Great Tit Pairs Research Articles

Comparison of nestling diet between first and second broods of great tits Parus major in urban and forest habitats

To understand why early broods tend to be more successful than late broods we investigated the nestling diet and reproductive success of great tit pairs that had both a first and a second brood in the same breeding season. We found that in forest habitats great tit parents delivered similar composition and amount of food per nestlings throughout the breeding season, resulting in similar nestling body mass and survival in both first and second broods. In urban habitats, however, although parents provided similar amounts of food to the second broods they tended to deliver fewer caterpillars. In parallel with this, we observed lower nestling survival in second urban broods than in first broods even though the body mass of surviving nestlings was similar to that of the first broods. These findings suggest that although parents produce smaller second broods in both habitats, they are able to compensate for lower food availability in forest habitats but not in urban habitats, thus leading to reduced food quality and lower offspring survival in urban second broods.

Anthropogenic noise alters the perception of a predator in a local community of great tits

Anthropogenic noise is increasing at an alarming rate in urban as well as rural ecosystems and detrimental effects have been reported on many animals relying on acoustic communication. Little is known regarding the impact of different types of noise on community level predator perception and breeding behaviour. Here, we presented a predator model alone or a predator combined with a specific noise type, such as traffic, a lawnmower or a chainsaw, to breeding pairs of great tits, Parus major , giving them the opportunity to perform mobbing behaviour that could provide public information to their neighbours. We then measured the provisioning behaviour of adjacent tit pairs, which were 50–150 m away from the focal pairs' nests. The adjacent breeding pairs decreased nest visits when focal birds were exposed to the sparrowhawk model or to the sparrowhawk model combined with the lawnmower noise. Neighbouring pairs did not reduce the rate of nest visits when focal birds were exposed to the hawk model combined with the traffic or the chainsaw noise. The distance from focal nests did not alter the noise effect on neighbouring pairs. We suggest that auditory masking is the most likely mechanism to explain why noise compromises public information. The chainsaw and traffic noise overlap with the frequency of mobbing calls and disrupt the transmission of auditory signals in the community. Further, when the hawk presentation was combined with the chainsaw noise, which was the type of noise that had the highest frequencies, the latency of mobbing by the focal birds was strongly increased, and the number of species involved in mobbing was reduced. This study demonstrates that the signalling function of mobbing is ineffective in noisy environments which ultimately impacts fitness via increased predation to adults or their offspring. • Focal great tits give mobbing calls in response to a sparrowhawk presentation. • Neighbouring pairs reduce nest visit rate when focal birds mob the hawk. • Neighbours do not respond to mobbing calls along with a chainsaw or traffic noise. • Acoustic masking by high-frequency noise disrupts the transmission of mobbing calls. • Low-frequency noise does not disrupt the transmission of public information.

Temperature has a causal and plastic effect on timing of breeding in a small songbird.

Phenotypic plasticity is an important mechanism by which an individual can adapt its seasonal timing to predictable, short-term environmental changes by using predictive cues. Identification of these cues is crucial to forecast the response of species to long-term environmental change and to study their potential to adapt. Individual great tits (Parus major) start reproduction early under warmer conditions in the wild, but whether this effect is causal is not well known. We housed 36 pairs of great tits in climate-controlled aviaries and 40 pairs in outdoor aviaries, where they bred under artificial contrasting temperature treatments or in semi-natural conditions, respectively, for two consecutive years, using birds from lines selected for early and late egg laying. We thus obtained laying dates in two different thermal environments for each female. Females bred earlier under warmer conditions in climate-controlled aviaries, but not in outdoor aviaries. The latter was inconsistent with laying dates from our wild population. Further, early selection line females initiated egg laying consistently ∼9 days earlier than late selection line females in outdoor aviaries, but we found no difference in the degree of plasticity (i.e. the sensitivity to temperature) in laying date between selection lines. Because we found that temperature causally affects laying date, climate change will lead to earlier laying. This advancement is, however, unlikely to be sufficient, thereby leading to selection for earlier laying. Our results suggest that natural selection may lead to a change in mean phenotype, but not to a change in the sensitivity of laying dates to temperature.

