Thiessen polygons as a model for animal territory estimation

Thiessen polygons are often used to model territory characteristics. However, information about the quality of Thiessen polygon‐based estimates is currently lacking. We used published data to investigate the match between Thiessen polygons and mapped bird territories regarding territory size, shape and neighbourhood. Although territory sizes and the number of neighbours were strongly correlated between these two methods, both parameters were overestimated by the Thiessen polygons. Therefore, caution is required when Thiessen polygons are used as a model for absolute values and when the assumptions of Thiessen polygons, such as formation of discrete territories and a contiguous study area, are not met.     

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.     

Couch potatoes do better: Delayed dispersal and territory size affect the duration of territory occupancy in a monogamous mammal

In territorial, socially monogamous species, the establishment and defense of a territory are an important strategy to maximize individual fitness, but the factors responsible for the duration of territory occupancy are rarely studied, especially in long‐lived mammals. A long‐term monitoring program in southeast Norway spanning over 18 years allowed us to follow the individual life histories of Eurasian beavers (Castor fiber) from adolescence in their natal family group to dispersal and territory establishment until the end of territory occupancy. We investigated whether territory size, resource availability, population density, and dispersal age could explain the duration of territory occupancy, which ranged from 1 to 11 years. The duration of territory occupancy was positively related to dispersal age, suggesting that individuals that delayed dispersal had a competitive advantage due to a larger body mass. This is in support with the maturation hypothesis, which states that an animal should await its physical and behavioral maturation before the acquisition of a territory. Further, we found that individuals that established in medium‐sized territories occupied them longer as compared to individuals in small or large territories. This suggests that large territories are more costly to defend due to an increased patrolling effort, and small territories might not have sufficient resources. The lifetime reproductive success ranged from zero to six kits and generally increased with an increasing duration of territory occupancy. Our findings show the importance of holding a territory and demonstrate that dispersal decisions and territory selection have important consequences for the fitness of an individual.     

The social organization of the Scarlet Robin Petroica multicolor and Flame Robin P. phoenicea in southeastern Australia: a comparison between sedentary and migratory flycatchers

Scarlet Robins Petroica multicolor and Flame Robins P. phoenicea occurred sympatrically during the breeding season in southeastern Australia. Scarlet Robins that occur territories in forest habitat remained there throughout the year, while those, mostly first-year birds, that occupied territories in grassland habitat were present only during the non-breeding season. Flame Robins were present for 9 months of the year and were territorial for 6 of those months. The migratory Flame Robin did not show more intense or prolonged territorial or courtship behaviour than the sedentary Scarlet Robin, despite having to obtain territories and mates when it returned each year. It is suggested that the similarities between the two species' social organizations arise partly from high adult survivorship (70–75%) and lengthy breeding seasons (4–5 months). High adult survivorship and scarcity of suitable breeding habitat also caused intense competition for territories and mates, and appears to have led to the evolution of bright, signalling plumages among female and young male Scarlet Robins. Scarcity of females and breeding habitat may have caused delayed maturation of plumage among male Flame Robins.     

Territory quality determines social group composition in Ethiopian wolves Canis simensis

1. We contrast the value of four different models to predict variation in territory size as follows: resource density (the ideal free distribution), population density, group size and intruder pressure (relative resource-holding potential). In the framework of the resource dispersion hypothesis, we test the effect of resource abundance and spatial variation in resource distribution on the age/sex composition of social groups. 2. We explore these drivers of territory size and group size/composition in Ethiopian wolves Canis simensis in the Bale Mountains, Ethiopia, using fine-scale distribution maps of their major prey species based on satellite-derived vegetation maps. 3. The number of adult males is correlated with territory size, while prey density, wolf population density and intruder pressure are not associated with territory size. On average, each additional adult male increases territory size by 1.18 km(2). 4. Prey abundance increases with territory size (average biomass accumulation of 6.5 kg km(-2)), and larger territories provide greater per capita access to prime foraging habitat and prey. 5. The age/sex composition of wolf packs is more closely related to territory quality than territory size. Subordinate adult females are more likely to be present in territories with greater proportions of prime giant molerat Tachyoryctes macrocephalus habitat (i.e. >80% of Web Valley territories and >20% in Sanetti/Morebawa), and more yearlings (aged 12-23 months) occur in territories with greater overall prey biomass. 6. Wolf packs with restricted access to good foraging habitat tend to defend more exclusive territories, having a lower degree of overlap with neighbouring packs. 7. The greater per capita access to prey in large groups suggests a strong evolutionary advantage of collaborative territorial defence in this species, although the relative costs of territorial expansion vs. exclusion depend upon the spatial distribution of resources. We propose a model whereby territory size is determined by the number of adult males, with the presence of subordinate females and yearlings dependent on the quality of habitat, and the abundance and distribution of prey, incorporated within territory boundaries.     

