Prior territorial responses and home range size predict territory defense in radio-tagged great tits

Abstract

Territorial animals often use signals to advertise territorial occupancy within their larger home ranges. Songbirds are among the best-studied territorial signaling taxa, and when competitors start singing during a territorial intrusion, residents usually show elevated spatial and vocal responses. These responses could be used by intruders and distant eavesdroppers to predict future responses or to compare responses across competitors. Yet, the extent to which responses of a resident to a territorial intrusion predict its future responses and its overall spatial behavior (home range) within a neighborhood is less well understood. We used wild great tits (Parus major) as a model species in repeated song playback trials, simulating territorial intrusions combined with radio-tracking before and during playback trials. The time spent close to the loudspeaker in response to an initial simulated intrusion predicted the same response variable during a second simulated intrusion on the next day, whereas singing activity during the first simulated intrusion did not predict singing during the second simulated intrusion. We also show that more explorative males (as determined by a novel environment test) and males with smaller home ranges sang more and spent more time near the loudspeaker in response to both simulated intrusions. Thus, by probing residents, intruders can obtain reliable information about subsequent response probabilities, while eavesdroppers from a distance, who can use auditory information only, would not receive sufficient predictive information. Our findings also suggest that males with larger home ranges are more tolerant toward intruders, which could reflect a trade-off between tendencies to respond strongly and to range widely. The lack of predictability of singing activity with regard to responses to future intrusions might explain why territorial animals continuously exchange vocal signals and regularly foray into neighboring territories, as a way to obtain regular information updates.

Significance Statement

Animals use experience from interactions with conspecifics in their future decision making, such as mate choice and strategies for conflict resolution. The value of such information depends in part on the predictability of the future behavior of that conspecific. In songbirds, territorial individuals respond to intruders by approach and signaling. Here, we tested in radio-tagged great tits (Parus major) if territorial responses are predictable and are affected by individual and environmental factors. We show that the time spent near the simulated intruder was more predictable than singing activity and that birds with larger home ranges showed weaker responses. These findings suggest that information based on such spatial responses is more useful for future decision making, as compared to vocal information, and that distant eavesdroppers will thus receive less reliable information. Limited predictability may explain why territorial animals continuously exchange vocal signals and foray into neighboring territories, providing opportunities for regular information updates.

Great tits do not compensate over time for a radio-tag-induced reduction in escape-flight performance

The use of biologging and tracking devices is widespread in avian behavioral and ecological studies. Carrying these devices rarely has major behavioral or fitness effects in the wild, yet it may still impact animals in more subtle ways, such as during high power demanding escape maneuvers. Here, we tested whether or not great tits (Parus major) carrying a backpack radio-tag changed their body mass or flight behavior over time to compensate for the detrimental effect of carrying a tag. We tested 18 great tits, randomly assigned to a control (untagged) or one of two different types of a radio-tag as used in previous studies in the wild (0.9 g or 1.2 g; ~5% or ~6-7% of body mass, respectively), and determined their upward escape-flight performance 1, 7, 14, and 28 days after tagging. In between experiments, birds were housed in large free-flight aviaries. For each escape-flight, we used high-speed 3D videography to determine flight paths, escape-flight speed, wingbeat frequency, and actuator disk loading (ratio between the bird weight and aerodynamic thrust production capacity). Tagged birds flew upward with lower escape-flight speeds, caused by an increased actuator disk loading. During the 28-day period, all groups slightly increased their body mass and their in-flight wingbeat frequency. In addition, during this period, all groups of birds increased their escape-flight speed, but tagged birds did so at a lower rate than untagged birds. This suggests that birds may increase their escape-flight performance through skill learning; however, tagged birds still remained slower than controls. Our findings suggest that tagging a songbird can have a prolonged effect on the performance of rapid flight maneuvers. Given the absence of tag effects on reproduction and survival in most songbird radio-tagging studies, tagged birds in the wild might adjust their risk-taking behavior to avoid performing rapid flight maneuvers.

