Birds are known to be able to actively shape their environment through their choices of habitat, behaviour or metabolic activities. Life-history theory suggests that parasites can generate selection pressures by affecting host health and, in natural habitats, Eurasian blue tits, Cyanistes caeruleus , are known for incorporating fresh herbal plants that contain volatile compounds into their nests to avoid parasites and their effects. In contrast, urban birds often come across resources not found in natural environments, including materials present in urban waste. There have been increasing reports that blue tits living in cities bring cigarette butts into their nests or build nests in cigarette ashtrays and butt bins. Tobacco, Nicotiana spp., is often cited as an efficient insect repellent and insecticide. In this study, we experimentally tested whether the inclusion of cigarette butts in nests (treatment 1), or replacing a natural nest with a sterilized, artificial, moss and cotton wool nest on the 5th and 10th day of the nestling period (treatment 2), affects components of nestling blue tit fitness: physiological condition, as indicated by blood levels of haemoglobin, glucose, haematocrit and body condition index (measured as the length of the wing of 13-day-old nestlings), in comparison with a control group. After the experimental procedures ended, we also collected all nest materials to extract, identify and count arthropod ectoparasites in the three treatment groups. Nestlings from broods supplemented with cigarette butts and nestlings in broods with artificial nests had significantly elevated haemoglobin and haematocrit, indicating improved physiological condition compared to the control group. The abundance of arthropods (ticks, mites, fleas and blowflies) in the nesting material varied depending on the experimental treatment, but, overall, ectoparasites were most numerous in natural nests, slightly less abundant in nests with cigarette butt addition and nearly absent in sterile nests. A reduction in blowfly larvae in nests with cigarette butts was observed compared with controls but this effect was only nearly significant. • Cigarette butts and sterile nests improved nestling blood parameters. • Sterile nests contained significantly fewer ectoparasites than natural nests. • Cigarette butts only marginally reduced blowfly and flea abundance.

Many predators avoid aposematic prey that possess conspicuous warning displays. This aversion can include innate and learned components, either of which could be exploited by profitable Batesian mimics that resemble aposematic prey. The perceptual bias hypothesis proposes that predators’ unlearned aversions may not correspond exactly with the patterns of aposematic prey, and that inaccurate Batesian mimics gain a fitness advantage by maximizing the unlearned aversion of their predators rather than their similarity to the model. This would provide a particular advantage when faced with naïve predators that have not yet had the opportunity to learn an association between the warning patterns and defences of the prey they encounter. We tested this hypothesis in the context of hoverflies that mimic stinging Hymenoptera: do naïve predators avoid wasp-like stimuli, and, if so, is this bias stronger when encountering inaccurate mimics? We presented domestic chicks, Gallus gallus domesticus, with arrays of 3D-printed stimuli each of which concealed a mealworm, Tenebrio molitor. Each array included control nonmimetic flies and alternative warning-coloured stimuli drawn from a set of wasps and honey bees (models), hoverflies (mimics) and artificially generated intermediates. The chicks had never encountered insect-like prey or stimuli (other than mealworms) prior to these trials. We found no difference in the order in which the stimuli were attacked in different treatments, but did find that chicks were slightly slower to open dishes with aposematic as opposed to control stimuli. Our results do not provide strong support for the perceptual bias hypothesis in the case of hoverflies, but we do find evidence of variation in the unlearned response towards insect stimuli dependent on appearance. These differences illustrate the importance of defensive signals, both honest and deceptive, in influencing predator behaviour even from the first encounter.

Territorial behaviour can facilitate the defence of resources but becomes disadvantageous when resources are mobile and move beyond territory boundaries. In such cases, animals may develop strategies to retain access to these resources within their territory. Here, we describe a novel behaviour in the territorial Mediterranean rainbow wrasse, Coris julis , which joins producer–scrounger aggregations with a goatfish species, Mullus surmuletus , that disturbs benthic prey during foraging. Wrasse remained within their territories and did not follow producers beyond these boundaries but repeatedly engaged in tactile interactions (brief physical contact using their ventral side and pelvic fins or caudal fin) with goatfish while they were present. Events involving tactile interactions were associated with longer feeding durations of goatfish and therefore, indirectly, a higher overall food uptake for wrasse. During these events, wrasse also had increased rates of food uptake, possibly due to the close proximity to the producer and the disturbed prey. In general, feeding with goatfish was beneficial for wrasse when compared to foraging alone. Although goatfish feeding durations were longer when tactile interactions occurred, their movement patterns through the environment were unaffected by the presence or tactile contact of wrasse. While a causal relationship between tactile contacts and length of feeding time could not be established, the underlying mechanism may involve a physiological effect on goatfish, such as by reducing stress or functioning as a reassuring safety cue. Our findings describe a novel example of tactile interactions in a marine system and may suggest a behavioural strategy that could increase access to food resources within territorial boundaries, ultimately mitigating a cost of territoriality. • Territorial scroungers show tactile interactions with heterospecific food-producers. • Tactile interactions were associated with longer feeding aggregations. • Scroungers did not follow mobile producers beyond their territorial boundaries. • Scrounging with producers resulted in higher food uptake than foraging alone. • Tactile contact further increased food uptake rates, likely due to close proximity.

Environmental stressors profoundly influence how animals learn and remember, yet different types of stress can trigger distinct behavioural and molecular outcomes. Understanding this is essential for decoding adaptive responses in ecological settings and also for informing translational models of stress-related cognitive dysfunction. The pond snail Lymnaea stagnalis offers a powerful platform for such investigations. Despite their simplicity compared to mammals, they exhibit higher-order associative learning, such as the Garcia effect, where a novel taste cue, when associated over a time gap (which can be hours) with sickness (induced by acute heat shock), causes the snails to avoid the novel food. The snails also show configural learning, which occurs when a positive (food cue) and negative stimulus (predator odour) are presented simultaneously, leading to the formation of a ‘fear landscape’, and the consequent avoidance of the positive food cue. In this study, we extend these existing paradigms by using biologically relevant stressors: a predator-attack-mimicking shell clip (ShC) and acute heat shock (HS). We found striking, stressor-specific differences in behavioural outcomes: ShC supported configural learning but not the Garcia effect, while HS induced the Garcia effect but failed to support configural learning. Along with behaviour, we show distinct transcriptional changes in the central ring ganglia, affecting expression levels of key targets including stress- and plasticity-related genes, serotonergic markers and enzymes of the endocannabinoid system. Together, these results suggest that different stressors establish unique internal stress states (‘fear’ versus ‘sickness’) that selectively shape learning pathways. This study highlights a critical principle in neuroscience: stress is not a monolith, and its impact on learning and memory depends on both its nature and the internal state it evokes. This finding holds direct relevance for understanding stress-related psychiatric disorders and maladaptive behaviours. • Stressor type determines if snails form configural learning or the Garcia effect. • Shell clip enables configural learning and heat shock supports the Garcia effect. • Stressors produce distinct transcriptional profiles in central ring ganglia. • Changes involve stress-, plasticity-, serotonin- and endocannabinoid-related genes. • The impact of stress on memory depends on the internal state produced.