Nest Territory Research Articles

The role of androgens in the trade-off between territorial and parental behavior in the Azorean rock-pool blenny, Parablennius parvicornis

Androgen hormones have been shown to facilitate competitive ability in courtship and territorial behavior, while suppressing paternal behavior. The rock-pool blenny, Parablennius parvicornis, provides an excellent model to study the proximate regulation of such a trade-off between territorial and parental behavior, because nest-holder males of this species display these behaviors simultaneously. A field study was carried out in which territorial nest holder males were either treated with long-lasting implants filled with 11-ketotestosterone (11-KT) or with control implants. Males treated with 11-KT showed a higher frequency of aggressive behavior, were more responsive to aggressive challenges, and were more persistent in aggressive behavior than control males. In addition, territories were larger in males treated with 11-KT than in controls. We found evidence for incompatibility between defense of a large territory and high levels of parental behavior. However, contrary to expectation, 11-KT did not suppress parental behavior. We suggest that trade-offs between territorial and parental behavior may not be regulated by androgen hormones but may result from a time constraint in the individual's activity budget.

Nest Success of Yellow Warblers in Willow Habitats: The Role of Surface Water and Snakes

Yellow Warblers (Dendroica petechia) are migratory songbirds found in high abundance in the tall willow (Salix boothii) habitats of Grand Teton National Park (GTNP). Willows are found in wet soils with high water tables and varying densities of exposed surface water. Dense surface area of water leads to thick, well foliated, continuous patches of S. boothii, which is the favored nesting habitat of Yellow Warblers. This water, however, also supports the favored prey base of the wandering garter snake (Thamnophis elegans vagrans), which has been known to prey on songbird nest contents when the opportunity arises. We hypothesized that nest territories containing high densities of surface water would also attract garter snakes, and increase the probability of nest failures. We found and monitored 28 Yellow Warbler nests, recorded their locations and fates (success or failure) and measured the density of surface water within each nest territory. We analyzed nest success with the logistic-exposure method coupled with comparisons of models with and without water density as an explanatory variable. Information theoretic model comparison consistently supported models with water density over those without. A significant correlation was found between water density within a nest territory and that nest's daily survival probability. The estimate of this effect was -0.049 (a logistic model parameter) with a standard error of 0.019. Expressed alternatively, each 5-meter per territory increase in waterway density decreases the odds ratio of nest survival by 21%. While water density provides a trade off between nesting habitat and predation pressure, other predation causes and temporal water density variation likely contribute to overall warbler productivity in important ways as well.

Territory fidelity, reproductive success and prey choice in the brown falcon, Falco berigora: a flexible bet-hedger?

The brown falcon, Falco berigora, is one of Australia's most common and widespread raptors, inhabiting a broad array of habitats and most climatic zones across Australia. We monitored a large, marked population (44–49 pairs) over three annual breeding seasons in southern Victoria. Reproductive parameters such as clutch size and the duration of parental care were constant across years. However, there were marked differences in brood size and the proportion of pairs breeding. Both sexes of falcons were found to have high territory and mate fidelity, with only 10% of members of each sex changing territories during the study. Falcons were flexible in their choice of nest sites, using a variety of tree species and even isolated nest trees. Nest sites and territories were regularly distributed throughout the study area, with the density of the population the highest on record for this species. The diet of the population as a whole was very broad, but each pair predominantly specialised on either lagomorphs, small ground prey, small birds, large birds or reptiles. Individuals that changed territory within the study area also switched their diet according to the predominant land-use within the new territory and thus prey availability. We argue that, at the population level, broad dietary breadth, flexibility in choice of nest site, and a conservative, static breeding strategy allows the species to persist in a broad range of environments, possibly through 'bet-hedging'. At the individual level, changeable dietary specialisation, high territory fidelity, strong year-round territorial defence, confining breeding to years when individual conditions were favourable and adjusting brood sizes when required appear to be the main strategies enabling brown falcons to thrive under a variety of conditions.

