Nocturnal playbacks reveal hidden differences in singing behaviour between populations of Robin Erithacus rubecula

Capsule Birds subjected to playbacks of conspecific song responded strongly by day in both Wales and Ireland, and strongly at night only in Wales.

Aims To test the responses of Robins to conspecific playbacks between Wales and Ireland and between day and night.

Methods We measured the responses of Robins to conspecific playbacks during daylight, and at night on territories illuminated by streetlights, and compared Robins in Wales and in Ireland.

Results Most Welsh Robins responded to playbacks at night, whereas Irish Robins rarely responded. Both populations responded equally during daylight.

Conclusion We show for the first time that different populations with similar daytime behaviours may differ in nocturnal behaviour. Such population differences in communication behaviour should be taken into account, for example when using acoustic playback to census populations.     

Territory structure of the Robin Erithacus rubecula outside the breeding season

On the basis of radio-tracking data from 51 individuals, the space use of Robins Erithacus rubecula outside the breeding season was non-uniform and multinuclear. On average, territorial Robins spent 78% of their time in one or more resource patches represented by bushes, which accounted for only 1% of a convex polygon encompassing each range. A suite of range polygons was developed to quantify range structure. One of these polygons showed similar size to, and high overlap with, defended-area polygons and so was a good estimate of territory. In addition, the area of bushes in each polygon suggested Robins occupied constant resource territories. There was undefended neutral ground between territories, which, although seldom visited, was crossed to achieve intrusions. Over the short term (days), Robins showed high fidelity to the resource patches they defended. Over the longer term (months), most individuals showed some degree of range shift. It is concluded that if the information concerning territories is to be maximized, empirically derived sampling protocols should be used when measuring territory parameters.     

Song and plumage effects on aggressive display by the European Robin Erithacus rubecula

The response was observed of territory-holding male European Robins ETithacus rubecula to red— and brown-breasted model Robins which were presented either simultaneously with tape-recorded Robin song, or with no song. Robins displayed and sang at the silent red-breasted model, but sang and displayed much more at the models that were presented with song. It is suggested that, while the red breast is one important feature of Robins eliciting aggressive behaviour, song is also important. The relationship of these findings to studies of stimulus features eliciting behaviour in other species is discussed.     

Trade-offs and seasonal variation in territorial defence and predator evasion in the European Robin Erithacus rubecula

The low incidence of intraspecific combat in territorial systems has traditionally been accounted for by theories that emphasize the bio-energetic advantages or the diminished risk of injury of threat display posture when compared with combat. Recently, however, it has been suggested that territory-holding passerines engaging in highly aggressive defensive behaviour are likely to pay a cost in terms of reduced vigilance for avian predators. To examine this further, two defensive options (combat and threat display) were evoked and observed in territorial European Robins Erithacus rubecula , in both winter and summer. It was found that Robins that engaged in combat during simulated territorial intrusion were significantly slower to react to a stuffed Sparrowhawk Accipiter nisus , than those eliciting threat display. Therefore, the decreased vigilance in escalated fighting may result in higher vulnerability to predation, and this might help, in part, to explain why threat display is favoured over direct combat. Moreover, the mean time to evade the predator was significantly longer in summer than in winter in threat-displaying Robins. In addition to this finding, a clear difference in the type of behaviour adopted between the seasons was observed, with lower incidence of combatative response during the winter months. This difference in response to the appearance of a predator is discussed in relation to reduced levels of safe cover, a reduction in the availability of alternative prey during the winter months and to seasonal fluctuations in plasma androgen levels.     

Age‐related sexual plumage dimorphism and badge framing in the European Robin Erithacus rubecula

Male and female European Robins Erithacus rubecula display their red breasts in year‐round territorial contests. Despite the clear signalling role of the red breast, little is known about its sexual dimorphism or trends in size when Robins age. We studied these patterns in resident and migrant Robins in a Mediterranean population. Both male and female first‐year Robins had smaller red breasts than second‐year individuals. Females, but not males, showed a reduction in badge area after the second year of life, leading to sexual dimorphism in red breast area in the oldest Robins. The grey fringe around the red breast showed a steady increase in width in males when ageing, but not in females, also leading to sexual dimorphism in this trait among oldest Robins. The contrast between the red breast and its surrounding grey fringe was higher than that between the breast and the back at both high and low environmental light intensities. This suggests that the grey fringe could function as a frame to highlight the perimeter of the red breast, and that the Robin’s plumage is equally suited to display in open areas and forest understoreys. Our study suggests that the extent of the red breast and its grey frame could confer information about the age and sex of the birds, and thus be used as a signal in territorial contests and mating decisions.     

Sex-determination of Continental European Robins Erithacus r. rubecula

Existing morphological sex-determination criteria for Continental European Robins proved inadequate when applied to birds captured in western Denmark. Only 49% of captured birds met with the wing-length criteria resulting in the classification of 2% males and 47% females respectively. This article presents a primer for correct sexing of Robins by amplification of a female-specific DNA element by a polymerase chain reaction (PCR) on genomic DNA. Further, two new criteria for sex-determination are proposed based on logistic regression and discriminant function analysis using morphological measures from birds with DNA-verified sex. Success rates of 80–81% for the new criteria were a considerable improvement compared with a success rate of only 36% for the old criteria.     