Effect of age, haemosporidian infection and body condition on pair composition and reproductive success in Great Tits Parus major

Several factors shape lifetime reproductive success, including genetic background, body condition, environmental conditions and ecological interactions such as parasitism. Adults often show higher reproductive success than their young conspecifics, especially in long‐lived bird species, and this may be explained by the cumulative effects of an increase in reproductive experience and the selection of high‐quality individuals from one year to the next. To test whether this pattern also exists in short‐lived bird species, we used 13 years of monitoring data from two Great Tit Parus major populations. The effects of male and female age on several reproductive parameters were analysed in 419 pairs of Great Tits, while accounting for body condition and infection by haemosporidian parasites. Reproductive success was mainly affected by the age‐class of males. Pairs containing a sub‐adult male fledged one‐third fewer chicks than pairs containing an adult male. The difference was not caused by variation in male fertility but could have been caused by better parental care provided by adult birds. In addition to lower reproductive success, first‐year males also had reduced access to mating compared with adult males, suggesting an avoidance of sub‐adult males by females. Nestling body condition was positively correlated with parental body condition, and the body condition of male and female members of breeding pairs was positively correlated. Finally, the number of fledged chicks was mainly affected by the infection status of males. These results temper our previously published results showing an effect of infection on Great Tit reproduction regardless of their sex. In our previous study, and as in most cases, the status of the partner was not taken into account and we show here that this is essential because it can lead to a biased interpretation of the results.

Incubating females signal their needs during intrapair vocal communication at the nest: a feeding experiment in great tits

Parental care is a key factor of breeding success and parents face evolutionary trade-offs between investment in current offspring and future reproduction and survival. Incubation in birds is energetically costly and the balance between parental and offspring's energetic needs is especially challenging when only one sex incubates, generally the female. In that case, males can contribute indirectly to incubation effort by feeding their mate and females may use begging behaviours to signal their needs to their visiting partner. The great tit, Parus major , is a good model species to test whether females use acoustic communication from the nest to signal their needs to their mate outside because females interact vocally with their mate during incubation. To test whether females use these vocal exchanges to communicate how hungry they are, great tit pairs were recorded on 2 days during incubation: 1 day with a feeder of mealworms in the nestbox and 1 day with an empty feeder. First, food supplementation increased females' nest attentiveness, revealing a decrease in foraging activity and consequently in females' needs. Second, females signalled their need for food both during their male's visits inside the nest but also during the vocal exchange immediately preceding the male's entrance by calling more and by modifying the frequency spectrum of their calls. So females' calling behaviour is an honest signal of need not only during their male's visits, but also before his entrance in the nest.

Interactive vocal communication at the nest by parent Great Tits Parus major

Although most bird species show monogamous pair bonds and bi‐parental care, little is known of how mated birds coordinate their activities. Whether or not partners communicate with each other to adjust their behaviour remains an open question. During incubation and the first days after hatching, one parent – generally the female – stays in the nest for extended periods, and might depend on acoustic communication to exchange information with its mate outside. The Great Tit Parus major is an interesting study system to investigate intra‐pair communication at the nest because males address songs to their mate while she is in the nest cavity, and females answer the male from the cavity with calls. However, the function of this communication remains unknown. In this study, we recorded the vocalizations and observed the resulting behaviour of Great Tit pairs around the nest at different breeding stages (laying, incubation and chick‐rearing). We observed vocal exchanges (vocalization bouts, alternated on the same tempo, between the female inside the nest and her male outside) in three contexts with different outcomes: (1) the female left the nest, (2) the male entered the box with food, and the female then used specific call types, (3) mates stopped calling but did not leave or enter the nest. The structure of vocal exchanges was globally stable between contexts, but females used calls with an up‐shifted spectrum during exchanges, at the end of which they left the nest or the male entered the nest. Birds vocalized more and at higher tempo during exchanges that ended up in feeding inside the nest. Birds also vocalized more during exchanges taking place during laying – a period of active mate guarding – than during incubation. We conclude that vocal exchanges could signal the females’ need for food and the males’ mate guarding behaviour, and discuss other possible functions of this communication.