Whistling is metabolically cheap for communicating bottlenose dolphins (Tursiops truncatus)

Toothed whales depend on sound for communication and foraging, making them potentially vulnerable to acoustic masking from increasing anthropogenic noise. Masking effects may be ameliorated by higher amplitudes or rates of calling, but such acoustic compensation mechanisms may incur energetic costs if sound production is expensive. The costs of whistling in bottlenose dolphins (Tursiops truncatus) have been reported to be much higher (20% of resting metabolic rate, RMR) than theoretical predictions (0.5-1% of RMR). Here, we address this dichotomy by measuring the change in the resting O₂ consumption rate (V̇ _O2 ), a proxy for RMR, in three post-absorptive bottlenose dolphins during whistling and silent trials, concurrent with simultaneous measurement of acoustic output using a calibrated hydrophone array. The experimental protocol consisted of a 2-min baseline period to establish RMR, followed by a 2-min voluntary resting surface apnea, with or without whistling as cued by the trainers, and then a 5-min resting period to measure recovery costs. Daily fluctuations in V̇ _O2  were accounted for by subtracting the baseline RMR from the recovery costs to estimate the cost of apnea with and without whistles relative to RMR. Analysis of 52 sessions containing 1162 whistles showed that whistling did not increase metabolic cost (P>0.1, +4.2±6.9%) as compared with control trials (-0.5±5.9%; means±s.e.m.). Thus, we reject the hypothesis that whistling is costly for bottlenose dolphins, and conclude that vocal adjustments such as the Lombard response to noise do not represent large direct energetic costs for communicating toothed whales.

Effects of ecotourism on eastern yellow robin (Eopsaltria australis) vocal behaviour

Ecotourism attempts to provide sustainable tourism with a focus on experiencing natural areas to foster environmental appreciation. However, it is often seen as a paradox, since associated infrastructure and activity can have detrimental effects on wildlife. One aspect that has been overlooked in this context is the potential impact of ecotourists on vocal behaviour, especially of birds. Many birds are susceptible to anthropogenic noise, avoiding noisy areas, or changing vocal activity within them. We used passive recording techniques to quantify vocal behaviours of a native Australian songbird, the eastern yellow robin, at an ecotourist resort to investigate how tourist presence might affect vocal behaviours. We found that during peak tourism seasonal periods, robins sang fewer songs, and these songs were shorter. However, this same pattern was observed on weekdays compared with weekends, when tourist numbers are generally lower. This seemingly contradictory pattern may be explained by the fact that maintenance of the resort grounds occurred predominantly on weekdays, with noise levels comparable to those detected during periods of high tourism. Thus, ecotourism infrastructure can have layered effects on bird vocal activity: tourist numbers as well as maintenance practices should be considered within the context of local wildlife conservation.

Context-dependent variability in blue whale acoustic behaviour

Acoustic communication is an important aspect of reproductive, foraging and social behaviours for many marine species. Northeast Pacific blue whales (Balaenoptera musculus) produce three different call types-A, B and D calls. All may be produced as singular calls, but A and B calls also occur in phrases to form songs. To evaluate the behavioural context of singular call and phrase production in blue whales, the acoustic and dive profile data from tags deployed on individuals off southern California were assessed using generalized estimating equations. Only 22% of all deployments contained sounds attributed to the tagged animal. A larger proportion of tagged animals were female (47%) than male (13%), with 40% of unknown sex. Fifty per cent of tags deployed on males contained sounds attributed to the tagged whale, while only a few (5%) deployed on females did. Most calls were produced at shallow depths (less than 30 m). Repetitive phrasing (singing) and production of singular calls were most common during shallow, non-lunging dives, with the latter also common during surface behaviour. Higher sound production rates occurred during autumn than summer and they varied with time-of-day: singular call rates were higher at dawn and dusk, while phrase production rates were highest at dusk and night.