Ecological causes of sex-biased parasitism in threespine stickleback

Males and females can differ in levels of parasitism and such differences may be mediated by the costs of sexual selection or by ecological differences between the genders. In threespine stickleback, Gasterosteus aculeatus, males exhibit paternal care and territorial nest defence and the costs of reproduction may be particularly high for males relative to females. We monitored levels of parasitism for 15 years in a population of stickleback infected by four different parasite species. Consistent with general predictions, overall parasite prevalence (total parasitism) was greater in males than in females. However, this excess did not occur for each species of parasite. Males had higher prevalence of a cestode Cyathocephalus truncatus and a trematode Bunodera sp. relative to females, while females had higher prevalence of a cestode Schistocephalus solidus and nematodes. This suggested ecological sources to differences in parasitism rather than reproductive costs and therefore we examined diet of unparasitized stickleback, predicting that differences in dietary niche would influence relative parasitism. This was partially confirmed and showed that female stomach contents had increased frequency of pelagic items, the major habitat for the primary host of S. solidus whereas males exhibited increased frequency of benthic items, the dominant habitat of C. truncatus and Bunodera. Temporal shifts in the extent and direction of differential parasitism among years between the sexes were associated with temporal shifts in dietary differences. Our results, combined with those in the literature, suggest that ecological differences between genders could be a more important component to patterns of parasitic infection in natural populations than currently appreciated.

Ecological causes of sex-biased parasitism in threespine sticklebac

Males and females can differ in levels of parasitism and such differences may be mediated by the costs of sexual selection or by ecological differences between the genders. In threespine stickleback, Gasterosteus aculeatus, males exhibit paternal care and territorial nest defence and the costs of reproduction may be particularly high for males relative to females. We monitored levels of parasitism for 15 years in a population of stickleback infected by four different parasite species. Consistent with general predictions, overall parasite prevalence (total parasitism) was greater in males than in females. However, this excess did not occur for each species of parasite. Males had higher prevalence of a cestode Cyathocephalus truncatus and a trematode Bunodera sp. relative to females, while females had higher prevalence of a cestodeSchistocephalus solidus and nematodes. This suggested ecological sources to differences in parasitism rather than reproductive costs and therefore we examined diet of unparasitized stickleback, predicting that differences in dietary niche would influence relative parasitism. This was partially confirmed and showed that female stomach contents had increased frequency of pelagic items, the major habitat for the primary host of S. solidus whereas males exhibited increased frequency of benthic items, the dominant habitat of C. truncatus and Bunodera. Temporal shifts in the extent and direction of differential parasitism among years between the sexes were associated with temporal shifts in dietary differences. Our results, combined with those in the literature, suggest that ecological differences between genders could be a more important component to patterns of parasitic infection in natural populations than currently appreciated.

Nest, but Not Egg, Fidelity in a Territorial Salamander

Egg recognition and subsequent egg brooding are costly forms of parental investment in many species of vertebrates. Life history factors, such as coloniality or risk of brood parasitism, may constrain egg recognition in vertebrates. Female red‐backed salamanders (Plethodon cinereus) from my study site are territorial and do not share nest sites with other females. They are terrestrial and neither they nor their eggs are likely to be displaced by environmental factors such as flooding. I experimentally tested, in the laboratory, the hypothesis that female red‐backed salamanders can discriminate between their own eggs and the eggs of unfamiliar females. Each female was allowed to move about a test chamber containing two clutches of eggs, one clutch with which it was found in the forest and one that had been found with a distant female. Most females remained with one clutch of eggs, which they brooded during the entire observation period. However, they did not significantly prefer to brood their own eggs over the eggs of another female. In a corollary field experiment, I tested whether brooding females that were displaced 1 m from their nest sites would return to their territories and commence brooding behaviour within 3 d. All 10 displaced females returned to their own nest within this time period and were found brooding their eggs. Because female red‐backed salamanders at my study site do not tend to share nest sites with other females and because their eggs remain in stationary nests, selection may not have favoured egg recognition. However, the results suggest that female salamanders indirectly recognize their own eggs by actively recognizing their territorial nest sites.

