Density effects on life-history traits in a wild population of the great tit Parus major: analyses of long-term data with GIS techniques

1. Population density often has strong effects on the population dynamics and reproductive processes of territorial animals. However, most estimates of density-dependent effects use the number of breeding pairs per unit area in a given season and look for correlations across seasons, a technique that assigns the same density score to each breeding pair, irrespective of local spatial variation. 2. In this study, we employed GIS techniques to estimate individual breeding densities for great tits breeding in Wytham Woods UK, between 1965 and 1996. We then used linear mixed modelling to analyse the effect of density on reproductive processes. 3. The areas of Thiessen polygons formed around occupied nestboxes were used to approximate territory size (necessarily inverse of breeding density). There were significant, independent and positive relationships between clutch size, fledging mass and the number of offspring recruited to the population, and territory size (all P < 0.001), but no effect of territory size on lay-date or egg mass. 4. Thiessen polygons are contiguous and cover all of the available area. Therefore, at low nest densities territory polygons were excessively oversized. Using a novel procedure to address this limitation, territory sizes were systematically capped through a range of maxima, with the greatest effect in the models when territories were capped at 0.9-2.3 ha. This figure approximates to the maximum effective territory size in our population and is in close agreement with several field-based studies. This capping refinement also revealed a significant negative relationship between lay-date and territory size capped at 0.9 ha (P < 0.001). 5. These density-dependent effects were also detected when analyses were restricted to changes within individual females, suggesting that density effects do not merely result from either increased proportions of low-quality individuals, or increased occupation of poor sites, when population density is high. 6. Overall, these results suggest that, in the current population, great tits with territories smaller than c. 2 ha independently lay smaller and later clutches, have lighter fledglings, and recruit fewer offspring to the breeding population. These analyses thus suggest a pervasive and causal role of local population density in explaining individual reproductive processes.     

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.     

Early comers occupy popular territories in a lycaenid butterfly,Chrysozephyrus smaragdinus (Lepidoptera: Lycaenidae)

Territorial activities of a lycaenid butterfly, Chrysozephyrus smaragdinus, were observed throughout its adult season to investigate the territorial dynamics. The territories occupied earlier in the season were abandoned later than later‐occupied territories, indicating that the site preference of male C. smaragdinus was largely maintained during the adult season. Three indexes of territory quality based on animal preference, occupation duration (OD), initial occupation date (IOD) and the number of contests (NOC), were highly correlated. Therefore, these preference‐based indexes are consistent and reliable, although they are indirect estimates of benefits obtained from territories. Males arriving at territory sites early in the season were more likely to hold territories of better quality when NOC was used as the index of territory quality. The positive relationship between territory quality and male arrival date is congruent with the result of a previous study showing that early comers tend to hold a territory whereas late comers do not. However, male arrival date was not related to territory quality when OD or IOD was used as an index of territory quality. One possible reason for this apparent discrepancy is that the coefficient of variation of NOC is larger than those of OD and IOD, and therefore NOC could evaluate territory quality more quantitatively than the other two indexes. To date, studies of territoriality in butterflies have not considered the quality of territories. More studies considering the quality of territories will provide new insights into butterfly territoriality.     

Field evaluation of abundance estimates under binomial and multinomial N-mixture models