Territorial responses to energy manipulations in the Anna hummingbird

Summary Territorial activity in the Anna hummingbird (Calypte anna) was measured while energy availability on the territory was varied. On days when energy availability was unlimited, residents defended highly exclusive territories primarily by energetically expensive defense behaviors. As energy availability decreased, exclusiveness declined gradually, relative use of energetically inexpensive defense increased, and owners spent less time on the territory.Territorial behavior also varied with short term depressions in energy availability: A lower percentage of intruders was chased and departures of an owner from its territory were more frequent shortly after feeding.When resource dispersion was increased without changing substantially total rewards per territory, chasing by owners increased.     

Causes and consequences of individual variation in territory size in the American red squirrel

For most territorial species that defend food or other resources, territory size is inversely related to resource density. However, in some food-based territorial species, larger territories are known to contain greater absolute resource availability. For these latter species, both the specific determinants of territory size, and the relationship between territory size and fitness, remain unclear. We predicted that in American red squirrels ( Tamiasciurus hudsonicus ), where larger territories tend to hold increased food resources, survival should be correlated positively with territory size and that both individual (body mass, sex, duration of territory ownership) and habitat-specific (population density) attributes would contribute to variation in territory size. We monitored survival and territory size of 58 squirrels during a two-year period using live-trapping and radio-telemetry. Survival and territory size were correlated positively, with each 0.1 ha decrease in territory size typically resulting in a 16.6% increase in mortality risk; mortality risk for squirrels with the smallest versus largest territories differed by up to 23-fold. Territory size was negatively related to local population density, but not to any attributes of individual squirrels, implying that, in this species, territory size may not be influenced strongly by individual quality. We conclude that while larger territories may confer a survival advantage to squirrels, territory size is unrelated to individual physical attributes, likely because costs associated with the acquisition of a larger or richer territory outweigh potential benefits of increased food access.     

Territorial behaviour of the Robin Erithacus rubecula: the importance of vegetation density

Robin Erithacus rubecula territories were mapped in an isolated copse of mixed deciduous woodland. An index of vegetation density, measured at numerous randomly chosen sites within the copse, was found to be higher in areas occupied by Robins. Both vegetation density and territory area were compared with measures of breeding success. In all cases vegetation density was correlated with breeding success and territory area was not. Possible evolutionary advantages to defending areas with dense vegetation are discussed.     

Determinants of territory size in the pomacentrid reef fish,Parma victoriae

Summary Factors governing the size of territories defended by the pomacentrid reef fish, Parma victoriae, were investigated, prompted by contradictory predictions in the literature concerning the effects of food supply and competitors. Observations were carried out over the non-breeding period (March–October) on a medium density population in which territories were partially contiguous. The territory size of adult fish varied between 3 and 26 m², and was inversely correlated with local densities of conspecifics. The same range in territory size was found for both males and females, which did not differ in the time they spent on territory defence and foraging activities. No correlation existed between territory size and the abundance of algal food, body size, age or time spent on territory defence. Also, there was little variation in territory size over time, despite seasonal changes in the abundance of food algae.Experimental reduction of food supplies on isolated territories of males and females had no effect on territory size. In a higher density habitat an experiment was carried out in which population density and food abundance were simultaneously manipulated. This showed that territory size was primarily determined by intraspecific interactions, as territories exhibited considerable increases in size upon removal of neighbours. No changes in the size of defended areas resulted from either artificial increases or decreases of food levels. There were also no changes in the time spent on defence of territories, foraging time or feeding rates associated with food manipulations or territory expansion, which suggested that food was not a limited resource. This conflicted with current theories proposed to explain territory defence and expansion. It is hypothesized that intraspecific interactions constrain territory size well below the optimum in terms of the abundance of preferred food algal species.     