Nest and mate choice in the red bishop (Euplectes orix) : female settlement rules

We investigated female settlement in a colony of red bishops (Euplectes orix), a territorial and highly polygynous weaverbird widely distributed over sub-Saharan Africa. An earlier study showed that male reproductive success is mainly determined by the number of nests a male builds in his territory, which appeared to be a good indicator of male quality. Because males provide no parental care or food resources within the territory, females sharing a territory do not compete for material resources and might therefore be expected to settle preferentially in territories of males that build many nests to gain the possible genetic benefit of high-quality offspring. An analysis of female settlement, however, revealed that females did not show a preference for territories of males with many nests and that the distribution of female breeding attempts with regard to the number of vacant nests within a territory could be explained best by random female settlement in 3 out of 4 years. Females settled more often than expected by chance (in 3 out of 4 years) in territories already containing occupied nests, indicating that resident females did not discourage settlement of additional females. However, sharing a territory with other females might impose costs in terms of an increased predation risk because nests in territories that contained other nests with young suffered from higher predation than nests in territories that did not contain other nests with young. Females therefore might trade the possible benefits of settling in territories of males with many nests against the costs of sharing a territory with other females. This might result in the mating pattern found with random female settlement and male reproductive success being directly proportional to the number of nests built. We discuss possible implications of this mating pattern for sexual selection on males. Key words: Euplectes orix, female choice, male quality, predation, random female settlement, sexual selection, weaverbirds. [Behav Ecol 11:378–386 (2000)] I n polygynous species with resource defense polygyny, territorial males provide resources that are important for reproduction such as food or nest sites within the territory to attract more than one female. Females often have to make a choice between a male without mate and an already mated male. If there is variation in the amount or quality of the resources provided by the males, it might pay for the female to choose an already mated male with, for example, a better territory, instead of an unmated male, because the advantage of higher territory quality outweighs the disadvantage of sharing the territory and the available food within the territory with another female. If the quality differences among territories are large, this will lead to high variation in male mating success (polygyny threshold model; Verner and Willson, 1966; Orians, 1969), and female mating preferences can be explained by direct benefits (e.g., better offspring survival resulting from higher food resources within the territory). In contrast, in polygynous species, where males provide females with little material resources, such direct benefits of female mate choice are less likely to fully explain female mating preferences. Instead, indirect mechanisms as proposed by ‘‘good genes’’ models of sexual selection may play a more important role. We studied sexual selection in the red bishop (Euplectes orix), a sexually dimorphic and polygynous weaverbird from Southern Africa. Males in breeding plumage establish and defend territories in reedbeds or bullrush stands around water, where they construct several nests during a breeding season to which they try to attract females. Most territories contain one or more empty nests at any time during the breeding

Recruitment and Demographic Variability in a Lagoonal Population of the Blenniid Fish Lipophrys Pavo

The demography of a lagoonal population of Lipophrys pavo over a four year period is described. This population exhibited high variability in recruitment and mortality and as a result, in age structure and density. During the study the average size at a given age decreased. This decrease was not linked to growth variation. Changes in demography and length structure are interrelated and could be linked to stressful environmental conditions and to reproductive patterns of this species.Lipophrys pavo (Risso, 1810) is a common species in shallow waters of the rocky littoral of the Mediterranean sea (Zander, 1972; Patzner, 1984). This species differs from other Blenniidae by its ability to colonize biotopes where hard substrates are scarce (Moosleimer, 1980; Almada et al., 1994; Ruchon, 1995), and by its great euryhaline and eurythermal tolerance limits (Marion, 1890; Paris & Quignard, 1971; Zander, 1972; Müller et al., 1973; Moosleitner, 1988; Patzner & Seiwald, 1988). This euryvalence has allowed L. pavo to colonize brackish and soft-bottom lagoons of the Gulf of Lion where it is the single Blenniidae present (personal observation). Lipophrys pavo is a gonochoric species exhibiting strong sexual dimorphism (Zander, 1975; Papaconstantinou, 1979). Males are territorial nest breeders during the spawning season (Patzner et al., 1986), and females are batch spawners (Fishelson, 1963a; Patzner, 1985a).