Assessing and modelling abundance from animal count data is a very common task in ecology and management. Detection is arguably never perfect, but modern hierarchical models can incorporate detection probability and yield abundance estimates that are corrected for imperfect detection. Two variants of these models rely on counts of unmarked individuals, or territories (binomial N-mixture models, or binmix), and on detection histories based on territory-mapping data (multinomial N-mixture models or multimix). However, calibration studies which evaluate these two N-mixture model approaches are needed. We analysed conventional territory-mapping data (three surveys in 2014 and four in 2015) using both binmix and multimix models to estimate abundance for two common avian cavity-nesting forest species (Great Tit Parus major and Eurasian Blue Tit Cyanistes caeruleus). In the same study area, we used two benchmarks: occupancy data from a dense nestbox scheme and total number of detected territories. To investigate variance in estimates due to the territory assignment, three independent ornithologists conducted territory assignments. Nestbox occupancy yields a minimum number of territories, as some natural cavities may have been used, and binmix model estimates were generally higher than this benchmark. Estimates using the multimix model were slightly more precise than binmix model estimates. Depending on the person assigning the territories, the multimix model estimates became quite different, either overestimating or underestimating the ‘truth’. We conclude that N-mixture models estimated abundance reliably, even for our very small sample sizes. Territory-mapping counts depended on territory assignment and this carried over to estimates under the multimix model. This limitation has to be taken into account when abundance estimates are compared between sites or years. Whenever possible, accounting for such hidden heterogeneity in the raw data of bird surveys, via including a ‘territory editor’ factor, is recommended. Distributing the surveys randomly (in time and space) to editors may also alleviate this problem.     

Determinants of territory size of the dusky gregory

To test the effect of food abundance and intruder pressure as determinants of territory size, the dusky gregory Stegastes nigricans were used as subjects on a coral reef in southern Taiwan during November to December 2000. Adults were used as intruders to provoke aggressiveness in a conspecific territory owner. The owner's maximum distance of attack (MDA) was used to delineate the territory size. While the owner of a territory appeared to defend a single boundary against different conspecifics, size variations among territories were evident. The effect of an intruder's identity on territory‐size regulation was not clear because for each territory examined, the MDA was found to be neither linked to the body size of the intruder nor to the amount of algae in the intruder's territory. Moreover, no significant differences were found between the MDAs that the owner maintained against neighbours and non‐neighbours even though, when conspecifics intruded in pairs, the probability was significantly higher for the first attack to be launched on a neighbour than on a non‐neighbour. Also when a neighbour and a non‐neighbour appeared simultaneously near the territory, the bite rate against the neighbour was also significantly higher. An inverse relationship between the amount of algae in the defended territory and the MDA of the owner indicates that food abundance might account for variations among territories. By contrast, territory size was not linked to the body size of the owner.     

Optimal patterns of scent marks in klipspringer (Oreotragus oreotragus) territories

The distribution of preorbital gland scent marks and dung middens within three territories of wild klipspringers in Zimbabwe are described. Nearest-neighbour analyses revealed that scent marks were distributed non-randomly and in a rough ring some distance within the territory boundary. Marking densities were greatest at about half the territory radius. In two territories, marking densities were shown to be sensitive to intrusion pressure at the periphery, being higher along contested boundaries than where territories were not contiguous. Marks were placed on branches facing neighbouring territories, where they are more likely to be detected, along contested boundaries but not in other areas. An analytical model is developed which tests the efficacy of scent-marking strategies along the continuum between extreme hinterland and extreme perimeter marking. This shows that the optimal position for a ring of scent marks is at 0.78 of the territory radius and is the product of a trade-off between maximizing the probability of mark detection by intruders and minimizing the cost of intrusion.     

The role of territory settlement,individual quality,and nesting initiation on productivity of Bell's vireos Vireo bellii bellii