Patterns of territory settlement and consequences for breeding success in the Northern Wheatear Oenanthe oenanthe

We examined the pattern of territory settlement and its consequences for breeding success in the Northern Wheatear Oenanthe oenanthe on Bardsey Island, Wales, during the breeding seasons of 1991-93. Males returned earlier than females, and older males returned earlier than first-year breeders. Although their boundaries shifted between years, the general location of territories was consistent during the three-year study. There was a high degree of fidelity to area and territory between years for both sexes. The order of territory settlement, from which a territory rank was calculated, was highly consistent for males between years irrespective of individual settlement patterns and territory fidelity. Patterns of territory settlement were less predictable for females, although there was a significant correlation between the mean territory ranks of paired males and females. There was a male-biased sex ratio in each year, and between 5% and 26% of males remained unpaired throughout the breeding season. Male mating status and breeding success were dependent on arrival date, territory rank and breeding density. Early-arriving, usually older, males were able to settle on territories first and were more likely to pair, while later-arriving individuals were more likely to remain unmated. These effects were consistent between years, and consequently territories could be classified as either preferred (accounting for proportionately more breeding attempts) or non-preferred. Territory quality as opposed to individual quality appeared to explain much of the variation in breeding success, and both sexes benefited by breeding on preferred territories through enhanced breeding success and an increased probability that their offspring would be recruited to the population.     

Intruder Pressure Affects Territory Size and Foraging Success in Asymmetric Contests in the Water Strider Gerris lacustris

The theory of evolutionarily stable strategies (ESS) in asymmetric contests predicts that the propensity of an individual to expose itself to risk during contests depends on the individual's resource-holding-potential (RHP) and on the value of the disputed resource (V) for the individual compared with that for an opponent. If encounters of a territory owner with individuals of high RHP and high food demands (V) increase in frequency, one should expect a decrease in total aggressiveness of the territory owner, and in consequence a decrease of its territory size. Such a decrease should result in a lower amount of food consumed by the territory owner. Using natural variability in RHP and V in Gerris lacustris, I experimentally tested these predictions. The average prey item has higher value (V) for reproductive female water striders (which probably transform most of their food into eggs), than for nonreproductive females and for males. Because males are smaller, they have lower RHP than females, as RHP depends on size. Thus the reproductive females are the class of individuals of high RHP and high food demands (high V). Most nonreproductive females defend food-based territories. I observed two groups of water striders in a seminatural laboratory setting. As predicted, there was a negative correlation between the rate of encounter with reproductive females and size of the territory, and a positive correlation between territory size and number of Drosophila flies consumed by the owner. Territories were smaller in the group with high rates of encounter between territory owners and reproductive females. Territory owners caught the same number of Drosophila flies as non-territorial individuals in this group. In contrast, in the group with fewer encounters between territory owners and reproductive females, territories were larger, and territory owners gained more food than non territorial water striders.     

Survival, site and mate fidelity in South Polar Skuas Catharacta maccormicki at Anvers Island, Antarctica

In 1974–1975, 34 adult South Polar Skuas Catharacta maccormicki were colour-ringed on 18 nest territories at Bonaparte Point, Anvers Island, near Palmer Station along the Antarctic Peninsula. Subsequently, the area was searched for these birds during the austral summers of 1975–1976 to 1984–1985 and in 1987–1988 and 1989–1990. Fifty-three percent were seen in 1984–1985, 32% in 1987–1988 and 21% in 1989–1990. Annual survival rate averaged 95% from 1974–1975 to 1984–1985; no sexual differences were detected (n = 28 of known sex). Strong territory and mate fidelity were apparent: 34 skuas averaged 1.1 nest territories and 1.7 mates each in 16 years. Only 4 of 34 individuals (all females) were known to change territories, and each territory change involved a change of mates. Although males showed higher territory fidelity than females (P < 0.01), most females (four of five) retained their territories when previous mates failed to return. Seventeen of 34 birds changed mates a total of 24 times; at least 20 mate changes followed the death or disappearance of the former mate. Males showed slightly higher mate fidelity than females (P < 0.04). Female South Polar and Brown Skuas Catharacta lonnbergi did not differ in territory or mate fidelity. From 1974–1975 to 1984–1985, 120 South Polar Skua chicks were ringed on 18 nest territories on Bonaparte Point: 17 were resighted in the Palmer area when they were 3–10 years old. All 15 returnecs were found within 3 km of their natal nest sites, and four of them occupied nest territories on Bonaparte Point.     