Factors related to aggressive nest protection behaviour: a comparative study of Holarctic waders

Data on 12 factors presumed to influence the distribution of aggressive nest defence in 111 species of waders (incubation-sharing by the parents, number of parents present near the nest, incubation time, nest habitat, breeding latitude, body mass, wing loading, wing structure, detectability on the nest, predator regime, coloniality and alternative prey) were collected from literature and field researchers. Body mass and number of parents present on the nest territory (within response range when avian predators appear) explain 50% of the variation in aggressive defence behaviour. The results support the notion that ecological conditions like predation pressure are important in shaping wader parental care systems, with implications for mating systems. Altogether, the investigated factors explain around 70% of the variation in the samples. Future research on the level of individuals is suggested in order to explain the remaining variation.

Effect of density on the reproductive behaviour of the territorial male demoiselle Chromis dispilus (Pisces: Pomacentridae)

Characteristics of nest sites and reproductive behaviour of territorial male demoiselles Chromis dispilus were examined at sites of low, medium and high population density in northeastern New Zealand, by use of SCUBA diving and a remote underwater video system. Nest territories were closer together at high density (relative to areas of low population density) and this was accompanied by an increase in the frequency of spawning sequences and territorial defence against conspecifics. Territorial males were more aggressive during periods of display and spawning than during egg guarding. Fish from areas of low density spent more time on courtship display (signal jumps) and territorial defence against fish of other species, than demoiselles from areas of high population density. Territories and nest sizes tended to be largest in areas of medium population density. As in other damselfishes, population density is a major determinant of the frequency and intensity of reproductive behaviours.

Nesting Henderson Reed-Warblers (Acrocephalus vaughani taiti) Studied by DNA Fingerprinting: Unrelated Coalitions in a Stable Habitat?

Using DNA fingerprinting we studied Henderson Reed-Warblers (Acrocephalus vaughani taiti), which are confined to Henderson Island in the central South Pacific. During the single study season, the birds had a well-defined nesting period from late August to early January. About one-third of nesting groups comprised three, not two adults. The members of trios, which could include two males and one female, or one male and two females, were unrelated. However, all members of trios contributed to incubation and/or feeding the young, whether or not they were parents of the chicks. The output of young per adult was slightly but not significantly higher in pairs than trios. We suggest a compensating advantage for members of trios: in the stable island habitat, young birds may be more readily able to secure a nesting territory when belonging to a trio than when in a pair. Received 16 August 1993,

Spatial Model of Territorial Competition and Population Dynamics in the Fire Ant Solenopsis invicta (Hymenoptera: Formicidae)

We developed a simulation model of monogyne red imported fire ant, Solenopsis invicta , populations that operates by competitive interactions of individual colony territories. This model describes six major aspects of colony lite history: nest founding, brood raiding, territory expansion, queen death, possible requeening, and colony death. Individual colony territories are represented by rectangles, whose sides change independently depending upon the presence and size of neighboring colonies. Growing colonies try to occupy free space, while retaining square territorial dimensions whenever possible. The growth of fire ant populations in the model compared well with available field data. The model predicted that competitive exclusion caused by territorial interactions should occur when areas reach 70–90% territory coverage. Several computer experiments were conducted with the model to examine the importance of colony growth rate, brood raiding activity, and other parameters on the relative success of sequentially founding colony cohorts. A 1-wk advantage in founding generally increased the long-term survival of young colonies by 40–80%. We also examined the factors affecting average coverage of an area by colony territories. Maximum coverage was achieved under a continuous influx of 50 queens/(ha·wk) or a single settlement of 1000 queens/ha; both values were much less than actual magnitudes of queen influx observed in the field.