Variation in habitat quality among territories within a heterogeneous patch should influence reproductive success of territory owners. Further, territory settlement order following an ideal despotic distribution (IDD) should predict the fitness of occupants if territory selection is adaptive. We recorded settlement order and monitored nests in territories occupied by individually marked Bell's vireos Vireo bellii bellii across a range of shrubland habitats in central Missouri, USA. We used an information theoretic approach to evaluate multiple hypotheses regarding the relationship between territory settlement order and seasonal territory productivity (productivity), which we define as the number of young fledged from all nest attempts in a territory. Territory settlement order and arrival date were not analogous and later arriving males displaced early settlers in 13 of 49 territories. Settlement order and lay date together were the best predictors of a territory's productivity; productivity decreased 2.08 young from earliest to latest settlement rank and lay date. Males that defended the same territory in successive years occupied territories with earlier settlement dates, but we found little evidence that age or prior ownership influenced productivity. Territory selection by male Bell's vireos was adaptive because males preferred to settle in territories that had high seasonal offspring production, but even though settlement rank was linked to territory quality, high productivity was only realized on high quality territory when also linked to early nest initiation date. While settlement rank was related to territory quality, obtaining a high quality territory had to be combined with early nest initiation to maximize productivity. We found support for the IDD hypothesis because the highest quality territories, (i.e. most productive), were settled earlier. Research that identifies high quality habitat by linking individual fitness with habitat characteristics may elucidate the importance of habitat quality, individual experience and temporal factors to productivity of Bell's vireos.     

Pairing strategies and mate choice in female robins Erithacus rubecula

Female European robins (Erithacus rubecula) adopted three different pairing strategies. These were, in order of frequency: moving on to a male's territory; fusing her territory with that of a neighbouring male; and being joined by a male on her territory. Although females were not free to choose between strategies during a particular season, individuals frequently changed strategy between different years. Their behaviour suggested that they were exercising mate choice, although the scope for such choice was limited by a strong tendency for a female to pair in areas where she had lived previously (her ‘Familiar Areas’). Males with large territories were more likely to pair and tended to pair earlier than those defending small territories. Female pairing strategy was also correlated with the area of their own territory. Although it is considered likely that some unidentified confounding variable was responsible for at least part of these relationships, possible causal explanations involving territory size are discussed. These include mate choice and a random settlement model. However the simplest explanation for the advantage enjoyed by males on large territories is that their territories were more likely to overlap with females' Familiar Areas.     

Territory Availability at Different Spatial Scales does not Affect Fighting Investment in Males of the Lycaenid Butterfly Strymon mulucha

In territorial species, rivals investment in fights over territories may increase when the availability of suitable areas for defense is low. This should occur because low territory availability may increase the costs to maintain and acquire territories. Although such process occurs in small spatial scales (local scale), territory availability in larger scales (regional scale) may also affect fighting investment, as losers should incur additional dispersing costs to find new territories. In this study, we used males of the hilltopping butterfly Strymon mulucha to evaluate the hypothesis that males should invest more in territorial fights when the costs to find new territories are higher (both at local and at regional scale). We timed male–male contests for territories located in 12 hilltops and measured male density per territory in each hilltop (local scale). We also quantified the distance between hilltops containing suitable areas for territories (regional scale). Male–male contests lasted 21 s on average, and copulations did not occur during the observations. The duration of contests was unrelated to the male density per territory or to the distance among hilltops, indicating that the investment in fights was unaffected by the availability of territorial sites, independent of the spatial scale. As male–male contests in S. mulucha are longer than the mean contest duration in other butterfly species and mating is extremely rare, we suggest that the value of each territory may be high enough to favor males that always invest as much as possible in contests.     

Defence, intrusion and the evolutionary stability of territoriality

Territorial behaviour can only be adaptive if its costs are outweighed by its benefits. Territorial individuals incur costs by defending their territories against intruders. Usually these intruders are assumed to be non-territorial floaters attempting to take over the whole territory or neighbours trying to extend the borders of their own territory. We instead investigate how costs and benefits of territorial behaviour are affected by neighbours which invade to steal resources on a territory.We show analytically that in the absence of defence intrusion into neighbouring territories always pays and that even if territories are defended intrusion levels can still be high. Using a more detailed simulation model we find that territory defence usually disappears from the population even if owners have a strong advantage over intruders in terms of fighting costs or foraging efficiency. Defence and thus territoriality can only be evolutionarily stable if fighting costs for the intruder relative to the productivity of the territory are very high or if crossing the borders between territories carries additional costs.Our results show that stealing of resources by neighbours can have a considerable effect on the evolutionary stability of territory defence and thus territoriality itself. A more mechanistic model of territorial behaviour is needed to incorporate these kinds of mechanisms into a general theory on the evolution of territoriality.     