Social behaviour in the non-breeding season in Great Tits Parus major and Willow Tits Poecile montanus: differences in juvenile birds’ route to territorial ownership, and pair-bond stability and mate protection in adults

Great Tits Parus major and Willow Tits Poecile montanus are small passerine monogamous species that live in dominance-structured flocks outside the breeding season. Within such flocks, males dominate females, and within sex, adults generally dominate juveniles (first-year birds). However, whereas Willow Tits maintain a yearlong defence of large territories and their flocks have a constant membership, the Great Tits maintain looser organizations of their flocks that vary in size, and the birds move around irrespective of territorial ranges. I compared the aggressive behaviour between juvenile conspecifics of Great and Willow Tits from July through April during 2004–2014, and the intra-pair behaviour of adults during November–February in 2011−2014 in a subalpine forest in Budal, central Norway. Two maxima of aggressive interactions were found among juveniles of both species: one in August–September and one in March–April. For Great Tits the autumn-interactions were probably due to competition for food, whereas the spring-interactions reflected competition for territorial space. For Willow Tits, the interactions in August – September were caused by competition for flock membership and ownership for a territory in March–April. Because flock membership in general is the only possible way to territory ownership, also the aggressive behaviour among juvenile Willow Tits in autumn is most likely linked to territorial behaviour. I found three circumstances of immediate benefits of the pair bond for adult Willow Tits that are consistent with the hypothesis of mate protection and that differed from that of Great Tit pairs: Willow Tits had a shorter intra-pair distance, females received less aggression, and they had a higher foraging rate when accompanied by their mate than did females of Great Tit pairs. One out of the five Great Tit pairs studied became divorced, whereas the bonds of three Willow Tit pairs were stable over at least three to four years.

Asymmetric seasonal nest site competition between Great Tits and House Sparrows

Using nest boxes with different sized entrances, we experimentally examined whether a large species of cavity breeder (House Sparrow, Passer domesticus) affects the nest box occupancy and breeding success of a smaller species (Great Tit, Parus major), and whether there are differences in the effects of competition during different parts of the breeding season. Great Tits occupied nest boxes regardless of the number of House Sparrows breeding in the vicinity. During the second part of the breeding season, the percentage of successful Great Tit pairs was negatively correlated with the occupation of nest boxes by the House Sparrows, in both the large- and small-entrance nest boxes. More Great Tit pairs bred and more young were fledged in the small- than large-entrance nest boxes. Great Tits occupied more large-entrance nest boxes during the first than the second part of the breeding season. This difference was probably due to House Sparrows occupying more large-entrance nest boxes during the second than first part of the breeding season. 74 % of the large-entrance nest boxes in which Great Tits built nests in both the first and second parts of the season were later occupied by House Sparrows. Great Tits preferred large-entrance nest boxes in the fall, when House Sparrows use only a few boxes for roosting, but not for breeding. The findings indicate that Great Tits are subject to interspecific competition with House Sparrows for nesting cavities, the intensity of which varies during the breeding season and is higher during the second part when more House Sparrows breed.