Territorial aggression and song of male European robins (Erithacus rubecula) in autumn and spring: effects of antiandrogen treatment

Male robins aggressively defending a feeding territory in winter have low levels of testosterone, while males defending a breeding territory in spring have elevated levels of testosterone. Song is an integrated part of territorial defense during both phases. We investigated whether testosterone is involved in the expression of these behaviors by treating free-living and captive male robins during both phases with the antiandrogen flutamide. Results suggest that, similar to species in which territoriality is restricted to the reproductive phase, aggressive defense of a breeding territory by male robins is facilitated by androgens. Territorial defense during the nonbreeding season, however, does not require androgenic activity. Singing frequency, on the other hand, was not significantly reduced during either phase by flutamide application. Since the quality of male song changes with season we suggest that the incorporation of sexual signals into male song during breeding depends on testosterone, but that the year-round production of territorial "keep out" signals is independent of testosterone.     

The decline of a peripheral population of the European robin Erithacus rubecula

This paper examines the potential effect of some factors involved in the decline of a winter population of the European robin Erithacus rubecula in southern Spain. The area receives extra‐Iberian immigrants that come into contact with resident conspecifics. Given this interspersed distribution, three hypotheses were tested to explain the decline: a) the depletion of the carrying capacity of the area during winter, b) the decline of the resident population, and c) the reduction of foreign individuals in this wintering area. Winter robin and fruit (a main winter food) abundances were assessed at 43 sites from 1999 to 2014, and the abundance of breeding robins was evaluated from 1998 to 2012. Rainfall and temperatures were recorded throughout this period and changes in forests occupied by breeding robins were assessed in 1999 and 2013. Results showed a decline in winter temperatures and an increase in fruit availability. Given that food increases could buffer the thermoregulatory costs of colder conditions, the depletion of the carrying capacity of the area was discarded as a main driver of robin decline. Spring counts showed a reduction of 45% in the breeding population that was not explained by changes in forest management. However, a significant increase in summer temperatures could boost the pervasive effect of drought on robins inhabiting this marginal area of the species' range. Foreign immigrants, despite the increase in breeding populations in most of Europe, did not buffer the decline caused by the reduction of residents. In fact, an analysis of ring recoveries reported in this area depicted a sharp decrease in extra‐Iberian wintering individuals. These results may be related to the effect of global warming on migratory schedules (the northwards retreat of wintering areas) and on the suitability for robins of local habitats in this peripheral region of the Palaearctic.     

Territoriality as a paternity guard in the European robin, Erithacus rubecula

To investigate the relative importance of paternity defences in the European robin we used behavioural observations, simulated intrusions and temporary male removal experiments. Given that paired males did not increase their mate attendance, copulation rate or territory size during the female's fertile period, the most frequently quoted paternity assurance strategies in birds were absent. However, males with fertile females sang and patrolled their territories more regularly, suggesting that territorial motivation and vigilance were elevated when the risk of cuckoldry was greatest. In addition, there was a significant effect of breeding period on response to simulated intrusions: residents approached and attacked freeze-dried mounts more readily in the fertile period. During 90-min removals of the pair male in the fertile period, neighbours trespassed more frequently relative to prefertile and fertile period controls and appeared to seek copulations with unattended females. When replaced on their territories, males immediately increased both song rate and patrolling rate in comparison with controls. We propose that male robins sing to signal their presence, and increase their territorial vigilance and aggression in the fertile period to protect paternity. Copyright 2000 The Association for the Study of Animal Behaviour.     

Nocturnal orientation of robins, Erithacus rubecula: birds caught during migratory flight are disoriented

For conventional experiments on the orientation behavior of migrant birds in funnels, either hand-raised birds or birds caught during resting periods at stopover sites are generally used. Topographic circumstances at the Alpine pass Col de Bretolet at the Switzerland/France border allow the capture of birds during active migratory flight during the whole night. These birds are in full migratory disposition. We expected them to orient in the seasonally appropriate direction more accurately than birds that had not experienced migration just before the funnel experiment. Experiments with robins, however, revealed a strong influence of the moon on the orientation behavior. The birds did not orient in the seasonally expected migratory direction but showed positive phototaxis, usually toward the lightest part of the funnel in the direction opposite to the moon. When the moon was absent the robins were disoriented. Sunset experiments with robins caught during the night before the experiment revealed a strong phototactic reaction toward the setting sun. As reasons for this poor orientation in the absence of light stimuli, the influence of the topography of the mountainous region and magnetic anomalies can be excluded. It is concluded that freshly caught birds are too stressed when tested immediately after capture or that the migration direction cannot be maintained. Testing of night migrants in complete darkness is also of disadvantage. Received: 23 December 1998 / Received in revised form: 16 July 1999 / Accepted: 30 July 1999     

Song characteristics and response to playback of male and female Robins Erithacus rubecula

Male and female Robins, from a population occupying deciduous woodland habitat, were recorded during autumn, winter and spring. Song characteristics were compared between individuals with respect to sex and reproductive success. For the parameters measured, male song did not vary with reproductive success; however, male song was more complex and less redundant than female song. Furthermore, male song in response to playback was more complex in response to female than to male song. Both males and females could discriminate between male and female song. Males approached playback of male song more readily in the autumn than in the spring. The possible significance of these results is discussed.     

