Territory quality and territorial behavior in two sympatric species of hummingbirds

Summary Changes in territorial behavior of blackchinned hummingbirds (Archilochus alexandri) in response to experimental changes in territory quality were investigated using artificial feeders and simultaneous, pair-wise observations of owners. Some of the responses of A. alexandri were similar to those documented by a previous study of the Anna's hummingbird (Calypte anna): A. alexandri defending rich territories spent more time on their territories, encountered a greater percentage of intruders, and chased more intruders than did A. alexandri defending poor territories. In contrast to C. anna, A. alexandri supplemented chases with energetically inexpensive threat vocalizations more extensively when territory quality was increased. This difference may be related to A. alexandri's more tenuous control of rich territories. When both species were observed on very poor territories, A. alexandri chased a greater percentage of intruders, consumed a greater proportion of available food, and obtained a greater net energy gain from their territories. When observed simultaneously on territories with ad lib food, both species defended highly exclusive territories but A. alexandri suffered higher intrusion pressure and spent more time and energy in defense. These interspecific differences in territorial behavior may facilitate sympatric coexistence of the two species through aggressive partitioning of flower patches according to productivity: the greater payoff received by C. anna on rich territories and A. alexandri on poor territories should favor a corresponding monopolization of rich areas by C. anna and poor areas by A. alexandri.     

The effect of prey consumption on territorial defense by harriers: differential responses to neighbors versus floaters

Summary Food consumption may reduce fighting intensity of territory owners by decreasing resource value (additional food cannot be consumed) and/or increasing fighting costs (food in the digestive tract may raise injury risks). A territorial harrier's (Circus cyaneus, adult females) decision to reduce its level of aggression should depend upon whether or not the intruder was a competitor for individual prey items (as are smaller male floaters) or for the territory proper (as are female floaters and especially female neighbors). Accordingly, following meals, aggressive intensity of owners was strongly reduced towards male floaters (more were ignored), slightly reduced towards female floaters (more were called at than chased), and remained unchanged towards neighbors (virtually all were chased). Hence, alterations in aggressive behavior of territory owners following food consumption may depend upon the type of intruder and the resource under contest (a food item or a territory).     

Predation risk and the territory value of cover: an experimental study

In an experiment on territorial brown trout (Salmo trutta), we addressed the novel hypothesis that protective cover increases the value of a territory in relation to the perceived level of predation risk. We predicted that territory holders should invest more resources defending territories with cover than territories without cover, and that defence should increase as predation risk increases. First, trout were allowed to establish ownership in territories with or without overhead cover. Second, predation risk was manipulated by simulating aerial predator attacks in half of the territories of each type, whereupon the preference for cover was estimated. Third, owners of the four types of territories were staged in dyadic contests against size-matched intruders. Territory owners showed a preference for cover, which increased further after simulated predator attacks. In subsequent contests, conflicts over territories with cover were settled faster than conflicts over territories without cover, which may suggest that the value of cover increases the motivational asymmetry between owner and intruder. Consistent with our hypothesis, owners of territories with cover were much more aggressive if they had been subjected to predator attacks the day before the conflict. These results suggest that territory owners are able to estimate the value of protective cover in response to variation in the level of predation risk in the habitat.Communicated by J. Krause     

Northern harriers on feeding territories respond more aggressively to neighbors than to floaters

Summary Territory owners often respond less aggressively towards intruding neighbors than towards intruding floaters, an observation termed the dear enemy phenomenon. Comparisons of territory owners' responses to intruding neighbors versus their responses to intruding floaters usually have been made for owners of multi-purpose and/or breeding territories. Here, I describe responses of female northern harriers Circus cyaneus (owners) on winter feeding territories towards three types of intruders (female neighbors, female floaters, and male floaters) and show that the dear enemy phenomenon does not occur. Owners' responses towards neighbors were more intense (mostly flights rather than calls) than responses towards female floaters, which in turn were more intense than responses towards male floaters. The greater intensity of owners' responses towards neighbors compared to owners' responses towards male and female floaters may be related to differences in the threat posed by each of the three intruder types in terms of fighting ability (RHP) and potential losses from intrusion. Hence, whether owners respond more aggressively towards neighbors or floaters, and whether the dear enemy phenomenon is observed, may depend upon the relative magnitude of threat presented by neighbors and floaters to owners in terms of fighting ability and potential losses from intrusion.     