Multifractal analysis reveals music-like dynamic structure in songbird rhythms

Music is thought to engage its listeners by driving feelings of surprise, tension, and relief through a dynamic mixture of predictable and unpredictable patterns, a property summarized here as “expressiveness”. Birdsong shares with music the goal to attract its listeners’ attention and might use similar strategies to achieve this. We here tested a thrush nightingale’s (Luscinia luscinia) rhythm, as represented by song amplitude envelope (containing information on note timing, duration, and intensity), for evidence of expressiveness. We used multifractal analysis, which is designed to detect in a signal dynamic fluctuations between predictable and unpredictable states on multiple timescales (e.g. notes, subphrases, songs). Results show that rhythm is strongly multifractal, indicating fluctuations between predictable and unpredictable patterns. Moreover, comparing original songs with re-synthesized songs that lack all subtle deviations from the “standard” note envelopes, we find that deviations in note intensity and duration significantly contributed to multifractality. This suggests that birdsong is more dynamic due to subtle note timing patterns, often similar to musical operations like accelerando or crescendo. While different sources of these dynamics are conceivable, this study shows that multi-timescale rhythm fluctuations can be detected in birdsong, paving the path to studying mechanisms and function behind such patterns.

Gelada vocal sequences follow Menzerath's linguistic law

Identifying universal principles underpinning diverse natural systems is a key goal of the life sciences. A powerful approach in addressing this goal has been to test whether patterns consistent with linguistic laws are found in nonhuman animals. Menzerath's law is a linguistic law that states that, the larger the construct, the smaller the size of its constituents. Here, to our knowledge, we present the first evidence that Menzerath's law holds in the vocal communication of a nonhuman species. We show that, in vocal sequences of wild male geladas (Theropithecus gelada), construct size (sequence size in number of calls) is negatively correlated with constituent size (duration of calls). Call duration does not vary significantly with position in the sequence, but call sequence composition does change with sequence size and most call types are abbreviated in larger sequences. We also find that intercall intervals follow the same relationship with sequence size as do calls. Finally, we provide formal mathematical support for the idea that Menzerath's law reflects compression-the principle of minimizing the expected length of a code. Our findings suggest that a common principle underpins human and gelada vocal communication, highlighting the value of exploring the applicability of linguistic laws in vocal systems outside the realm of language.

Are simakobu (Simias concolor) loud calls energetically costly signals?

OBJECTIVES

Male vocal displays play an important role in sexual selection in many species. If there are costs or constraints, calls may convey honest information about the caller. We studied the loud calls of male simakobu (Simias concolor), a sexually dimorphic primate that resides in one-male groups, on Siberut Island, Indonesia.

MATERIALS AND METHODS

We combined behavioral and ecological data with acoustic analysis to determine the influence of energy costs and status on calling effort. Based on all-day follows (n = 107) and loud-call recordings (n = 186), we determined how calling rates and durations varied in relation to short-term (time of day, temperature, travel distance) and long-term (fruit availability, physical condition) measures of energy availability and expenditure.

RESULTS

We found that males avoided calling at the hottest times of the day, and produced shorter calls at higher temperatures, supporting the hypothesis that calls are influenced by thermoregulation costs. A peak of longer duration calls just before sunrise, however, could not be explained by energy constraints, but we hypothesize that this pattern is related to the fact that dawn calls occur in choruses, motivating males to call longer than their neighbors. Contrary to predictions, calling rates increased with decreasing fruit availability and did not appear to be energetically costly.

DISCUSSION

We hypothesize that calling rates instead reflect strategies involving feeding and/or mate competition, signaling the motivation, rather than fighting ability, of the caller. Overall, results only weakly support the hypothesis that the timing and duration-but not rate-of simakobu loud calls are influenced by energy costs. Am J Phys Anthropol 161:44-52, 2016. © 2016 Wiley Periodicals, Inc.