Individual variation in avian reproductive physiology does not reliably predict variation in laying date

Most animals reproduce seasonally. They time their reproduction in response to environmental cues, like increasing photoperiod and temperature, which are predictive for the time of high food availability. Although individuals of a population use the same cues, they vary in their onset of reproduction, with some animals reproducing consistently early or late. In avian research, timing of reproduction often refers to the laying date of the first egg, which is a key determinant of fitness. Experiments measuring temporal patterns of reproductive hormone concentrations or gonadal size under controlled conditions in response to a cue commonly assume that these proxies are indicative of the timing of egg laying. This assumption often remains untested, with few studies reporting both reproductive development and the onset of laying. We kept in total 144 pairs of great tits (Parus major) in separate climate-controlled aviaries over 4years to correlate pre-breeding plasma luteinizing hormone (LH), prolactin (PRL) and gonadal growth with the timing of laying. Individuals varied consistently in hormone concentrations over spring, but this was not directly related to the timing of gonadal growth, nor with the laying date of the first egg. The timing of gonadal development in both sexes was similarly not correlated with the timing of laying. This demonstrates the female’s ability to adjust the onset of laying to environmental conditions irrespective of substantial differences in pre-laying development. We conclude that stages of reproductive development are regulated by different cues, and therefore egg laying dates need to be studied to measure the influences of environmental cues on timing of seasonal reproduction.

Increasing Temperature, Not Mean Temperature, Is a Cue for Avian Timing of Reproduction

Timing of reproduction in temperate-zone birds is strongly correlated with spring temperature, with an earlier onset of breeding in warmer years. Females adjust their timing of egg laying between years to be synchronized with local food sources and thereby optimize reproductive output. However, climate change currently disrupts the link between predictive environmental cues and spring phenology. To investigate direct effects of temperature on the decision to lay and its genetic basis, we used pairs of great tits (Parus major) with known ancestry and exposed them to simulated spring scenarios in climate-controlled aviaries. In each of three years, we exposed birds to different patterns of changing temperature. We varied the timing of a temperature change, the daily temperature amplitude, and the onset and speed of a seasonal temperature rise. We show that females fine-tune their laying in response to a seasonal increase in temperature, whereas mean temperature and daily temperature variation alone do not affect laying dates. Luteinizing hormone concentrations and gonadal growth in early spring were not influenced by temperature or temperature rise, possibly posing a constraint to an advancement of breeding. Similarities between sisters in their laying dates indicate genetic variation in cue sensitivity. These results refine our understanding of how changes in spring climate might affect the mismatch in avian timing and thereby population viability.

Spring phenology does not affect timing of reproduction in the great tit (Parus major)

Many seasonal breeders adjust the timing of reproduction in response to year-to-year variations in supplementary environmental cues, amongst which ambient temperature is thought to be most influential. However, it is possible that for species such as the great tit (Parus major L.), phenological cues from sprouting vegetation and the consequent abundance of invertebrate prey, although dependent on temperature, may provide supplementary environmental cues per se. This hypothesis was investigated in breeding pairs of great tits kept in outdoor aviaries. In spring, experimental pairs were provided with access to leafing birch branches and caterpillars as a visual food cue, while control pairs were provided with non-leafing branches. Observations were made on the onset of laying and on concentrations of plasma luteinizing hormone (LH) at regular intervals to monitor changes in reproductive function. The onset of egg laying was not advanced by the presence of leafing branches and caterpillars. LH concentrations increased during the course of the study, but phenological cues did not affect plasma LH levels in females and males. Early spring vegetation, such as the leafing of birch branches, and the appearance of caterpillar prey do not appear to play a significant role in fine-tuning the onset of egg laying in great tits.

Effects of urban matrix on reproductive performance of Great Tit (Parus major) in urban woodlands