Food availability,intrusion pressure and territory size: an experimental study of Anna's hummingbirds (Calypte anna)

Theoretical considerations implicate food availability and intrusion pressure as important determinants of territory size, but empirical studies have led to contradictory conclusions about cause-and-effect relationships among these three variables. To investigate this problem, we provided patches of electronically controlled artificial flowers, which were defended by male Calypte anna. Food availability was experimentally manipulated, and intrusion rate and territory size were calculated from behavioral observations of the territory owner. Changes in both food availability and intrusion rate were found to be significantly correlated with changes in territory size under certain conditions. Intrusion rate, which was influenced by food availability, was negatively associated with territory size so long as food availability was high. This association persisted even after possible effects of food availability were controlled statistically. Food availability was negatively correlated with territory size only when intrusion rates were high and after owners had been defending territories for 3 days. As food availability and intrusion rate increased, owners increasingly restricted their defense to the patch itself; partial regressions revealed a significant association for intrusion rate but not food availability. When intrusion rate was low and food availability varied from low to high levels, no relationship was observed between food availability and territory size, apparently because of opposing influences of food abundance on territory size. Correspondence to: P.W. Ewald     

Dominance in feeding territories relates to foraging success and offspring growth in brown skuas <Emphasis Type="Italic">Catharacta antarctica lonnbergi</Emphasis>

Territoriality should lead to strict dominance, as territory holders typically control access to resources and exclude others from their use. In feeding territories, dominance should be reflected in foraging success and ultimately in reproduction differences; however, these successive links have rarely been made explicit. Therefore, we investigated a population of brown skuas Catharacta antarctica lonnbergi, in which only part of the breeding population occupied feeding territories within penguin colonies. We identified the dominance hierarchy and determined the foraging success of the participants in fights for access to penguin carcasses within the territories. Furthermore, we monitored offspring growth from parents with and without feeding territories. Our results indicated a clear dominance hierarchy with territorial birds in their own territory dominating over territorial breeders from other territories, non-territorial breeders and non-breeding birds. However, territory owners could not completely exclude others from access to food. Foraging success was positively related to dominance scores: The dominant territory owners received 63% of a carcass, whereas non-territorial pairs could get less than 10%. The link between foraging success and offspring development was less clear: Although male chicks of non-territorial parents suffered from lower growth rates and, thus, delayed fledging, there were no such differences in female chicks. Territoriality in skuas did not imply a complete occupation of food, but guaranteed optimal growth conditions for offspring. Non-territorial individuals were forced to search for alternative resources, and the restricted access to the preferred food resulted in inferior conditions for offspring development, making this foraging strategy less rewarding.     

Correlation between nectar supply and aggression in territorial honeyeaters: causation or coincidence?

Summary A fundamental prediction of food-based economic models of territoriality is that animals will not defend territories if food is so abundant that defense will not improve access to food. Several studies of nectar-feeding birds support this prediction, with territoriality being rare or absent in years when nectar was particularly abundant. However, these results could potentially be an artefact of changes in bird density with nectar availability, and in at least some cases the correlations between territory defense and nectar availability could be purely coincidental. This paper reports the first experimental test of whether cessation of territory defense in nectar-feeding birds results from a direct response to abundance of nectar. New holland honeyeaters Phylidonyris novaehollandiae and white-cheeked honeyeaters P. nigra show pronounced changes in their levels of territorial aggression over the 7–8 months that they breed. These changes are predictable from economic considerations in that the birds are least aggressive in the months when nectar is extremely abundant. I tested whether the birds were responding to changes in nectar availability by providing sugar-water feeders at neutral locations that were easily accessible to territory holders, but far enough away from territories that intrusion rates were unaffected. I tested for responses at two time scales feeders were put out for 48-h periods in 1987, and were left out continuously from January to October 1988. The only effect was that territory holders visited feeders instead of flowers when floral nectar was scarce. They continued to defend their territories aggressively at those times, showed seasonal changes in aggressiveness similar to birds on a site without feeders, and did not shift their territories toward feeders. I conclude that the observed changes in aggressiveness are not responses to changes in nectar availability, and suggest alternative explanations.     

Influence of territoriality on adult density in two rockfishes of the genus Sebastes

Nonterritorial Sebastes carnatus and S. chrysomelas existed, along with territorial individuals, at 3 tagging sites off southern California, USA, which were monitored for nearly 1 yr. To test the hypothesis that territoriality affected adult density in these species, territorial fish were removed and the subsequent utilization of vacated territories by other fish was monitored. Intrusion of other fish into vacated territories increased significantly in 90% of the removals. Other fish colonized both the feeding and sheltering parts of the vacated territories, indicating that the previous owners had successfully defended both parts of their territories. Many of the colonizers had already possessed territories; they expanded their territories or moved into presumably better havitat. Several previously-nonterritorial fish also moved into vacated areas, and at least some of them appeared to establish territories. These fish, then, had previously been capable of establishing territories, but were prevented from doing so by resident territory holders. Thus territoriality, rather than such other factors as predation or low recruitment, limited the number of territorial fish at each site. However, territorial fish did not inhibit the settlement of larval recruits, and the relative mortality rates of older territorial vs nonterritorial fish were not determined. Thus the question of whether territoriality was a major factor controlling total density remains unresolved.     

