important in several aspects of avian life histories (Caro & Balthazart 2010). For example, birds can recognize their nest using chemical cues (Mínguez 1997, O'Dwyer et al. 2008, Bonadonna & Bretagnolle 2002, Bonadonna et al. 2003a,b, 2004, de Leon et al. 2003, Caspers & Krause 2011) and they are able to discriminate the scent of their partners from the scent of other conspecifics (Bonadonna & Nevitt 2004, Jouventin et al. 2007). Therefore, chemical cues may play a role in social behaviour (Hagelin 2007). Chemical cues appear to also be useful in the relationship between birds and their environment. For example, birds can use the sense of smell to discriminate aromatic plants (Petit et al. 2002, Mennerat et al. 2005, Gwinner & Berger 2008), to orientate and navigate (Bonadonna et al. 2004, Wallraff 2004, Nevitt & Bonadonna 2005) and to forage (Hutchison & Wenzel 1980, Nevitt et al. 1995, Marples & Roper 1996, Kelly & Marples 2004, Cunningham et al. 2008).

The ability to use chemical cues to ascertain predator presence has been documented in a great number of taxa (Apfelbach et al. 2005), but scarcely in birds (Kats & Dill 1998, Hagelin 2007, but see Amo et al. 2008, Roth et al. 2008). However, using chemical cues to assess predation risk could be important in birds, especially in species that use habitats where visual detection of predators is impaired (Amo et al. 2008). This is the case in hole-nesting birds such as the Great Tit Parus major, that use cavities for breeding and roosting, where they may encounter predators such as mustelids. The chemical cues of predators provide a first assessment of predation risk. This may be especially important when survival depends on ascertaining predator presence and avoiding an encounter with a predator. In this case, natural selection may favour the innate detection of predator chemical cues. Animals may also have the ability to learn to detect and recognize chemical cues of predators, since this ability may be important when confronted with new predators. Naïve animals of several species are known to discriminate the chemical cues of their predators (van Damme et al. 1995, Dalesman et al. 2007, Sundermann et al 2008). However, whether the recognition of predator chemical cues in birds is innate or learned remains unknown. To our knowledge, the only contexts where the ability to recognize and respond to chemical cues has been proved to be innate in birds is in a foraging context (Bonadonna et al. 2006) and in the discrimination of the aromatic plants that some species introduce in the nests (Gwinner & Berger 2008).

Although recognition of the chemical cues of predators may confer an advantage to prey as it allows an early assessment of predation risk, it can also lead to an overestimation of risk if the animal continues to avoid an area when the predator is no longer present (Kats & Dill 1998). Therefore, once an animal has detected predator chemical cues, it should evaluate the ‘real’ risk of predation that these cues are signalling, to perform an adequate anti-predatory response. Furthermore, the response to predator chemical cues may be the result of the trade-off between the anti-predator responses appropriate to the risk perceived and other requirements such as foraging, reproduction (Lima & Dill 1990, Sih 1992) or the avoidance of costs of the antipredatory response itself (Ydenberg & Dill 1986, Sih 1997). Changes in cost-benefits may explain variation in the anti-predatory response of the same individual, as well as differences between individuals differing in sexes or ages that may have a different solution to the trade off between avoiding predation and other requirements (Lima & Dill 1990). However, individual animals often behave in a way that distinguishes them from other members of their species of the same sex, state and age class, and understanding the source of this variation is fundamental to evolutionary studies (Bennett 1987, Wilson 1998).

Individuals of many animal species are known to differ consistently in several aspects of their behaviour, such as aggressiveness, activity, exploration, risk-taking, fearfulness and reactivity (Wilson et al. 1994, Gosling & John 1999, Réale et al. 2007). Suites of these consistent traits are also referred to as animal personality, behavioural syndromes, coping styles or temperament (Koolhaas et al. 1999, Sih et al. 2004, Groothuis & Carere 2005, Réale et al. 2007) and are comparable to how humans differ in personality.