In urban areas the density of breeding tits (Paridae) is generally higher and reproductive performance lower compared to rural areas. To explain these landscape differences several hypotheses have been proposed, e.g. differences in habitat quality, inter- and intra-specific competition, predation and food abundance. How breeding performance of birds within remnants of natural vegetation in urban areas is affected by adjacent matrix has been less studied. We performed an experimental study in four urban woodlands surrounded by three types of habitat matrix: residential, high-rise building and grassland/golf course. We placed 300 nest boxes for tits (great tit Parus major and blue tit Cyanistes caeruleus) in 15 transects that were 200 m long, extending from the adjacent matrix 150 m into urban woodland. Density of breeding great tit pairs was highest in residential areas and hatch date was earlier in the residential habitat compared to the other two habitats, however, nestling condition was lower in residential areas. Hatching date was earlier but hatching spread (heaviest nestling/lightest nestling) higher in the three types of urban matrix than inside the urban woodlands. In contrast to previous large-scale comparisons of urban and rural matrix, we almost exclusively found differences in qualitative measures (nestling condition and hatchling spread) at the small scale in which this study was conducted. Adjacent matrix affect great tits breeding in remnant urban woodlands, thus we suggest that management of the surrounding matrix should be included in conservation plans for urban woodlands.

Calcium effects on life-history traits in a wild population of the great tit (Parus major): analysis of long-term data at several spatial scales

Calcium is an essential micronutrient for birds during egg formation and for skeletal development in nestlings. Habitat level studies suggest that birds breeding in low-calcium areas may be limited in the size or number of eggs they lay and in the quality of their nestlings. However, as birds forage non-randomly and may travel considerable distances to acquire calcium, describing different breeding environments in terms of their calcium availability is problematic. Here we explore the spatial relationships between 300-fold variation in soil calcium and the life-history traits of ca. 6,000 pairs of great tits breeding in a single continuous woodland over 41 years. Controlling for other habitat differences, we found strong positive associations between soil calcium, clutch size and recruitment at spatial scales of over 300 m from each nestbox, suggesting that females may have been travelling inter-territorially to acquire calcium during egg formation. Soil calcium near each nestbox (mean distance = 58 m) was a strong positive predictor of mean fledgling mass, suggesting that local calcium was more important during nestling stages. We found no effect of soil calcium on lay-date or egg mass. This study is the first to provide evidence that small woodland passerines are limited by calcium availability at several different spatial scales. However, experimental work is necessary to test the causality of these spatial patterns.

Nestling begging intensity and parental effort in relation to prelaying carotenoid availability

Carotenoids are antioxidants playing major roles in physiological functions at various stages of an animal’s life. Female birds deposit large amounts of carotenoids into their eggs. Carotenoids are, however, a limiting resource, and females are expected to balance carotenoid deposition into the eggs with their utilization for themselves. Carotenoid availability is thus likely to determine both the levels of yolk carotenoids and maternal care during rearing. Carotenoids have been shown to benefit the embryo and the growing nestling, and it can be hypothesized that an increase in carotenoid availability during laying leads to higher nestling condition and competitive ability. We manipulated carotenoid availability to great tit pairs prior to and during egg laying and later partially cross-fostered chicks at hatching. During the rearing period, we measured how carotenoid availability affected nestlings begging behavior and male and female feeding effort. We also manipulated the ectoparasite load, predicting that carotenoid supplementation would help adults and nestling to cope with parasites. Nestlings hatched from eggs laid by carotenoid-supplemented females and raised in small broods begged more intensely. Nestlings in small deparasitized broods also begged more actively. The feeding effort of control females increased with brood size, whereas the feeding effort of carotenoidsupplemented females was high whatever the brood size. Male feeding effort was unaffected by our treatment. Our results support the hypothesis that maternally derived carotenoids increase nestling begging behavior and hence competitive ability. They further suggest that carotenoid availability determines the level of parental investment and can mediate trade-offs between life-history traits. Key words: begging, food provisioning, maternally derived carotenoids, parasites, parental care, reproductive investment. [Behav Ecol]