Interference asymmetries among age-sex classes of rufous hummingbirds during migratory stopovers

Summary Three age-sex classes of rufous hummingbirds (Selasphorus rufus) overlap temporally and defend feeding territories during migratory stopovers in the Sierra Nevada of California. We demonstrate that these classes differ in their ability to secure and maintain high-quality feeding territories for refueling, and that these differences result in differences in resource use. Data on acquisition of territories, territory characteristics, and responses of territory owners to intruders suggest that several mechanisms are involved in determining dominance, involving sex- and age-related differences in wing disc loading, coloration, and experience. We discuss the implications of these results for understanding intraspecific variation in migration strategies. Correspondence to: F.L. Carpenter     

Diversionary displays of paternal stickleback

Summary In Crystal Lake, British Columbia, male threespine stickleback (Gasterosteus aculeatus) perform a diversionary display that deters cannibalistic conspecifics from entering their territories and destroying their nests or consuming offspring. In this population, the display consists of males swimming directly out of their territories and rooting aggressively in the substratum, mimicking stickleback feeding on nests of others, or digging for nesting materials. The display is elicited solely by conspecifics, and usually by benthic foraging groups of 2–300 individuals. It usually is elicited when groups are approaching the territory directly. Otherwise the male typically joins the group or remains still in his territory until the group leaves. This observation suggests that males are capable of discriminating conditions under which the display is most likely to be successful. The display is absent in a population in Garden Bay Lake, British Columbia, in which benthic foraging groups are absent. In Crystal Lake the males themselves do not risk predation by the groups, and the tendency of males to perform the display does not change with reproductive condition. Comparison with diversionary displays performed in two additional allopatric populations suggests that diversionary displays in this species incorporate elements of other behavioral sequences displaced to the display, and that variation in the structure of the display exists among populations.     

Playback and speaker-replacement experiments on song-based neighbor,stranger, and self discrimination in male Red-winged Blackbirds

Summary Two experiments were conducted to deterine whether territorial male Red-winged Blackbirds discriminate among neighbor, stranger, and self songs. In the first experiment, recordings of the three classes of songs were played to territorial males. Responses to playback of stranger song, as measured by song rate, intensity of Song Spread display, and closest approach, were significantly stronger than to playback of neighbor song. In addition, stranger song elicited significantly more intense Song Spread displays than did self song.In the second experiment, territorial males were removed from their territories and each was replaced with speakers broadcasting recorded songs of a stranger, and of the removed male. Broadcasts of the removed male's own songs were more effective in discouraging trespassing by neighboring male redwings than were broadcasts of songs of a stranger. However, the two broadcasts did not differ in their ability to deter nonneighboring male redwings from flying through or trespassing within the speaker territory.We suggest that neighbor-stranger discrimination in the Red-winged Blackbird develops as a consequence of associative learing.     

The reproductive behavior of Anthidium manicatum (Hymenoptera: Megachilidae) and the significance of size for territorial males

Summary Males of the wool-carder bee, Anthidium manicatum, patrol clumps of garden plants. Females of this species visit these plants for pollen, nectar, and pubescence; they also mate there. Females are polyandrous, with intervals between copulations as short as 35 s. Patrolling males defend their territories (0.1–1.3 m²) against other males and against other species of flower-visiting insects. Honey bees may be rendered unable to fly by the attacks of A. manicatum.Territory owners perform exploratory flights to other males' territories, changing territories often (median ownership 4–7 days; maximum 30 days) and flying up to 450 m to establish new territories. Territorial usurpations are nearly always by larger males.Female visitation rate is significantly correlated with number of flowers on a territory. The head size of territory-owner males shows significant correlation with territorial quality (measured by number of flowers, not area) and thus with number of female visits and copulatory opportunities. Some males fail to maintain territories and instead attempt to forage and copulate in other males' territories while the owners are otherwise occupied. Nonowner males are significantly smaller than owners, forage less often and from fewer flowers, and achieve significantly fewer copulations than owners. Females, however, do not reject smaller, nonowner males at a higher rate than they do larger, owner males; their choice for male size appears to be indirect, based instead on choice of food resource.The interval between a copulation and the male's next attempt with a different female is not shorter than that involving the same female. Males do not escort just-mated females about their teritories, as observed in Anthidium maculosum. Territorial behavior in this species most likely evolved through intrasexual competition for reproductive success which led to sexual dimorphism. The defense of a resourcebased territory is the mechanism used by a male to maximize his reproductive potential.     