Winter territory prospecting is associated with life‐history stage but not activity in a passerine

Finding a high quality territory is essential for many animals to reproduce successfully. Despite its importance for fitness, we know little about the process of territory prospecting in wild birds, and whether individual traits and behaviours, such as personality, co‐vary with territory prospecting. Here, we use long‐term data from a wild, insular house sparrow Passer domesticus population to test three hypotheses about territory fidelity and prospecting: 1) house sparrows show high territory fidelity between years and also during winter. 2) Individuals will prospect for a breeding territory during their first winter whereas older, more experienced individuals will keep a territory from previous years and will, therefore, show no or reduced winter territory prospecting. 3) More active behavioural types will prospect more than less active behavioural types. We use data from four winters from automatically, daily recorded nest‐box visits of 188 birds of known age. The number of nest‐boxes that each individual visited within each winter was used as a proxy of winter territory prospecting. We show that house sparrows visit multiple nest‐boxes during their first winter, whereas older individuals keep territories year‐round and, potentially because of this, indeed show reduced winter territory prospecting. Activity was not associated with the number of nest‐boxes visited. Further research is needed to investigate whether time of territory and mate acquisition differs among individuals and the possible effect on lifetime fitness.     

Intertidal territoriality and time-budget of the surgeonfish, Acanthurus lineatus, in American Samoa

Synopsis The herbivorous surgeonfish Acanthurus lineatus aggressively maintained feeding territories in the surf zone of the outer reef flat in American Samoa. Intertidal territories were re-established each morning, as well as after displacement by low tides or rough surf. Day-to-day site fidelity of recognizable individuals was high: 99.9% return rate per day for adults (15–20 cm), 99.6% for juveniles (8–13 cm), and 97.2% for recruits (2.5–5 cm). Fish fed on turf algae primarily in the afternoon (80% of available time), and spent 10% of their time on active territorial defense and 2–13% of their time on forays from their territory. On average, a fish defended its territory 1900 times daily and took 17 000 bites (= 7400 bites m^–2 d^–1), but rough surf reduced feeding by 60% and defense by 75%. High territorial defense requirements significantly reduced feeding rates. Although the distribution and behavior of this species in Samoa was in large part similar to that reported for it elsewhere (Australia, Indian Ocean), there were notable differences: in Samoa A. lineatus densities within colonies were greater (0.4 fish m^–2), territory size was smaller (2.3 m²), and defense rate against intruders was greater (2.5 attacks min^–1). These differences in Samoa may be related to their smaller body size, greater abundance or increased food supply caused by hurricane damage to reefs which has enhanced the algal turfs that A. lineatus feeds upon.     

Ontogeny of territoriality in the desert clickerLigurotettix coquilletti (Orthoptera: Acrididae)

Field observations and experiments showed that settlement prior to reproductive maturation strongly influenced territorial success in the desert clicker(Ligurotettix coquilletti), an acridid grasshopper in which males defend individual host plants (Larrea tridentata bushes) as mating arenas. High tendencies to move and reposition among the host plants were displayed in two distinct episodes. The first occurred during early nymphal development, and it resulted in many individuals settling on high-qualityLarrea bushes. Prior work showed thatL. coquilletti that fed on foliage from these specific bushes exhibited higher relative growth rates and therefore were expected to eclose sooner. Early-eclosing males enjoyed a priority advantage in defending high-quality bushes as mating territories, and they consequently encountered more females and mated more frequently. Despite the distribution of most insects on high-quality bushes at the time of eclosion, though, a second movement episode occurred shortly thereafter. This reshuffling contrasts markedly with the site fidelity of mature adult males, most of which settle on mating territories, and it may function as a means of examining an enlarged sample of potential sites. The above results imply that territorial success of maleL. coquilletti does not result from retaining sites defended by the parents. This point is also supported by the finding that females do not oviposit particularly close to theLarrea bushes in which they reside regardless of their quality; most egg pods are deposited in bare soil midway between the bushes. Nonetheless, certain oviposition sites may be conducive to earlier hatch, and this can lead to earlier eclosion and ultimately to defense of a valuable territory. Therefore, the parental generation, through selective oviposition, may yet influence the success of their male offspring.     