A simulation study of subterranean termites' territory formation

The process through which the Formosan subterranean termite, Coptotermes formosanus Shiraki, establishes territory was simulated using a lattice model in order to understand how such territories are formed from their seeds-founding pairs that fall to the ground every year. The model incorporated, summer–winter cycles, and fourteen years were simulated. Simulated pairs fell to random sites within the lattice space at the beginning of every summer, and their territories grew during the summer season, and shrunk during the winter season. Fourteen years were sufficient for territory size and shape to become stable over time. The simulation revealed that only pairs introduced at t = 0 had established large territories by the end of the simulation (t = 14). Pairs that were introduced later expanded their territory a little at first, but ultimately shrunk back into a single-cell-sized or small-sized territory. This means that stable-state territory size is mostly determined by when that territory was initially established.     

Endangered species diversity ‘hot spots’ in Russia and centers of endemism

Geographic distributions of all 125 endangered and threatened animals and all 70 endangered flowering plant species were compiled and mapped by administrative territory (republics, states, provinces, and districts) of the Russian Federation, based on distributional data in the official Red Data Books of the R.S.F.S.R. [Eliseev et al. (eds) (1985) Red Book of the R.S.F.S.R.: Animals. Rosleskhoz Publishing, Moscow; Golovanov et al. (eds). (1988). Red Book of the R.S.F.S.R.: Plants. Rosagprom Publishing, Moscow]. Territories were ranked using an algorithm that gives highest rank to the single territory with the greatest number of endangered species present, then iteratively assigns the next highest rank to the territory containing the greatest number of species not found in a territory of higher rank. This algorithm minimizes the number of territories necessary to include one population of each endangered species, but may designate very low rank to a territory that holds high numbers of endangered species if many of those species are found in a territory of higher rank. When different sets of species were used to determine territory rank with this algorithm the only significant correlation between rank lists was between ranks generated when all species are used and when only species endemic to single territories were used. Mountainous territories on Russia's southern borders held the greatest numbers of endangered species and were highly ranked whether or not species occurring outside of Russia were excluded from the ranking algorithm. The Maritime state, Krasnodar state, Dagestan republic, Sakhalin province and the Jewish province all are centers of endangered species diversity that together contain 50 or more of endangered species for multiple taxonomic groups; this was the Dobson et al. [(1997) Science 275: 550–553] definition of an endangered species hot spot. These are all mountainous territories on Russia's southern border. Precise territorial ranking within the mountainous southern regions of the Southern Far East, Caucasus, and Altai/Sayan regions of southern Siberia was highly influenced by species endemic to single territories.     

Complex effects of habitat loss on Golden Eagles Aquila chrysaetos

The development of commercial forests presents potential threats to large raptors that rely on prey caught in open country. We examined the effect of afforestation of breeding habitat used by a population of Golden Eagles Aquila chrysaetos in Scotland where, over the last 50 years, extensive stands of exotic conifers have been planted. Using data for 31 years on territory occupancy and breeding success, together with spatiotemporally dynamic mapping of forest cover and predicted areas of territory‐use in a Geographical Information System, we examined relationships between forest cover and Eagle ecology at landscape and individual territory scales. Several territories were abandoned during the earliest phases of forest planting, but relatively few were apparently lost to later plantings. Territories with poorer breeding productivity appeared to be more vulnerable to abandonment than territories with better breeding productivity. At the landscape scale, temporal differences in breeding productivity were negatively related to the extent of forest cover, although productivity of individual territories showed no clear relationship with forest cover. Several territories with less than a 5% increase in forest cover experienced reduced productivity; however, territories least constrained by neighbouring pairs of Eagles showed an increase in productivity. Territories experiencing the greatest increases in forest cover showed a greater use of spatially separated nest‐sites by occupying pairs. Hence, pairs that were less constrained by neighbours appeared to compensate for loss of open habitat by shifting their territory‐use, whereas pairs that were more constrained could not compensate for open habitat loss and suffered reduced productivity (and, probably in some cases, abandoned the territory). We suggest that simple guidelines based on the extent and locations of habitat loss are inadequate when predicting effects on large territorial raptors such as Golden Eagles. Consideration should also be given to the ‘quality’ of a territory or occupying pair, as well as the extent to which territory‐use is constrained by neighbouring pairs or other ‘unsuitable habitat’ which may have been affected by previous episodes of open habitat loss.     

