John N. Mager

Male common loons, Gavia immer, communicate body mass and condition through dominant frequencies of territorial yodels

We investigated whether male-specific territorial ‘yodels’ communicate information about individual size and condition in the common loon. Individuals in better condition and of larger body mass, but not larger structural body size, produced lower-frequency yodels, and changes in dominant frequencies of yodels between years reflected changes in male body mass and condition. An acoustic playback experiment indicated that potential receivers vocalized sooner and more often in response to low-frequency yodels, a possible indication that dominant frequencies of the yodel may communicate condition-dependent fighting abilities. Physiological constraints associated with mass and/or condition may preserve honest signalling by preventing small individuals from producing low-frequency yodels.

Changing territories, changing tunes: male loons, Gavia immer, change their vocalizations when they change territories

Male common loons produce a territorial vocalization called the yodel. Each male loon has a characteristic yodel that is stable from year to year and differs from that of other male loons on other lakes. Of 13 male loons whose yodels we recorded before and after they changed territories, 12 substantially changed their yodels either the first or second year on the new territory. Furthermore, this change increased the difference between the new residents yodel and that of the previous resident. This result implies that loons not only change their vocalizations as the birds change territory, but also that the new owner is familiar with the yodel of the resident that it replaces.

Fatal battles in common loons: a preliminary analysis

Theoretical models predict that lethal contests should take place only when animals have severely limited breeding opportunities. Indeed, fatal fighting appears to occur routinely in only a handful of species that fit this mould. Here we report that 16–33% of all territorial evictions in male common loons, Gavia immer, are fatal for the displaced owner; in contrast, females seldom fight to the death for territories despite frequent territorial evictions. Since loons are long-lived and have ample reproductive options, they differ starkly from other fatal-fighting species. Several factors might contribute to lethal combat in loons, including: (1) the high value of territories to males, (2) a steady loss of condition among male residents, which could lead individuals with poor reproductive prospects to invest heavily in a current reproductive attempt, and (3) an inability of males defending tiny lakes to escape aggressive usurpers, owing to extremely high wing loading. The difficulty of detecting fatal contests in the field and the tendency of scientists to underestimate the behavioural impact of rare events leave open the possibility that fatal contests are a more widespread behavioural pattern than currently thought.

MALE COMMON LOONS SIGNAL GREATER AGGRESSIVE MOTIVATION BY LENGTHENING TERRITORIAL YODELS

Abstract We examined two critical predictions of the hypothesis that male Common Loons (Gavia immer) communicate greater aggressive motivation by increasing the number of repeat syllables within their territorial yodels. We observed (from >3,500 hrs of field observations of 58 males) the probability that territorial interactions escalated from territorial flyovers by intruders to stereotyped ‘social gatherings’ to escalated fights between residents and intruders was positively correlated to the number of repeat syllables given by individually-banded males. Males yodeling during these escalated contests often assumed the upright ‘vulture’ posture rather than the usual ‘crouch’ posture, reflecting an escalated aggressive motivational state. Territorial pairs responded sooner and with more threat and alarm vocalizations to playback yodels that contained more repeat phrases. This reflected a greater willingness to attack by residents to perceived intrusions by males of higher aggressive motivational state. Our study demonstrates the ability of loons to communicate greater aggressive motivation by lengthening acoustic territorial threat signals, which not only may be important for conveying imminent attack, but may also reflect important tactics for individuals of poorer fighting ability to deter territorial evictions. Our results also raise questions regarding what receiver-dependent and receiver-independent selective factors are responsible for maintaining signal honesty in this non-oscine bird.

Nest platforms increase aggressive behavior in common loons

Artificial manipulations of habitat, such as those that incorporate adding nesting boxes or platforms for birds, often enhance the breeding success of threatened animals. However, such alterations are likely to have unintended behavioral and ecological effects that might negatively impact the target species or others in its community. We conducted a controlled study to investigate the effect of artificial nesting platforms on aggressive behavior and reproductive success of male common loons (Gavia immer). Males residing on territories to which platforms were added produced longer territorial “yodels” (reflecting willingness to escalate a contest), experienced increased levels of confrontation and aggression with territorial intruders, and experienced increased rates of territorial displacement by intruders. Surprisingly, males of treatment territories also experienced lower productivity. Therefore, in addition to providing novel empirical support of sequential assessment models of animal contests that predict contest escalation with increasing resource quality, this study is one of a few to show that tools used to mitigate habitat loss can negatively impact reproductive fitness in a threatened species.

Macrogeographic Variation in the Body Size and Territorial Vocalizations of Male Common Loons (Gavia immer)

Abstract Geographic variation in the body mass and acoustic parameters of territorial ‘yodels’ recorded from male Common Loons (Gavia immer) were assessed for individuals breeding on territories across the eastern United States. Multivariate analyses incorporating male body mass, body size, the acoustic parameters of yodels, and geographic latitude and longitude indicated that males inhabiting lakes in northwestern regions were smaller and produced higher-frequency yodels. These relationships strengthen previous observations of clinal geographic variation in loon body size and vocal behavior across North America, but also support the hypothesis that the dominant frequencies of yodels are in part influenced by male body size. Therefore, the frequencies loons use for long-distance communication are apparently influenced, at least in part, by those selective forces responsible for shaping optimal body size.