Metabolic incentives for dishonest signals of strength in crustaceans

To reduce the potential costs of combat, animals may rely upon signals to resolve territorial disputes. Signals also provide a means for individuals to appear better than they actually are, deceiving opponents and gaining access to resources that would otherwise be unattainable. However, other than resource gains, incentives for dishonest signalling remain unexplored. In this study, we tested the idea that unreliable signallers pay lower metabolic costs for their signals, and that energetic savings could represent an incentive for cheating. We focused on two-toned fiddler crabs (Uca vomeris), a species that frequently uses their enlarged claws as signals of dominance to opponents. Previously, we found that regenerated Uca vomeris claws are often large but weak (i.e. unreliable). Here, we found that the original claws of male Uca vomeris consumed 43% more oxygen than weaker, regenerated claws suggesting that muscle quantity drives variation in metabolic costs. Therefore, it seems that metabolic savings could provide a powerful incentive for dishonesty within fiddler crabs.

Energetics of the acrobatic courtship in male golden-collared manakins (Manacus vitellinus)

In lek mating systems, females choose mates through indicators of quality, which males may exhibit by their performance of courtship displays. In temperate regions, displaying seasons are brief (one to two months), whereas in the tropics courtship seasons may be prolonged. Moreover, in temperate-breeding animals lekking behaviour can be energetically demanding, but little is known about the energy costs of lekking in tropical animals. Daily, over the course of a nearly seven-month-long breeding season, male golden-collared manakins (Manacus vitellinus) of Panamanian rainforests perform acrobatic courtship displays that markedly elevate heart rates, suggesting that they require high energy investment. Typically, animals of tropical lowland forests (such as manakins) exhibit a 'slow pace of life' metabolic strategy. We investigated whether male manakin courtship is indeed metabolically costly or whether the birds retain a low daily energy expenditure (DEE), as seen in other tropical species. To assess these questions, we calibrated manakin heart rate against metabolic rate, examined daily lek activity and, using telemetry, obtained heart rates of individual wild, lekking male manakins. Although metabolic rates peak during courtship displays, we found that males actually invest minimal time (only approx. 5 min d(-1)) performing displays. As a consequence, the DEE of approximately 39 kJ d(-1) for male manakins is comparable to other lowland tropical species. The short, intense bursts of courtship by these birds make up only approximately 1.2% of their total DEE. Presumably, this cost is negligible, enabling them to perform daily at their arenas for months on end.

Simulated birdwatchers' playback affects the behavior of two tropical birds

Although recreational birdwatchers may benefit conservation by generating interest in birds, they may also have negative effects. One such potentially negative impact is the widespread use of recorded vocalizations, or “playback,” to attract birds of interest, including range-restricted and threatened species. Although playback has been widely used to test hypotheses about the evolution of behavior, no peer-reviewed study has examined the impacts of playback in a birdwatching context on avian behavior. We studied the effects of simulated birdwatchers’ playback on the vocal behavior of Plain-tailed Wrens Thryothorus euophrys and Rufous Antpittas Grallaria rufula in Ecuador. Study species’ vocal behavior was monitored for an hour after playing either a single bout of five minutes of song or a control treatment of background noise. We also studied the effects of daily five minute playback on five groups of wrens over 20 days. In single bout experiments, antpittas made more vocalizations of all types, except for trills, after playback compared to controls. Wrens sang more duets after playback, but did not produce more contact calls. In repeated playback experiments, wren responses were strong at first, but hardly detectable by day 12. During the study, one study group built a nest, apparently unperturbed, near a playback site. The playback-induced habituation and changes in vocal behavior we observed suggest that scientists should consider birdwatching activity when selecting research sites so that results are not biased by birdwatchers’ playback. Increased vocalizations after playback could be interpreted as a negative effect of playback if birds expend energy, become stressed, or divert time from other activities. In contrast, the habituation we documented suggests that frequent, regular birdwatchers’ playback may have minor effects on wren behavior.

