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Avian nest design varies depending on environmental factors but may also be influenced by between‐species interactions. In the brood parasitism context, hosts may evolve nest architectures that may limit parasite access to the nest cup, reduce parasite laying success or hinder parasite chick success. Therefore, nest characteristics may reduce the likelihood or minimise the costs of being parasitised. The common redstart Phoenicurus phoenicurus is a regular host of the common cuckoo Cuculus canorus, for which cuckoo eggs are often laid outside the nest cup, resulting in low effective parasitism rates. This allowed us to evaluate variation in host nest design and test whether nest design characteristics correlate with brood parasitism likelihood and cuckoo laying success (i.e. cuckoo egg laid in the nest cup versus outside the nest cup). While recording brood parasitism events in two distant redstart populations, we documented nest cup characteristics, such as internal dimensions, materials used and nest cup position, along with the nest‐box dimensions. Cuckoo parasitism likelihood was lower for redstart nests in cavities with smaller entrances, for redstart nests with smaller nest cups and with nest cups that were built level to the rim material. For parasitised nests, cuckoo laying success was lower at redstart nests with nest cups placed further from the cavity entrance. Our results suggest a conditional process, where the cavity entrance size first prevents brood parasites access, then the cup size and the cup level in reference to the rim material affect the cuckoo choice, and finally, the nest cup position hinders cuckoo's laying success. The use of multiple nest design strategies may explain the current low effective parasitism rates in this system. Host nest design may serve as a frontline defence that could shape parasite's preferences, and consequently host nest characteristics.

Breeding habitat choice and investment decisions are key contributors to fitness in animals. Density of individuals is a well-known cue of habitat quality used for future breeding decisions, but accuracy of density cues decreases as individuals disperse from breeding sites. Used nests remain an available information source also after breeding season, but whether such information is used for breeding decisions is less well known. We experimentally investigated whether migratory, cavitynesting pied flycatchers (Ficedula hypoleuca) prospect potential breeding sites after breeding season and use old nests as a cue for future breeding decisions. In late summer 2013, forest sites were assigned to four treatments: (1) sites including nest boxes with old nests of heterospecifics (tits), (2) sites including suitable but empty nest boxes, (3) sites with unsuitable nest boxes, or (4) sites without any nest boxes. In the following year, we investigated pied flycatcher habitat choice and reproductive investment according to these “past” cues while also controlling for additional information sources present during settlement. Flycatchers preferred sites where tits had been perceived to breed in the previous year, but only if great tits were also currently breeding in the site and had a relatively high number of eggs. Old flycatchers avoided sites previously treated with suitable but empty cavities, whereas young flycatchers preferred sites where tits had apparently bred in the previous year. Also egg mass, but not clutch size or clutch mass, was affected by the combination of past treatment information and current tit abundance.

Being an obligate parasite, juvenile common cuckoos Cuculus canorus are thought to reach their African wintering grounds from Palearctic breeding grounds without guidance from experienced conspecifics but this has not been documented. We used satellite tracking to study naïve migrating common cuckoos. Juvenile cuckoos left breeding sites in Finland moving slowly and less consistently directed than adult cuckoos. Migration of the juveniles (N = 5) was initiated later than adults (N = 20), was directed toward the southwest-significantly different from the initial southeast direction of adults-and included strikingly long Baltic Sea crossings (N = 3). After initial migration of juvenile cuckoos toward Poland, the migration direction changed and proceeded due south, directly toward the winter grounds, as revealed by a single tag transmitting until arrival in Northwest Angola where northern adult cuckoos regularly winter. Compared to adults, the juvenile travelled straighter and faster, potentially correcting for wind drift along the route. That both migration route and timing differed from adults indicates that juvenile cuckoos are able to reach proper wintering grounds independently, guided only by their innate migration programme.