Female breeding dispersal in wolverines,a solitary carnivore with high territorial fidelity

Individuals’ fidelity to an area provides insight into the stability of the spatial and temporal organisation of animals. Territorial fidelity is often influenced by reproductive success, age, the dispersion and predictability of resources, and intraspecific competition. We examined between-year territorial fidelity in wolverines (Gulo gulo), using location data from 47 individuals collected during 1993–2013 in northern Sweden, to assess the stability of the spatial organisation of this solitary carnivore. For females, we also determined residency status from 1 year to the next. The study population is characterized by a stable distribution of resident individuals, with both males and females showing higher fidelity at the total territory level compared to more intensively used core areas. In 86% of the yearly residency status estimates (n?=?122), the female remained stationary. In the remaining 14% of the cases, females either vacated their territory (8% of residency statuses), or expanded into a neighbouring territory (6% of residency statuses). We documented six cases of breeding dispersal, representing one of the few known cases of breeding dispersal in long-lived large mammals. We suggest that this high territorial fidelity is enabled by wolverines’ caching and scavenging behaviours, which buffer the unpredictable and large spatiotemporal variation in resource abundance in this low-productivity area. Breeding dispersal may occur due to competition for high-quality territories in this saturated population, where females are forced to abandon their territory by competitors or bequeath territories to offspring. This study further highlights the complexity of the social and spatial dynamics for solitary carnivores.     

Winter territoriality of Mute Swans Cygnus olor

There was considerable variation in winter territoriality among Mute Swan pairs around Oxford, though most pairs maintained their territories for at least half the winter. Variation was associated with temperature and with territory quality pairs left their territories most commonly during the coldest months of the winter, and pairs on good territories, with abundant vegetation and accessible pasture. stayed longer on territory than those on poor territories with sparse vegetation and little pasture. As in other species, exclusive territorial defence was maintained at intermediate levels of resource availability, and flocks were able to settle on a few particularly good territories. Circumstantial evidence suggested that winter territories may be maintained not only to provide an adequate food supply for the owners but also to ensure access to a brerding site by preventing other individuals from taking over the area.     

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.     

The sociality of solitary foragers: a model based on resource dispersion

Under versions of what may broadly be called the Resource Dispersion Hypothesis (RDH) several authors have concluded that territory size and group size are limited respectively, and independently, by the dispersion and richness of patches of food. This paper presents a model that shows how the frequency distribution of resources available per unit time within a territory may permit the formation of groups even in the absence of any functional advantage to any individual from the presence of another. In this model, animals (called primary occupants) occupy territories containing sufficient resources to meet or exceed their requirements for a critical proportion of feeding periods. The availability of these resources is described in terms of their mean richness and their heterogeneity, and plots of these parameters indicate the circumstances within which individuals may share the minimum territory with the primary occupants. The model shows how, under plausible conditions of resource dispersion, a territory that provides almost total food security for two occupants could also provide, at no cost to the original occupants, substantial food security for an additional group member, even if it never used the same food patches as the originals. It is not therefore, as is sometimes supposed, a necessary condition for the RDH that members of a group often forage simultaneously in the same patch. Thus the model describes ecological circumstances whereby groups could evolve amongst species whose members neither forage communally, nor even meet frequently.     

Resource Defence or Exploded Lek? – A Question of Perspective

It becomes increasingly obvious that animal mating systems cannot be classified into distinct categories, but transitions between mating system classes are continuous. Positioning a certain mating system at this continuum is often not straightforward, however. Depending on which characteristic is considered, a mating system may end up at very different positions on this gradient. Here, we explore the potential conflict between mating system classifications that may arise when they are based on different criteria by investigating the mating system of the cichlid fish Simochromis pleurospilus in which males defend small patches of homogeneously distributed food resources (turf algae) vigorously against food competitors, but they allow specific females to use them. We hypothesized that male defence may generate high‐quality feeding patches serving to attract females, and hence male territoriality constitutes a form of courtship. Our field data show that males selectively allow approximately one‐third of the visiting females to feed on their territory and that females preferentially feed in male territories and usually sample several territories successively. As males protect food patches against other algae grazers and guard females from harassment by food competitors, females gain nutritional benefits from visiting male territories. Hence, males appear to generate essential resources for females, which is the key feature of resource‐defence mating systems, although the distributions of resources and of males and females are characteristics of an exploded lek.