Social facilitation of selective mortality

Territorial defense by breeders influences access to resources near defended nest sites by intruder species and may have indirect effects on other species within the territory, leading to local patchiness in distribution patterns. The present study demonstrates that adult males of a damselfish, Pomacentrus amboinensis, indirectly facilitate the increased survival of conspecific juveniles through the territorial defense of their nesting site from potential egg predators. Moreover, male territoriality results in a shift in the selectivity of predation on newly settled juveniles. We monitored the fate of pairs of predator-naive, newly settled P. amboinensis placed inside and outside nesting territories. Individuals within a pair differed in size by approximately 1 mm and were tagged for individual identification. Away from male territories larger juveniles had greater survival, while within territories, larger juveniles suffered higher mortality. Behavioral observations indicated that the moonwrasse Thalassoma lunare, a predator of benthic eggs and small fishes, had reduced access to juveniles within male territories, while another predator on small fishes, the dottyback Pseudochromis fuscus, had unobstructed access to male territories. Experimental removal of P. fuscus indicated that the shift in the direction of phenotypic selection on newly settled juveniles was the indirect effect of aggression by nest-guarding male damselfish, which resulted in differential access to male territories by these two predators of small fishes. Evidence suggests that behavioral interactions between the resident community and intruders will influence patchiness in selective pressures imposed on benthic prey by influencing both the composition of predator types that can access the prey resource and their relative abundance. How this spatial and temporal patchiness in predator pressure interacts with spatial patchiness of recruiting prey will have a major influence on the resulting distribution of juveniles and their phenotypic traits.     

Dominions and site-fixed aggressive behavior in breeding male Leucorrhinia intacta (Odonata: Libellulidae)

Summary We report aggressive spacing behavior in male dragonflies, Leucorrhinia intacta, that is characterized by variations in the probability of chasing conspecific male intruders within a defined area around a male's perch. The chase probability depends on the total intruder pressure and the behavior and distance of the intruder from the perched male (Fig. 2). This nonexclusive, site-fixed area has been called a dominion.We also examined the distribution of intruders among the various behavior-distance categories. Chase rates (per 15 min) were correlated mostly with number of intruders that hovered (rather than flew) close to the territorial male. Finally, we examined the impact of the changing chase probabilities on the variation in aggressive interactions as a function of intruder pressure. We conclude that, to some extent, territorial males stabilized defense costs across a variety of intruder pressures by maintaining dominions.     

Territoriality among male red-winged blackbirds

Summary We tested two hypotheses to explain territorial dominance in male birds. Male red-winged blackbirds were removed from their territories for 7 d and then released after replacement owners had held their territories 2 to 7 d. Original owners regained territories from short-term replacements, but could not defeat 6 to 7 d replacements. This outcome suggests that replacement males relinquished their territories to persistent original owners after 2 to 3 d of ownership because the territory lacked sufficient value to them, but not after 7 d, when its value was greater. This result supports the Value Asymmetry Hypothesis of territorial dominance and provides strong evidence in birds that differences in the extent of knowledge of or investment in an area and, hence, willingness to escalate contests, contribute to territorial dominance.     

Territorial defense in the bannertail kangaroo rat (Dipodomys spectabilis): footdrumming and visual threats

Summary Footdrumming and visual displays associated with territorial defense in the bannertail kangaroo rat (Dipodomys spectabilis) were studied in the field in Arizona. We predicted that foot-drumming functioned like bird song in territorial advertisement. Rats were tested in their territories for their responses to the presentation of four different stimuli: a playback of airborne footdrumming patterns (audio); a stuffed and mounted kangaroo rat in an aggressive posture (visual); a combination of both (audio-visual); and a playback of cricket stridulation as a control. Kangaroo rats investigated all experimental stimuli equally by becoming alert, approaching, and retreating (Fig. 2). Rats, therefore, seemed to assess the amount of threat and footdrummed at higher rates and sooner in response to the more threatening visual stumulus of a stuffed rat than to the apparently less threatening ones of a airborne footdrumming playback or cricket stridulation (Fig. 1). We conclude that rats footdrum at long ranges to repel a potential intruder and also at close ranges when territories are threatened by a persistent intruder. Chase and attack, as in territorial birds, are the final lines of defense.     