Brood parasite – host systems continue to offer insights into species coevolution. A notable system is the redstart Phoenicurus phoenicurus parasitized by the ‘redstart‐cuckoo’ Cuculus canorus gens. Redstarts are the only regular cuckoo hosts that breed in cavities, which challenges adult cuckoos in egg laying and cuckoo chicks in host eviction. We investigated parasitism in this system and found high overall parasitism rates (31.1% of 360 redstart nests), but also that only 33.1% of parasitism events (49 of 148 eggs) were successful in laying eggs into redstart nest cups. The majority of cuckoo eggs were mislaid and found on the rim of the nest; outside the nest cup. All available evidence suggests these eggs were not ejected by hosts. The effective parasitism rate was therefore only 12.8% of redstart nests. Redstarts responded to natural parasitism by deserting their nests in 13.0% of cases, compared to desertion rates of 2.8% for non‐parasitized nests. Our egg parasitism experiments found low rates (12.2%) of rejection of artificial non‐mimetic cuckoo eggs. Artificial mimetic and real cuckoo eggs added to nests were rejected at even lower rates, and were always rejected via desertion. Under natural conditions, only 21 cuckoo chicks fledged of 150 cuckoo eggs laid. Adding to this low success, is that cuckoo chicks are sometimes unable to evict all host young, and were more likely to die as a result compared to cuckoo chicks reared alone. This low success seems to be mainly due to the cavity nesting strategy of the redstart which is a challenging obstacle for the cuckoo. The redstart‐cuckoo system appears to be a fruitful model system and we suggest much more emphasis should be placed on frontline defences such as nest site selection strategies when investigating brood parasite–host coevolution.

Background Breeding site choice constitutes an important part of the species niche. Nest predation affects breeding site choice, and has been suggested to drive niche segregation and local coexistence of species. Interspecific social information use may, in turn, result in copying or rejection of heterospecific niche characteristics and thus affect realized niche overlap between species. We tested experimentally whether a migratory bird, the pied flycatcher Ficedula hypoleuca , collects information about nest predation risk from indirect cues of predators visiting nests of heterospecific birds. Furthermore, we investigated whether the migratory birds can associate such information with a specific nest site characteristic and generalize the information to their own nest site choice. Results Our results demonstrate that flycatchers can use the fate of heterospecific nesting attempts in their own nest site choice, but do so selectively. Young flycatcher females, when making the decision quickly, associated the fate of an artificial nest with nest-site characteristics and avoided the characteristic associated with higher nest predation risk. Conclusions Copying nest site choices of successful heterospecifics, and avoiding choices which led to failed attempts, may amplify or counter effects of nest predation on niche overlap, with important consequences for between-species niche divergence-convergence dynamics, species coexistence and predator-prey interactions.

Food-supplementation experiments conducted on terrestrial vertebrates have usually shown the importance of food as an agent in increasing reproductive success. However, to date, the number of large-scale food supplementation experiments carried out on top predators has been limited. The Eurasian Eagle Owl (Bubo bubo) is the largest nocturnal avian predator in the boreal and temperate regions of Eurasia. Here, we study the diet composition and breeding success of this top predator in relation to the location and distance to rubbish dumps, which sustain large numbers of brown rats Rattus norvegicus, one of the main prey items for owls. The number of these rubbish dumps (hereafter dumps) increased in Finland during the period between the 1960s and the 1990s, whereafter their number was steeply reduced, leading to a remarkable decline in rat populations. A similar trend in the Eagle Owl population largely followed this development, raising the question of whether this country-wide \"supplementary feeding experiment\" was behind this phenomenon. The proportion of rats in the diet of the owls was high, on average, 70% of prey items near the dumps decreasing to around 10-15% at a 6-7 km distance from the dumps. Correspondingly, the number of 3-4-week-old offspring declined in relation to the distance from the dump, so that 0.5 less chicks were recorded at a distance of 10 km from the dump. During the study, the overall density of nests near the dumps was 2.7 times higher than those recorded further away. The availability of extra food material within the dumps (and rats therein) probably induced an increase in the Finnish Eagle Owl population, which saw a decline after dumps were closed.