Here, we explore the use of chemical cues of predators for predation risk assessment when selecting cavities for roosting by a hole-nesting songbird, the Great Tit. Firstly, we aimed to determine whether the ability to detect predator chemical cues for roosting selection is innate in birds. Roosting may be an important behaviour for overwinter survival, as one of the benefits of sleeping in a cavity is a decrease in energetic costs during cold nights (Walsberg 1986, Webb & Rogers 1988). Birds actively select warmer and thermally more stable roosting places (Velky et al. 2010). Furthermore, birds also avoid roosting in cavities containing the signals of predators (mammal fur and mangled feathers) (Ekner & Tryjanowski 2008). Roosts also help to decrease the risk of predation by owls, but increase the risk of predation by mammals such as mustelids (Dhondt et al. 2010). Differences in the costs and benefits of roosting in different populations may have exerted different selection pressures that have led to population differences in the use of cavities during the night (Dhondt et al. 2010). Dhondt et al. (2010) showed that Blue Tits Cyanistes caeruleus from populations where owls are scarce and mammals abundant did not use cavities for roosting, while birds from populations where the relative abundances of predators is the opposite did. Therefore, if natural selection favours the use of cavities while the costs of this behaviour are lower than its benefits, it may also favour the ability to assess the costs of roosting, such as the innate detection of chemical cues of predators. Thus, we hypothesized that the response to predator chemical cues may be innate in birds, as it may be subjected to strong selection. We expect this because mustelid predators can be hidden in cavities where visual detection is difficult, and once the bird has entered a cavity containing a predator survival is unlikely. Secondly, we aimed to examine whether the personality of individuals influences the response to predator chemical cues in order to understand individual variability in anti-predatory behaviour. In the Great Tit, birds from wild populations can be placed along an axis ranging from slow to fast explorers in a novel environment (Verbeek et al. 1994, Drent et al. 2003). This is correlated with differences in their reaction to novel objects (Verbeek et al. 1994), aggressiveness (Verbeek et al. 1996), recovery time and behaviour after lost contests (Verbeek et al. 1999), foraging behaviour (Drent & Marchetti 1999, Marchetti & Drent 2000) and reactions to stress (Carere & van Oers 2004, Fucikova et al. 2009). Artificial selection combined with cross fostering resulted in clear evidence for a genetic basis of these traits (heritabilities between 20 and 50%; Drent et al. 2003, van Oers et al. 2004b) and in natural populations variation in personality was found to be under natural (Dingemanse & Réal 2005) and sexual selection (van Oers et al. 2008). The personality of birds is also related to the response of birds to a risky situation, with fast Great Tits being more prone to return to a risky feeding place than slow individuals (van Oers et al. 2004a). Therefore, we hypothesize that birds of different personality types may also differ in their anti-predatory behaviour when exposed to predator chemical cues, with slow birds exhibiting a greater anti-predatory response than fast birds.

METHODS

Study species

We used 20 hand reared captive Great Tits (8 females and 12 males) belonging to the 4th generation of a bidirectional artificial selection program for fast and slow (7 fast and 13 slow) exploratory behaviour (for details see Drent et al. 2003). All birds were 1 year old. Birds used in the experiments were naïve to predator odour, as they were maintained in captivity since 10 days after hatching. Furthermore, nestboxes where nestlings were reared during the first 10 days of their life were frequently inspected and neither predation events nor signals of predator visits were detected.

Birds were housed individually in cages of 0.9 m (L) × 0.4 m (W) × 0.5 m (H), with wooden bottom, top, side, and rear walls, a wire-mesh front, and three perches. The bottom was covered with sawdust. They were kept under natural winter daylight augmented with fluorescent light tubes and provided with ad libitum water, sunflower seeds, a commercial dry mixture (proteins, trace elements, minerals, and vitamins), a fresh mixture of raw heart and live mealworms. A week before the experiment, the Great Tits were released into the experimental aviary individually for one hour to decrease stress due to novelty of the environment during the experiment. Birds did not exhibit any sign of stress and were healthy during the experiment.

Figure 1.

The aviary where the experiment was carried out.

RESULTS

There were no differences in the behaviour of males and females in any of the variables considered (P > 0.05 in all cases), or in the number of males and females that slept outside the nestbox or inside any nestbox in the cologne (Chi-squared test: χ²₁ = 1.20, P = 0.27) and predator treatments (χ²₁ = 0.59, P = 0.44). The sexes did not differ in their choice of nestbox in the cologne (χ²₁ = 0.03, P = 0.86) and predator treatments (χ²₁ = 1.89, P = 0.17). Therefore, we did not include the sex of birds in subsequent analyses.

During the first hour of observed behaviour, all birds visited at least one of the nestboxes. More birds approached the experimental nestbox before the control nestbox when the experimental box contained predator scent (12 vs. 5) compared to when the experimental box contained cologne scent (7 vs. 13) (lmer: χ²₁ = 4.65, P = 0.03). There were no significant differences between personality types (lmer: χ²₁ = 0.54, P = 0.46), and also the interaction was not significant (lmer: χ²₁ = 1.98, P = 0.16). The proportion of visits to the experimental nestbox did not differ between treatments (lmer: χ2₁ = 1.52, P = 0.22) or personality type (lmer: χ²₁ = 0.14, P = 0.71). The interaction was also not significant (lmer: χ²₁ = 1.69, P = 0.19).