Population and sex differences in antipredator responses of breeding fathead minnows (Pimephales promelas) to chemical stimuli from garter snakes (Thamnophis radix andT. sirtalis)

We conducted a predator bite survey on a population of fathead minnows (Pimephales promelas) considered to be under substantial predation pressure by western plains garter snakes (Thamnophis radix). Scarring, due to failed predation attempts by garter snakes and crayfish (Orconectes virilis), was observed significantly more often in breeding males than in breeding females and nonbreeding minnows. Likely, territorial nest defense under the edges of rocks along the water's edge, a habitat occupied by crayfish and frequented by snakes, caused the breeding males to be differentially vulnerable to predation. Under controlled laboratory conditions, breeding males from this population exhibited an antipredator response to chemical stimuli from live snakes (T. sirtalis andT. radix) significantly more often than breeding female minnows from the same population and breeding minnows of both sexes from a population that was presumed to be under lower predation pressure from snakes.

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.

Patterns of nest site, territory, and mate switching in black-billed magpies (Pica pica)

A colour-marked population of black-billed magpies (Pica pica) was studied in Edmonton, Alberta, from 1982 to 1988. Between years, magpies changed nest sites within their territories significantly more often than they remained in the same site. Both sexes showed equally strong territory fidelity, with 72% of the males and 70% of the females nesting within their territories of the previous year. Breeding success in the previous year did not determine whether magpies switched nest sites within territories or changed territories, or how far they moved between years. In 1 year (1987), breeding failure (during a snowstorm) influenced the switching of neither nest sites nor territories. Although there was remarkable mate fidelity in some pairs (paired for 5–7 years), other magpies changed mates up to three times over the 7 years of the study. Both sexes showed a similar tendency to switch mates; 50% of the males and 63% of the females changed mates at least once in 7 years. Mate switching was independent of reproductive success in the previous year. There was a positive association between mate fidelity and territory fidelity in males: males that changed territories were also likely to change mates. These results are compared with those from other studies of black-billed magpies and discussed in the context of life-history strategies.

Theoretical considerations of density-dependent spacing of a mound-building ant ( Formica altipetens)

Nests of the mound-building ant, Formica altipetens (Hymenoptera: Formicidae), exhibit a pattern of density-dependent spacing. This pattern is observed across 12 small sites (0·0625 and 0·16 ha) and one larger site (2 ha), where F. altipetens nest density varies from 0·01–0·10 nest m−2. Nests are clumped on sites where the average density is less than 0·02 nests m−2. Nests are regularly dispersed on all sites where the average nest density is greater than 0·02 nests m−2. Nest spacing is increasingly regular on sites where there are more ant nests. A simple model that includes both clumped dispersal and local competition during nest establishment also yields a density-dependent spacing pattern. The model consists of two basic parameters: the dispersal distance (D) and the nest territory size (T). A parameterization of the model where D = 10 m and T = 2·5 m explains 75·7% of the empirical variance in an index of nest spacing (the index of dispersion, R, from the Clark & Evans nearest neighbor distance analysis). The spatial patterns of F. altipetens differ quite strongly from “null” versions of the model (no competition with either random dispersal or clumped dispersal). Thus, a model that includes both competition and clumped dispersal explains the spatial arrangement of a mound-building ant in both a qualitative and a quantitative sense.

Changes in plasma levels of gonadal steroids during spawning behaviour in territorial male demoiselles Chromis dispilus (Pisces: Pomacentridae) sampled underwater

Males of the demoiselle Chromis dispilus (a species endemic to New Zealand) were hand-netted from nest territories by scuba divers and blood sampled in situ. Reproductive condition and behaviour prior to capture were compared with plasma levels of testosterone (T) and 17α,20β dihydroxy-4-pregnen-3-one (17,20βP). Spawning cycles of about 7 days, consisting of 1–2 days of spawning activity followed by 4–5 days of brooding behaviour, were observed. Plasma levels of T and 17,20βP were elevated (4–6 and 1–1.5 ng · ml −1, respectively) in fish that were engaged in courtship display, spawning, or guarding freshly spawned eggs. Levels of both hormones fell to <1 (T) and <0.4 ng · ml −1 (17,20βP) at the end of spawning episodes and remained low through the subsequent brooding period. T levels increased slightly toward the end of the brooding phase. Samples taken at 2- to 3-hr intervals during daylight hours showed similar correlation of steroid levels and activity; however, there was a marked fall in 17,20βP in spawning fish with the onset of shelter seeking at dusk. Values returned to high levels with the resumption of spawning the next day. Plasma steroid levels were not consistently correlated with reproductive condition (degree of spermiation). It is concluded that there is a strong correlation between plasma T and 17,20βP levels and the male spawning cycle, but not brooding or territorial behaviour per se.