Prey availability affects territory size,but not territorial display behavior,in green anole lizards

The availability of food resources can affect the size and shape of territories, as well as the behaviors used to defend territories, in a variety of animal taxa. However, individuals within a population may respond differently to variation in food availability if the benefits of territoriality vary among those individuals. For example, benefits to territoriality may differ for animals of differing sizes, because larger individuals may require greater territory size to acquire required resources, or territorial behavior may differ between the sexes if males and females defend different resources in their territories. In this study, we tested whether arthropod abundance and biomass were associated with natural variation in territory size and defense in insectivorous green anole lizards, Anolis carolinensis. Our results showed that both male and female lizards had smaller territories in a habitat with greater prey biomass than lizards in habitats with less available prey, but the rates of aggressive behaviors used to defend territories did not differ among these habitats. Further, we did not find a relationship between body size and territory size, and the sexes did not differ in their relationships between food availability and territory size or behavioral defense. Together, these results suggest that differences in food availability influenced male and female territorial strategies similarly, and that territory size may be more strongly associated with variation in food resources than social display behavior. Thus, anole investment in the behavioral defense of a territory may not vary with territory quality.     

The influence of food abundance,food dispersion and habitat structure on territory selection and size of an Afrotropical terrestrial insectivore

Most tropical insectivorous birds, unlike their temperate counterparts, hold and defend a feeding and breeding territory year-around. However, our understanding of ecological factors influencing territory selection and size in tropical insectivores is limited. Here we examine three prominent hypotheses relating food abundance, food dispersion (spatial arrangement of food items), and habitat structure to territoriality in the Usambara Thrush Turdus roehli. We first compared leaf-litter macro-invertebrate abundance and dispersion, and habitat structure between territories and random sites. We then examined the relation between these same ecological factors and territory size. Invertebrate abundance and dispersion were sparsely and evenly distributed across our study system and did not vary between territories and random sites. In contrast, habitat structure did vary between territories and random sites indicating the Usambara Thrush selects territories with open understorey and closed overstorey habitat. Invertebrate abundance and dispersion within territories of the Usambara Thrush were not associated with habitat structure. We believe the most likely explanation for the Usambara Thrush’s preference for open understorey and closed overstorey habitat relates to foraging behavior. Using information-theoretic model selection we found that invertebrate abundance was the highest-ranked predictor of territory size and was inversely related, consistent with food value theory of territoriality.     

Correlation between male size and territory quality: consequence of male competition or predation susceptibility?

Territory characteristics correlate with male characteristics in several species. This can result from male competition for the best territories, or from males varying in their ability to pay other costs of territoriality, such as predation risk costs. In a population of threespine sticklebacks, Gasterosteus aculeatus , we found the biggest males to defend the biggest territories with a low structural complexity and a high female encounter rate. By experimentally manipulating competition intensity and habitat structure, we show that both male competition and predation exposure influenced the distribution of territories among males. Males increased the size of their territory when a neighbouring male was removed, whereas they reduced their territory when habitat complexity and cover from predators were reduced, with large males reducing their territory size less than smaller males. This suggests that large males occupy large, open territories both because of their superior competitive ability and because of their either lower predation susceptibility or higher risk-taking. Large, open territories were beneficial in mate attraction and male competition and predation exposure therefore biased mating opportunities towards large males. This suggests that cost of territoriality to males may reduce mate choice costs to females by securing that large males are encountered more often than small males, and by providing an additional cue, territory quality, which indicates which males are worth inspecting.     