Compression as a universal principle of animal behavior

A key aim in biology and psychology is to identify fundamental principles underpinning the behavior of animals, including humans. Analyses of human language and the behavior of a range of non-human animal species have provided evidence for a common pattern underlying diverse behavioral phenomena: Words follow Zipf's law of brevity (the tendency of more frequently used words to be shorter), and conformity to this general pattern has been seen in the behavior of a number of other animals. It has been argued that the presence of this law is a sign of efficient coding in the information theoretic sense. However, no strong direct connection has been demonstrated between the law and compression, the information theoretic principle of minimizing the expected length of a code. Here, we show that minimizing the expected code length implies that the length of a word cannot increase as its frequency increases. Furthermore, we show that the mean code length or duration is significantly small in human language, and also in the behavior of other species in all cases where agreement with the law of brevity has been found. We argue that compression is a general principle of animal behavior that reflects selection for efficiency of coding.

Limited condition dependence of male acoustic signals in the grasshopper Chorthippus biguttulus

In many animal species, male acoustic signals serve to attract a mate and therefore often play a major role for male mating success. Male body condition is likely to be correlated with male acoustic signal traits, which signal male quality and provide choosy females indirect benefits. Environmental factors such as food quantity or quality can influence male body condition and therefore possibly lead to condition-dependent changes in the attractiveness of acoustic signals. Here, we test whether stressing food plants influences acoustic signal traits of males via condition-dependent expression of these traits. We examined four male song characteristics, which are vital for mate choice in females of the grasshopper Chorthippus biguttulus. Only one of the examined acoustic traits, loudness, was significantly altered by changing body condition because of drought- and moisture-related stress of food plants. No condition dependence could be observed for syllable to pause ratio, gap duration within syllables, and onset accentuation. We suggest that food plant stress and therefore food plant quality led to shifts in loudness of male grasshopper songs via body condition changes. The other three examined acoustic traits of males do not reflect male body condition induced by food plant quality.

Nutritional correlates and mate acquisition role of multiple sexual traits in male collared flycatchers

The information content of a sexual signal may predict its importance in a multiple signal system. Many studies have correlated sexual signal expression with the absolute levels of nutrient reserves. In contrast, the changes of nutrient reserves associated with signal expression are largely unknown in the wild due to technical limitations although they are important determinants of signal information content. We compared two visual and eight acoustic sexual traits in male collared flycatchers to see whether the nutritional correlates of expression predict the role of the signal in sexual selection. We used single point assays of plasma lipid metabolites to estimate short-term changes in nutritional state in relation to sexual trait expression during courtship. As a measure of sexual selection, we estimated the relationship with pairing latency after arrival in a 4-year dataset. Males which found a mate rapidly were characterized by large wing and forehead patches, but small song strophe complexity and small figure repertoire size. Traits more strongly related to pairing latency were also more closely related to changes in nutrient reserves. This indicates a link between signal role and information content. Small wing patches and, surprisingly, complex songs seemed to indicate poor phenotypic quality and were apparently disfavoured at mate acquisition in our population. Future studies of the information content of sexual traits, especially dynamic traits such as song, may benefit from the use of plasma metabolite profiles as non-invasive indicators of short-term changes in body condition.

Predators and the breeding bird: behavioral and reproductive flexibility under the risk of predation