Dominions and desert clickers (Orthoptera:Acrididae): influences of resources and male signaling on female settlement patterns

Summary Previous field work on the grasshopper Ligurotettix coquilletti revealed that females were not evenly distributed among male mating territories, Larrea tridentata (creosote) bushes, but were clustered at particular locations. These sites generally harbored several signaling males simultaneously and also possessed foliage preferred by the insects as a food source, this preference being based on the relative concentrations of various extra-foliar compounds. The clustering of females, therefore, could result from a preference for specific bushes because of the resources (i.e., food) available there and/or an orientation to groups of males per se. Here, we present the results of 3 field experiments in which we controlled the spatial distribution and intensity of male signals using a computer-operated system of loudspeakers and monitored the movement of individually marked females released in the study area. When male song was identical at high and low quality territories (all having single loudspeakers), females still aggregated at the high quality sites, indicating that variability in host plant quality alone may be sufficient to promote a skewed distribution of females. Among high quality territories, females did not discriminate between sites with one versus three loudspeakers (all broadcasting the same signal), but displayed a strong preference for sites (all having single loudspeakers) with a high intensity signal over a low intensity one. Field measurements showed that the songs of grouped males were more intense than those of lone males, implying that the signaling of grouped males may have enhanced the settlement of females at the bushes harboring male groups above and beyond that influenced by territory quality alone. We conclude that female attraction to host plants is influenced primarily by male signaling, whereas their subsequent retention is more dependent on territory quality. An experiment on male settlement failed to show an aggregative tendency, suggesting that male groups form through the passive accumulation of individuals at high quality sites.     

Signal use by collared lizards, Crotaphytus collaris: the effects of familiarity and threat

Many lizard species use lateral compressions of the body during agonistic encounters. I investigated the signal value of the frequency at which these displays are presented and how that rate is affected by familiarity and threat. The response of resident collared lizards (Crotaphytus collaris) to tethered strangers, neighbors on the correct boundary, and neighbors displaced to the opposite boundary, was recorded by counting the number of lateral displays directed at the intruder and the number of subsequent acts of escalated aggression. There was no relationship between size asymmetry of the opponent and the rate of lateral display or aggression, nor was there a relationship between body size and the rate of lateral display or aggression. However, there was a high positive correlation between lateral display rate and aggression, suggesting that the rate of lateral displays is a conventional signal of motivation to attack. The highest rates of display and aggression were directed toward displaced neighbors, somewhat less toward strangers, and the least toward neighbors at the correct boundary. The ratio of aggressive acts to lateral displays followed the same pattern, presumably because the perceived threat to the resident decreases in the same order. Taken together these data suggest that collared lizards are able to assess the threat of an opponent and signal motivation to respond aggressively towards that opponent.Communicated by S. Downes     

Female encounter rates and fighting costs of males are associated with lek size in Drosophila mycetophaga

Male costs and benefits associated with male display size in field populations of an Australian lekking Drosophila species were examined. Results suggested that male mating success increased with display size, since matings appeared to be more common in large displays, and since the probability of males encountering a female increased as displays contained more males. Female encounter probabilities did not increase once about 20 males or more were present on a display. Male size and fighting costs tended to increase with display size. The distribution of males among displays did not follow the ideal free distribution in the sense that each male did not have equal mating opportunity per unit time. Deviation from an ideal free distribution may have been due to female preference for mating in aggregations rather than with solitary males, since in a field experiment females were more willing for mating in an aggregation of five males than with solitary males. Received: 22 May 1997 / Accepted after revision: 1 November 1997     

Spatial scale of local breeding habitat quality and adjustment of breeding decisions

Experimental studies provide evidence that, in spatially and temporally heterogeneous environments, individuals track variation in breeding habitat quality to adjust breeding decisions to local conditions. However, most experiments consider environmental variation at one spatial scale only, while the ability to detect the influence of a factor depends on the scale of analysis. We show that different breeding decisions by adults are based on information about habitat quality at different spatial scales. We manipulated (increased or decreased) local breeding habitat quality through food availability and parasite prevalence at a small (territory) and a large (patch) scale simultaneously in a wild population of Great Tits (Parus major). Females laid earlier in high-quality large-scale patches, but laying date did not depend on small-scale territory quality. Conversely, offspring sex ratio was higher (i.e., biased toward males) in high-quality, small-scale territories but did not depend on large-scale patch quality. Clutch size and territory occupancy probability did not depend on our experimental manipulation of habitat quality, but territories located at the edge of patches were more likely to be occupied than central territories. These results suggest that integrating different decisions taken by breeders according to environmental variation at different spatial scales is required to understand patterns of breeding strategy adjustment.