Food-supplementation experiments conducted on terrestrial vertebrates have usually shown the importance of food as an agent in increasing reproductive success. However, to date, the number of large-scale food supplementation experiments carried out on top predators has been limited. The Eurasian Eagle Owl (Bubo bubo) is the largest nocturnal avian predator in the boreal and temperate regions of Eurasia. Here, we study the diet composition and breeding success of this top predator in relation to the location and distance to rubbish dumps, which sustain large numbers of brown rats Rattus norvegicus, one of the main prey items for owls. The number of these rubbish dumps (hereafter dumps) increased in Finland during the period between the 1960s and the 1990s, whereafter their number was steeply reduced, leading to a remarkable decline in rat populations. A similar trend in the Eagle Owl population largely followed this development, raising the question of whether this country-wide \"supplementary feeding experiment\" was behind this phenomenon. The proportion of rats in the diet of the owls was high, on average, 70% of prey items near the dumps decreasing to around 10–15% at a 6–7 km distance from the dumps. Correspondingly, the number of 3–4-week-old offspring declined in relation to the distance from the dump, so that 0.5 less chicks were recorded at a distance of 10 km from the dump. During the study, the overall density of nests near the dumps was 2.7 times higher than those recorded further away. The availability of extra food material within the dumps (and rats therein) probably induced an increase in the Finnish Eagle Owl population, which saw a decline after dumps were closed.

Breeding habitat choice based on the attraction to other species can provide valuable social information and protection benefits. In birds, species with overlapping resources can be a cue of good quality habitats; species with shared predators and/or brood parasites can increase joint vigilance or cooperative mobbing, while raptors may provide a protective umbrella against these threats. We tested whether the migratory common redstart (Phoenicurus phoenicurus) is attracted to breed near active nests of the great tit (Parus major), a keystone-information source for migrant passerine birds, or a top predator, the northern goshawk (Accipiter gentilis). This system is unique to test these questions because the redstart is a regular host for the common cuckoo (Cuculus canorus). Therefore, we also evaluated other possible benefits coming from the heterospecific attraction, especially in terms of reducing brood parasitism risk. We monitored redstart occupancy rates, onset of breeding, reproductive investment, and followed nest outcomes in terms of brood parasitism, nest predation risk and overall reproductive success. Redstarts avoided breeding near goshawks, but showed neither attraction nor avoidance to breed next to great tits. Both neighbours neither reduced brood parasitism risk nor affected overall nesting success in redstarts. Redstarts may not use heterospecific attraction for settlement decisions, as associations with other species can only exist when some benefits are gained. Thus, environmental cues may be more important than social information for redstarts when breeding habitat choice. Other front-line defence strategies may have a better impact reducing breeding negative interactions, such brood parasitism.

Brood parasitism incurs severe fitness costs for hosts. Diverse host adaptations to avoid parasitism exist at various stages of the host breeding cycle, but the literature suggests that egg-stage defenses are the most evolved. Fitness costs of parasitism would be minimized if hosts avoided parasitism prior to egg laying, but it remains unclear whether hosts are able to reduce parasitism risk via informed habitat choices. We conducted a playback experiment to examine the capability of forest passerine birds to perceive vocal cues of the Common Cuckoo (Cuculus canorus) to estimate local parasitism risk and adjust their breeding-habitat choices accordingly. The density of open-nesting host species was considerably lower in sites of high perceived parasitism risk than in control sites. Hosts thus seem able to reduce parasitism risk through informed breeding-habitat choices, but this behavior is restricted to open-nesting hosts. Cavity-nesting hosts did no+t respond to the simulated increase in parasitism risk, nor did bird species that are not known as regular Common Cuckoo hosts. Informed habitat selection as an adaptation against parasitism may have important implications for parasite–host coevolutionary interactions, via reduced selection for later-stage host adaptations, and for bird community structure in general.

Social information use for decision-making is common and affects ecological and evolutionary processes, including social aggregation, species coexistence, and cultural evolution. Despite increasing ecological knowledge on social information use, very little is known about its genetic basis and therefore its evolutionary potential. Genetic variation in a trait affecting an individual’s social and nonsocial environment may have important implications for population dynamics, interspecific interactions, and, for expression of other, environmentally plastic traits. We estimated repeatability, additive genetic variance, and heritability of the use of conspecific and heterospecific social cues (abundance and breeding success) for breeding site choice in a population of wild collared flycatchers Ficedula albicollis. Repeatability was found for two social cues: previous year conspecific breeding success and previous year heterospecific abundance. Yet, additive genetic variances for these two social cues, and thus heritabilities, were low. This suggests that most of the phenotypic variation in the use of social cues and resulting conspecific and heterospecific social environment experienced by individuals in this population stems from phenotypic plasticity. Given the important role of social information use on ecological and evolutionary processes, more studies on genetic versus environmental determinism of social information use are needed.