A growing body of work suggests that breeding birds have a significant capacity to assess and respond, over ecological time, to changes in the risk of predation to both themselves and their eggs or nestlings. This review investigates the nature of this flexibility in the face of predation from both behavioural and reproductive perspectives, and also explores several directions for future research. Most available work addresses different aspects of nest predation. A substantial change in breeding location is perhaps the best documented response to nest predation, but such changes are not always observed and not necessarily the best strategy. Changes in nesting microhabitat (to more concealed locations) following predation are known to occur. Surprisingly little work addresses the proactive avoidance of areas with many nest predators, but such avoidance is probably widespread. Individual birds could conceivably adopt anti-predator strategies based on the nest predators actually present in an area, but such effects have yet to be demonstrated. In fact, the ways in which birds assess the risk of nest predation is unclear. Nest defence in birds has historically received much attention, but little is known about how it interacts with other aspects of decision-making by parents. Other studies concentrate on predation risk to adults. Some findings suggest that risk to adults themselves influences territory location, especially relative to raptor nests. An almost completely unexplored area concerns the sorts of social protection from predators that might exist during the breeding season. Flocking typical of the non-breeding season appears unusual while breeding, but a mated pair may sometimes act as a "flock of two". Opportunistic heterospecific sociality may exist, with heterospecific protector species associations more prevalent than currently appreciated. The dynamics of singing during the breeding season may also respond to variation in predation risk, but empirical research on this subject is limited. Furthermore, a few theoretical and empirical studies suggest that changes in predation risk also influence the behaviour of lekking males. The major influence of predators on avian life histories is undoubtedly expressed at a broad phylogenetic scale, but several studies hint at much flexibility on an ecological time scale. Some species may forgo breeding completely if the risk of nest predation is too high, and a few studies document smaller clutch sizes in response to an increase in nest predation. Recent evidence suggests that a female may produce smaller eggs rather than smaller clutches following an increase in nest predation risk. Such an increase may also influence decisions about intraspecific brood parasitism. There are no clear examples of changes in clutch/egg size with changes in risk experienced by adults, but parental responses to predators have clear consequences for offspring fitness. Changes in risk to adults may also influence body mass changes across the breeding season, although research here is sparse. The topics highlighted herein are all in need more empirical attention, and more experimental field work whenever feasible.

Sex-specific timing of mate searching and territory prospecting in the nightingale: nocturnal life of females

Formal models have shown that diel variation in female mate searching is likely to have profound influence on daily signalling routines of males. In studies on acoustic communication, the temporal patterns of the receivers' signal evaluation should thus be taken into account when investigating the functions of signalling. In bird species in which diel patterns of signalling differ between males singing to defend a territory or to attract a mate, the diel patterns of mate and territory prospecting are suggested to depend on the sex of the prospector. We simulated newly arriving female nightingales (Luscinia megarhynchos) by translocating radio-tagged females to our study site. The mate-searching females prospected the area mostly at night, visiting several singing males. The timing of female prospecting corresponded to the period of the night when the singing activity of unpaired males was higher than that of paired males. In contrast to females, territory searching males have been shown to prospect territories almost exclusively during the dawn chorus. At dawn, both paired and unpaired males sang at high rates, suggesting that in contrast to nocturnal singing, dawn singing is important to announce territory occupancy to prospecting males. In the nightingale, the sex-specific timing of prospecting corresponded to the differential signalling routines of paired and unpaired males. The temporal patterns in the behaviour of signallers and receivers thus appear to be mutually adapted.

Singing in the shade: song and song posts of northern cardinals near nesting Cooper’s hawks

Animals may compensate for increased predation risk by changing the behaviours that make them vulnerable. In this study, we compared the singing and perching behaviours of northern cardinals ( Cardinalis cardinalis L., 1758) living within 100 m of nesting Cooper’s hawks ( Accipiter cooperii (Bonaparte, 1828)) with that of cardinals in similar control sites. Cardinals sang similar amounts at hawk nests and at control sites, but they sang from less exposed perches when near Cooper’s hawk nests. These results suggest that singing from an exposed perch is both more dangerous and more effective for northern cardinals.