Consequences of the settlement of migrant European Robins Erithacus rubecula in wintering habitats occupied by conspecific residents

In wintering areas where migrant birds meet sedentary conspecifics, early settlement of local residents in the best habitat patches might reduce the availability of suitable sites for arriving migrants. We studied how sympatric migratory and sedentary European Robins Erithacus rubecula occupy two wintering habitats of different quality (forests and shrublands) in southern Spain, and how such a distribution affects individuals of each population sector. In September, before migrants arrived, Robins were only found in forests, and they had already saturated these habitats, so that rather than increasing Robin abundance in these habitats, the arrival of migrants caused a massive occupation of the previously vacant shrublands. During the winter, we captured Robins and identified them as migrants or residents using a discriminant function based on morphological traits. Residents always predominated in forests, and migrants in shrublands, but through the winter around 35% of residents (mainly juveniles) moved to shrublands, having been replaced by some migrants in forests. Although food was more abundant in shrublands, Robins had better body condition in forests, suggesting that other factors determined habitat preferences (e.g. shelter availability or food diversity, which were higher in forests). In addition, we observed a greater variance in body mass relative to body size in forests, suggesting that energy management was less constrained in this habitat (for example owing to a lower exposure to predators or a higher food predictability). Our results suggest that sedentary Robins benefit from an early occupation of the best habitats in the wintering grounds, forcing migrants to colonize apparently less suitable sites. This would explain the persistence of these small southern populations despite the yearly flooding of the area by huge numbers of migrant conspecifics.     

Winter segregation of migrant European robins Erithacus rubecula in relation to sex, age and size

Birds often show some form of social segregation during winter, both at large geographical scales (a consequence of differential migration) and at the regional or local level, when comparing different habitats or micro-habitats. However, our understanding of the mechanisms underlying such patterns is still poor. These issues have been rarely investigated in migratory Old-World passerines, particularly with respect to differences between the sexes. In this study, we show that female European robins Erithacus rubecula (sexed by molecular techniques) greatly outnumber males in southern Iberia, which confirms that this species is a differential migrant with a strong latitudinal segregation of the sexes. Furthermore, sex, age and body size influence the habitat distribution of robins in winter. Subordinate birds (females, juveniles and small individuals) were generally more common in habitats with a greater shrub development, and comparatively scarce in woodlands with relatively little undergrowth. Birds wintering in woodlands were in better condition (assessed by breast-muscle scoring) than birds wintering in shrubland. These results are consistent with the hypothesis that proposes that social dominance, mediated by differences in size and experience, is important in determining the habitat segregation of sex and age classes. The alternative hypothesis (habitat specialization), although not specifically supported by our findings, cannot be ruled out on the basis of the available evidence.     

The effect of yellow and blue light on magnetic compass orientation in European robins, Erithacus rubecula

To analyze the wavelength dependency of magnetic compass orientation, European robins were tested during spring migration under light of various wavelengths. Under 565-nm green light (control) the birds showed excellent orientation in their migratory direction; a 120° deflection of magnetic North resulted in a corresponding shift in the birds' directional tendencies, indicating the use of the magnetic compass. Under 443-nm blue light, the robins were likewise well oriented. Under 590-nm yellow, however, oriented behavior was no longer observed, although the activity was at the same level as under blue and green light. The spectral range where magnetic orientation is possible thus differs from the range of vision, the former showing parallels to that of rhodopsin absorption. The interpretation of the abrupt change in behavior observed between 565 green to 590 yellow is unclear. There is no simple relationship between magnetoreception and the known color receptors of birds. Accepted: 17 December 1998     

Frugivory of transient and wintering European robins Erithacus rubecula in a Mediterranean region and its relationship with ornithochory

Stomach contents of 209 dead and droppings of 57 mist-netted migrating or wintering European robins Erithacus rubecula in a Mediterranean area in southern France showed the following seasonal trends: (1) a dominance of invertebrates in early autumn, (2) a dominance of fruits in mid-autumn, (3) acorns being important in midwinter, and (4) a strong dominance of invertebrates at the end of winter and in early spring.
The fruits of 23 plant species had been consumed. The most widespread and abundant ones were also the most commonly taken ( Rubia peregrina at the regional level and Viburnum tinus at a local one). The size of these fruits ranged from 4 to 12 mm. mostly between 6 and 8 mm. Owing to the amount of fruits consumed and the numbers of birds involved, the European robin is one of the few main dispersers of the numerous fleshy-fruited plant species of the Mediterranean area. Nevertheless, a correct evaluation of its role is only possible within the dense network existing between these plant species and all their dispersers (mainly birds but also mammals).