Territory settlement in common loons: no footholds but age and assessment are important

Many animals face the task of locating and settling on a territory where they can produce offspring. Over the past 36 years, theoretical and empirical studies have provided growing support for the ‘foothold hypothesis’, which attempts to explain territorial settlement of long-lived animals. The hypothesis maintains that a young prebreeder lives within or intrudes into a cluster of breeding territories, accumulates site-dependent dominance there, then outcompetes other prebreeders for a territory within the cluster when it becomes available. We examined patterns in territorial intrusion and settlement among prebreeders of known age and natal origin to test the foothold hypothesis in the common loon, Gavia immer. We tested two other hypotheses for territory settlement: the maturation hypothesis, which posits that animals await physical and/or behavioural maturity before territory acquisition; and the assessment hypothesis, which maintains that prebreeders intrude into territories to assess fighting ability of territory owners, one of which they ultimately evict. We found no evidence for footholds in loons: prebreeders focused their intrusions within roughly 10 clustered territories, but intruded infrequently into the lake on which they later settled. Furthermore, prebreeders that waited years to usurp a territory had reproductive success no different from those that settled more rapidly on a vacant territory. The maturation hypothesis, in contrast, was supported in both sexes: prebreeders showed a sharp increase in fighting ability with age, and males exhibited age-related increases in mass and tendency to confront territory owners. The assessment hypothesis also gained support because intruders interacted extensively with owners and intruded frequently after territorial turnovers. Our study adds to a small but growing number of studies that fail to support the foothold hypothesis for territory settlement and support the conclusion that modes of territory acquisition might be more varied than previously thought.

Dynamics of an Aggressive Vocalization in the Common Loon (Gavia immer): A Review

Abstract. Unlike most waterbirds, Common Loons (Gavia immer) have a dynamic vocal repertoire that includes the high-amplitude wail, tremolo, and yodel. This paper is a review of the acoustic structure of the yodel, an aggressive warning signal only given by male Common Loons. The context in which males yodel is described along with the possible adaptive functions of this signal. The yodel is the most acoustically complex vocalization of the Common Loon and contains a wealth of information about the signaler. Suites of frequency and time elements of the yodel appear to communicate information about the identity of the signaler, which may be important for neighbor-stranger, mate, and kin recognition. The peak frequencies of the final note of the introductory phrase and repeat phrases also appear to communicate the condition-dependent fighting ability. Finally, the number of repeat phrases a male adds to its yodel appears to communicate the aggressive motivation, or the willingness a male Common Loon has to escalate a contest. Under various contexts not necessarily unique to Common Loons, these functions may be mutually beneficial to signalers and conspecific and heterospecific receivers, and evoke a number of interesting questions regarding the function of this dynamic signal. Such dynamic vocal signals are rare among waterbirds, and among ornithologists and behaviorists alike elicit questions regarding the conditions that maintain signal honesty among birds communicating fighting ability and aggressive state within the same vocal signal.

Habitat and Landscape Suitability as Indicators of Bird Abundance in Created and Restored Wetlands

We used aerial photography, field measurements, and bird surveys to evaluate 7 Ohio mitigation wetlands for their capacity to support avian guilds at both local and landscape scales. At the local scale, we assessed each wetland with habitat suitability indices (HSI) for eight wetland-dependent bird species as indicators for four guilds: wading, diving, dabbling, and emergent dependent. We characterized landscapes within 2.5 km of each wetland by measuring the buffer width, road density, connectedness, and anthropogenic land development. The changes in landscape variables over time were determined by comparison of aerial photos taken near the time of wetland construction and near the time of this study. Bird abundance data were poorly correlated with HSI scores but were well described with logistic models of buffer width, wetland area, and road density. Our results suggest that landscape variables are better predictors of bird abundance than HSI scores for these guilds in these wetlands.

Introduction: An Overview of Loon Research and Conservation in North America

Abstract. A workshop titled The Status of Gavia: Conservation in Black and White was held at the 5th North American Ornithological Conference on 14 August 2012, in Vancouver, British Columbia. The last such North American meeting focused on loons was at the American Ornithologists Union meetings in Minneapolis in 1997. During the interim 15 years, our knowledge of this group of diving birds in North America has increased significantly. From the 2012 workshop, as well as from contributions of authors unable to attend, 15 papers are presented in this special issue under five broad headings: behavior, life history and population ecology, movements and migration, habitat and landscape requirements and contaminants. Some highlights include the first data on sex ratios in Common Loon (Gavia immer) chicks, the first adult survival estimates for Red-throated Loons (G. stellata), and first reports of mercury exposure in Yellow-billed Loons (G. adamsii) from Alaska and Canada. In addition, a new long-distance migration record for the Common Loon, a landscape assessment of Common Loons in Massachusetts, and oil concentrations in loons wintering in Barataria Bay, Louisiana (one of the areas hit hardest by the Deepwater Horizon Oil Spill) are documented. We hope this collection of papers will be useful to researchers and wildlife managers in North America and abroad.