Edge Effects in the Great Tit: Analyses of Long‐term Data with GIS Techniques

In contemporary fragmented landscapes, edges are commonplace, and understanding the effects of edge environments is thus essential for the conservation of forest communities. The reproductive output of forest passerines is often reduced close to forest edges. Possible explanations include overcrowding by conspecifics, elevated rates of predation, and the occurrence of lower-quality habitat and/or individuals at forest edges. We attempted to separate these processes by examining edge effects in the absence of nest predation and by effectively controlling for differences in breeding density and the quality of habitats and individuals. We used an edge distance index (EDI), which accounts for the number and distribution of edges in close proximity to a breeding location, to help explain variation in breeding density, nesting success, and reproductive traits of 8308 pairs of Great Tits (Parus major) breeding between 1965 and 2005, in Wytham, near Oxford, United Kingdom. Results from linear mixed modeling confirmed higher breeding density and a higher proportion of immigrant individuals at forest edges. Nevertheless, independently of these effects, we also found that birds laying later, with smaller clutches but larger eggs, were typical of edge environments. The number of offspring recruited to the breeding offspring per breeding attempt was also reduced at edges, both directly and mediated through changes in clutch size and laying date. Edge effects on life histories were detectable within individual females and up to 500 m from the woodland edge. Woodland edges are increasingly common in contemporary fragmented landscapes. Therefore these results, which suggest a pervasive effect of edges on reproduction, are of considerable importance to the management and conservation of forest communities.

Nest design and the abundance of parasitic Protocalliphora blow flies in two hole-nesting passerines

:Ectoparasites dwelling in bird nests regularly reduce reproductive success and condition of breeding birds. Thus, establishing the factors that determine the abundance of ectoparasites is important for better understanding of reproductive trade-offs and life history evolution in birds. A recent hypothesis states that interspecific differences in the abundance of ectoparasites may be caused by nest composition. For example, great tits (Parus major) have nests made of mosses and fur, whereas Ficedula flycatchers have nests made of grasses, bast, and bark, and tits are more infested by nest-dwelling ectoparasites than flycatchers. We swapped nests between pairs of great tits and collared flycatchers (F. albicollis) during egg-laying or early incubation and counted parasitic Protocalliphora blow flies at the end of breeding to test this hypothesis experimentally. We controlled statistically for habitat (oak versus spruce forest), brood size, season, year, and mean nestling weight before fledging. We found a significant effect of bird species (tit > flycatcher), habitat (oak > spruce), and year. There was no effect of nest type. Consequently, the hypothesis ascribing the different abundance of ectoparasites in great tits and collared flycatchers to different nest composition was not supported by our study.

Pair bond and breeding success in Blue TitsParus caeruleusand Great TitsParus major

Data from 939 nests of the Blue TitParus caeruleusand 1008 nests of the Great TitP. majorfrom nestboxes provided in superabundance in mixed forest study sites between 1976 and 2001 were analysed to examine the effects of mate retention on breeding success and the relationship between mate fidelity and site fidelity. Most birds retained their former partner (76% in Great Tits and 65% in Blue Tits). The probability of a pair divorcing was affected by male age in Great Tits, divorce being more likely in pairs with first‐year males. Great Tit pairs breeding together for a second season bred earlier, but had no higher breeding success than pairs breeding together for the first time. In Blue Tits laying date and start of incubation tended to be earlier in pairs breeding together for a second season, but hatching and fledging dates were not earlier than in other pairs. Great Tit pairs breeding together for two consecutive seasons bred earlier in the second season than in the first, but breeding success did not differ significantly between years. In both species, breeding performance did not differ between pairs that divorced after a season and pairs that stayed together. Thus breeding success did not determine whether a pair divorced or bred together again. Neither Blue Tits nor Great Tits improved their breeding performance through divorce. Blue Tit females even had fewer fledglings in the year after divorce than in the year before. Mate retention affected breeding site fidelity. Blue Tit females had greater breeding dispersal distances between consecutive years when re‐mating than when breeding again with the same mate. In Great Tits both males and females dispersed more when re‐mating than when retaining the former partner, suggesting that mate retention increased the chance of retaining the breeding site. In both species, breeding dispersal distances did not differ between pairs that divorced and pairs in which one mate disappeared. Because no major advantage of mate retention was evident, we suggest that mate retention evolved under different conditions than those found in study sites with high breeding densities and a superabundance of artificial nesting sites.