Heat transfer and the energetic cost of singing by canaries Serinus canaria

Sexually selected displays, such as male passerine bird song, are predicted to be costly. However, most measurements calculated the rate of oxygen consumption during singing using respirometry have shown that bird song has a low energetic cost. Since birds are reluctant to sing when enclosed inside a respirometry chamber, the energetic cost of singing could differ from that under more normal circumstances. We used heat transfer modelling, based on thermal images, to estimate the energetic cost of singing by canaries (Serinus canaria) that were not enclosed in respirometry chambers. Metabolic rate calculated from heat transfer modelling was 0.70 +/- 0.02 W (N = 10 birds) during singing, which was 14+/-5% greater than during standing (0.62 +/- 0.02 W). The energetic cost of singing did not differ significantly from that measured previously using respirometry when we took into account that birds sang for a greater proportion of the time during the current experiments. These conclusions were not sensitive to potential errors in the heat transfer model. Heat transfer modelling would be especially useful to obtain measurements of the energetic cost of activities that animals do not perform readily inside respirometry chambers, such as singing in birds.

The effect of energy reserves on social foraging: hungry sparrows scrounge more

Animals often use alternative strategies when they compete for resources, but it is unclear in most cases what factors determine the actual tactic followed by individuals. Although recent models suggest that the internal state of animals may be particularly important in tactic choice, the effects of state variables on the use of alternative behavioural forms have rarely been demonstrated. In this study, using experimental wind exposure to increase overnight energy expenditure, we show that flock-feeding house sparrows (Passer domesticus) with lowered energy reserves increase their use of scrounging (exploiting others' food findings) during their first feed of the day. This result is in accordance with the prediction of a state-dependent model of use of social foraging tactics. We also show that scrounging provides less variable feeding rates and patch finding times than the alternative tactic. These latter results support the theoretical assumption that scrounging is a risk-averse tactic, i.e. it reduces the risk of immediate starvation. As the level of energy reserves predicts the use of social foraging tactics, we propose that selection should favour individuals that monitor the internal state of flock mates and use this information to adjust their own tactic choice.

Reproductive State, but Not Testosterone, Reduces Immune Function in Male House Sparrows (Passer domesticus)

The immune system requires energetic and nutritional resources to optimally defend organisms against pathogens and parasites. Because resources are typically limited, immune function may require a trade-off with other physiologically demanding activities. Here, we examined whether photoperiodically induced seasonal states (breeding, molting, or nonbreeding) affected the cutaneous immune response of captive male house sparrows (Passer domesticus). To assess immune function in these birds, we injected the mitogen phytohemagglutinin (PHA) into the patagium and measured the resulting wing web swelling. Molting and nonbreeding birds had similar immune responses to PHA injection. However, males in a breeding state showed lower immune responses than both molting and nonbreeding birds even though they did not actually breed. We tested whether this decrease in the PHA swelling response in birds in a breeding state was due to elevated plasma concentrations of testosterone (T) by administering T to birds in a nonbreeding state. Contrary to some evidence in the literature, T did not suppress the response to PHA in house sparrows. Our data show that passerine birds show seasonal modulation in immune function, even in benign environmental conditions. However, even though T is often cited as a strong immunosuppressant, it is not fully responsible for this seasonal modulation.

Causes and consequences of song amplitude adjustment in a territorial bird: a case study in nightingales

Vocal amplitude, one of the crucial factors for the exchange of acoustic signals, has been neglected in studies of animal communication, but recent studies on song variation in Common Nightingales Luscinia megarhynchos have revealed new insights into its importance in the singing behavior of territorial birds. In nightingales song amplitude is not maximized per se, but is individually regulated according to the level of masking background noise. Also, birds adjust their vocal intensity according to social variables, as in male-male interactions. Moreover, during such interactions, males exploited the directionality of their songs to broadcast them in the direction of the intended receivers ensuring the most effective signal transmission. Studies of the development of this typical long-range signaling suggest that sound level is highly interrelated with overall developmental progression and learning, and thus should be viewed as an integral part of song ontogeny. I conclude that song amplitude is a dynamic feature of the avian signal system, which is individually regulated according to the ecological demands of signal transmission and the social context of communication.