When one of the nestboxes contained predator scent, more birds did not use any nestbox and slept outside than when one of the nestboxes contained cologne (lmer: χ²₁ = 17.86, P < 0.0001; Table 1). There were no differences in this behaviour associated with the personality of the birds (lmer: χ²₁ = 0.02, P = 0.90) and also the interaction between treatment and personality was not significant (lmer: χ²₁ = 0.10, P = 0.75), indicating that birds with different personality types did not react differently to the presence of predator scent.

Within birds that used a nestbox, there was no difference in the number of birds that slept inside the control or the experimental nestbox between treatments (lmer: χ²₁ = 1.18, P = 0.28; Table 1), neither did personality affect this choice (lmer: χ²₁ = 2.81, P = 0.09). The interaction between treatment and personality was not significant (lmer: χ²₁ = 1.35, P = 0.25), indicating that birds of different personality did not differ in their reaction to the treatment.

Table 1.

Number of birds (by type, ‘fast’ or ‘slow’ birds) that spent the night in the control nest-box, experimental nest-box or outside any nest-box in the ‘Cologne’ or ‘Predator’ treatments.

DISCUSSION

Our results provide evidence that birds can detect the chemical cues of predators and use them to assess the level of predation risk while selecting cavities to roost in. When there was predator scent in one of the nestboxes, more birds slept outside a nestbox. These results agree with the experiment of Ekner & Tryjanowski (2008) in natural conditions that shows that small hole nesting passerines, mainly Great Tits, prefer to roost in control nestboxes compared to nestboxes containing signals of predators and predation (mammal fur and mangled feathers). Our results are also in accordance with previous studies on Blue Tits (Amo et al. 2008) and House Finches (Roth et al. 2008), which show that birds can detect and use chemical cues of predators in a reproductive and a foraging context, respectively. We did not find any significant effect of the cologne treatment on choosing a nestbox for roosting, despite the fact that birds are known to exhibit aversive responses to unknown odours (Jones et al. 2002; reviewed in Roper 1999). Therefore, this suggests that birds are responding to specific predator chemical cues but not to a new scent or a non-biological scent, comparable to previous results using different controls such as bird scent (Amo et al. 2008) or non-predatory mammal scent (Roth et al. 2008). However, we did not find differences in the use of nestboxes between control and experimental nestboxes, which suggests that birds perceived the whole aviary as a risky area. This result may be explained because the minimum home range of the European Polecat is 60 hectares (Blanco 1998), and therefore, the presence of predator chemical cues in one of the nestboxes may indicate an elevated risk of predation at both boxes inside the aviary. Our results provide the first evidence that the chemical detection of predators is innate in birds, as it has been previously found in other taxa (Turner et al. 2006, Sundermann et al. 2008).

Our results show that individual differences in the response to predator chemical cues could not be explained by the personality of the individual. This is despite the results of a previous study that showed that, in a foraging context, the personality of birds helped to explain individual differences in anti-predatory behaviour, with slow Great Tits returning later to a feeding place after being startled than fast individuals (van Oers et al. 2004a). Birds were released in the aviary at least one and a half hours before sunset, when they usually explore potential sites for roosting (Veký et al. 2010) and therefore, they had time to assess the risk of predation by using chemical cues and to respond to it. Furthermore, more birds approached the experimental nestbox first when it contained predator chemical cues than when it contained a control scent; i.e. they inspected the predator-scented nestboxes before nestboxes containing a control scent. They thereby perched on the hole of the nestbox, looking around and inside. This inspecting behaviour may allow birds to identify the odour source and assess predator presence. This predator inspection behaviour has been described in many prey fish (Brown & Dreier 2002), but birds are also known to approach predators during nest defence (Regelmann & Curio 1983). This result indicates that Great Tits were able to detect the chemical cues of predators from outside the nestbox, in agreement with previous findings (Amo et al. 2008). Also, after the first visit, both cologne and predator scented nestboxes were visited similarly. The personality of birds did not influence which nestbox was visited for the first time and the proportion of visits to the experimental nestboxes or the choice between either sleeping inside or outside one of the nestboxes. However, we may have incurred Type II error due to the low sample size of each personality type used in the experiment, therefore, the low power of our test in relation to personality does not allow us to draw strong conclusions. Thus, the factors affecting the anti-predatory behaviour of birds that perceived the chemical cues of predators remain unclear. Further research is needed to try to disentangle the causes of individual variation in the anti-predatory responses of birds to predator chemical cues.