Green woodhoopoe Phoeniculus purpureus territories remain stable despite group-size fluctuations

Cooperatively breeding groups may be constrained in size by the territory available to them, or territories may be expanded to accommodate extra group members. Here, we show that there was no relationship between the number of adult green woodhoopoes Phoeniculus purpureus in a group and the size of its territory. Furthermore, territories were remarkably stable between seasons, with no significant changes in area, despite fluctuating group sizes. These results suggest that food was not limiting at the group sizes found in this study: sufficient resources were available within existing territories for groups that were expanding in size. Following an increase in group membership, a larger proportion of the available area was utilised. Groups also used a larger area in the non-breeding season compared to when breeding: in the latter instance, foraging was concentrated in the vicinity of the nest.     

Nestsite selection by male loons leads to sex-biased site familiarity

1. The concept that animals benefit from gaining familiarity with physical spaces is widespread among ecologists and constitutes a theoretical pillar in studies of territory defence, philopatry and habitat selection. Yet proximate causes and fitness benefits of site familiarity are poorly known. 2. We used data from marked common loons Gavia immer breeding on 98 territories over 14 years to investigate the 'win-stay, lose-switch rule' for nestsite placement (if eggs hatch, reuse nestsite; if predator takes eggs, move nestsite). Males controlled nest placement in this species: pairs used the rule if both members remained the same from the previous nesting attempt or if only the male remained the same but not if only the female remained the same. 3. By means of the nesting rule, male common loons benefited from site familiarity, increasing nesting success by 41% between their first and third years on a territory. In contrast, females exhibited no increase in nesting success with increased territorial tenure. 4. Owing to site familiarity, a male loon competing for a breeding territory faces a considerable 'familiarity deficit' compared with the male breeder already established there. The familiarity deficit probably explains why resident animals often fight hard to retain familiar territories, when challenged, and why animals of many species tend to remain on familiar territories rather than moving when territories of higher intrinsic quality become available nearby.     

Predicting occurrence of wolf territories in Scandinavia

We used logistic regression to compare a set of habitat features inside known Scandinavian wolf Canis lupus territories with the same habitat features in areas outside known territories, but still close enough to be available for wolf colonization. In addition, we analysed changes in habitat variables over time within wolf territories. Wolf territories had lower densities of roads, built-up areas and open land than areas outside wolf territories, but there was no difference in the density of the wolves' main prey, elk Alces alces . The logistic regression model classified 79% of Scandinavia outside the reindeer husbandry area as suitable wolf habitat, that is with a probability of wolf occurrence >0.5. The proportion of built-up areas within the wolf territory decreased as the 'borders' of the wolf territory changed over time. Our model had a reasonably high predictive power, with correct classification in 90% (18 of 20) of the observed wolf territories in the study area. Polygons, randomly distributed outside the observed wolf territories, were correctly classified as not being occupied by wolves in 85% of the cases (17 of 20). This allows a more effective use of resources to, for example, prevent wolf depredation on livestock and dogs.     

Historical effect in the territoriality of ayu fish

Ayu fish form algae-feeding territories in a river during a non-breeding (growing) season. We build a cost-benefit theory to describe the breakdown and formation of territory. In the early stage of a growing season, all fish hold territories at low densities. Once all territory sites are occupied, excess fish become floaters. When fish density further increases, a phase transition occurs: all the territories suddenly break down and fish form a school. In contrast, when the fish density is decreased, territories are suddenly formed from the school. Both theory and experiments demonstrate that ayu should exhibit a historical effect: the breakdown and formation processes of territory are largely different. In particular, the theory in formation process predicts a specific fish behavior: an “attempted territory holder” that tries to have a small territory emerges just before the formation of territory.