Breeding of the Peregrine FalconFalco peregrinus:I. Weather, Nest Spacing and Territory Occupancy

SummaryOlsen, Penny D. & Olsen, Jerry (1988). Breeding of the Peregrine Falcon Falco peregrinus: I. Weather, nest spacing and territory occupancy. Emu 88, 195–201.The density of pairs of Peregrine Falcons occupying nest sites near Canberra was high (minimum of one pair per 343 sq. km). Nest sites were scarce. Falcon pairs were spaced no closer than 2 km apart and were clumped where there were suitable cliffs; within these clumps the spacing of cliffs used for nesting was fairly regular while that of all cliffs was random. Over 12 years, occupancy of nest sites was high and stable with between 82 and 100% of sites occupied annually by at least one Falcon. Variation in occupancy by pairs, and therefore breeding density, was not significantly correlated with the change in occupancy from the previous year (a density-dependent influence). Heavy rain during August, in particular, and October reduced occupancy of nest territories by pairs and this reduced the potential breeding population (a density-independent influence). Occupancy of territories by pairs was significantly correlated with rain intensity, which accounted for 77% of the variation in occupancy. Flooding of nests, more than a reduction in food (prey availability, hunting success, etc.), appeared to result in lower occupancy by pairs in wet years.

The breeding ecology and diet of the Hobby Falco subbuteo in southern England

Brecding densities, nest spacing, breeding habitat, productivity and diet are compared for Hobbies Falco subbuteo in the New Forest (heathland and largely coniferous woodland), river valley farmland and largely cultivated chalk downland. Regular spacing of nests was characteristic of the three areas. The New Forest had the highest density and lowest mean nearest neighbour distance for nests. Nest areas tended to be traditional. A high proportion of clutches were laid in disused nests of Carrion Crows Corvus corone which favoured the crowns of Scots pine Pinus sylvestris. Except on downland, the availability of such sites is unlikely to provide a constraint on Hobby distribution. In the New Forest, woodland clearings and valley mires were a feature of nest territories. There was no evidence of a habitat‐related difference in fledging success.Division of labour between sexes is marked. The male is responsible for provisioning the female throughout most of the breeding season as well as the young. During the incubation period many woodland passerines were taken but when feeding nestlings Hobbies tended to specialize on Swallows Hirundo rustica, House Martins Delichon urbica and, where they had access to more urban areas, Swifts Apus apus. By breeding relatively late, Hobbies take advantage of the availability of the juveniles of such prey on which to feed their young. Large insects supplemented the diet before egg‐laying and when the Hedged young were learning to hunt.

Breeding and Dominance Among the Heron Island Silvereyes Zosterops Lateralis Chlorocephala

The Heron Island population of Silvereyes with a known age distribution was followed through the breeding season. Its breeding cycle is described and the data on the 1969/70 season are used to find out how the dominance status of individual birds in winter affects their breeding performance. Most floaters were subordinate first-year birds and among the breeders older birds tended to produce more fledglings than younger birds. Clutch size was nearly constant and variation in brood size was not dependent on age or dominance of parents. The young of the intermediate class of parents had the greatest rate of survival till the winter. The dominant birds tended to nest in food-rich areas in crowded conditions, but the survival rate of their young was not related to the area of their origin. The dominant parents made the greatest contribution and the subordinate parents the least to the number of young surviving till the following breeding season. Thus selection with respect to dominance favoured the dominant birds during the high- density phase of 1969/70. Being dominant in the breeding context means that it enables the individual to establish a nesting territory which offers easy access to concentrated food. Territoriality appears to limit the number of nesting territories in the high-density area. Territorial defence is thought to play an important role in the protection of the nest; this is more effective if parents forage closer to the nesting area.