Do sibling tits (Parus major, P. caeruleus) disperse over similar distances and in similar directions?

We studied dispersal movements by sibling pairs of great tits, Parus major, and blue tits, P. caeruleus, in a patchy environment, in order to test whether siblings are more similar in dispersal than expected by chance. Because of possible common environmental effects due to the heterogeneity and finiteness of the study area, we compared the similarity among siblings with the similarity between each sibling and an unrelated bird that fledged in the same patch and year, as close to the siblings' nest as possible. Siblings of both species were not more similar in dispersal distance than they were to control birds. However, great tit siblings dispersed in similar directions compared to control birds, and this result was not affected by the degree of matching between sibling and control birds. As a consequence, siblings ended up breeding at closer distances from one another than control birds. Heritability values calculated from parent-offspring regressions were close to zero, suggesting that there is no additive genetic variance for dispersal distance or dispersal direction. We propose that similarity in dispersal direction originates from association of siblings during dispersal or during activities that influence the choice of direction, such as postfledging family movements. Our results show that non-independence in the choice of dispersal direction by siblings may influence small-scale kin structure in this population with high local recruitment.

Rival imprinting: interspecifically cross-fostered tits defend their territories against heterospecific intruders

Failure to recognize conspecifics in social interactions such as mate choice and aggressive encounters will often result in reduced fitness. Studies on mate choice show that the ability to recognize conspecifics as mates is not universally present at birth, but often needs to be learned. In contrast, little is known about the ontogeny of intrasexual species recognition. To test whether learning influences the recognition of sexual rivals, we compared the aggressive response towards intruders of interspecifically cross-fostered individuals and controls reared by conspecific parents. We simulated territorial intrusion by presenting either a caged individual or playback song near the nest of breeding pairs of great tits, Parus major, and blue tits, P.caeruleus. Great tits reared by blue tit parents responded much more to blue tit stimuli than did great tit controls, and furthermore showed stronger responses to blue tit stimuli than to those of their own species. Blue tits reared by great tits responded much more to great tit stimuli than did blue tit controls. In contrast, blue tits cross-fostered to coal tits, P.ater, did not respond more to coal tits than did blue tit controls. There was a species difference in the response to conspecifics: blue tits cross-fostered to great tits responded more to conspecifics than did cross-fostered great tits. The results were similar for males and females. We conclude that learning influences intrasexual species recognition in these tits. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Breeding territory size affects fitness: an experimental study on competition at the individual level

Summary1.Descriptive studies have shown that the annual mean fecundity and survival in bird populations decline as density increases. Experimental studies in which breeding density has been manipulated show that density causally affects reproduction in some but not other species.2.In a 3‐year study on great titsParus majorwe manipulated density by removing about one‐third of territorial great tit pairs from half the study area each year. The removal resulted in an increase in territory size of the remaining birds in the experimental sub‐area.3.Fecundity and survival did not differ between the experimental low‐density and the control sub‐areas. The experiment thus did not show a causal relationship between breeding density and fecundity and survival at the level of the population.4.The data were analysed further with respect to individual territory size, the level at which competition is assumed to operate. Pairs in the experimental sub‐area enlarged their territories. We argue that variation in territory enlargement is not caused by individual quality.5.Both pre‐manipulation territory size and territory enlargement positively affected the probability that a territorial pair nested, the growth rate of their chicks, the number of fledglings that recruited into the breeding population and the survival of the territorial adults. No effect of territory enlargement was observed on clutch size, nor on the probability that pairs started a second brood.6.The positive effect of the experimental territory enlargement argues for a causal effect of territory size on these reproductive parameters, independent of individual quality.7.An effect of the density reduction on fecundity may not be detectable at the population level, either due to the large variation in territory sizes within the experimental treatments or to the negative effect of the experimentper se, which counter balances the positive effect of territory enlargement.