Olfactory navigation versus olfactory activation: a controversy revisited.

In the early 1970s, Floriano Papi and colleagues proposed the olfactory-navigation hypothesis, which explains the homing ability of pigeons by the existence of an odor-based map acquired through learning. This notion, although supported by some observations, has also generated considerable controversy since its inception. As an alternative, Paulo Jorge and colleagues formulated in 2009 the olfactory-activation hypothesis, which states that atmospheric odorants do not provide navigational information but, instead, activate a non-olfactory path integration system. However, this hypothesis is challenged by an investigation authored by Anna Gagliardo and colleagues and published in the current issue of the Journal of Comparative Physiology A. In this editorial, the significance of the findings of this study is assessed in the broader context of the role of olfaction in avian navigation and homing, and experiments are suggested that might help to finally resolve the olfactory-navigation versus olfactory-activation controversy.

Nestsite selection by male loons leads to sex-biased site familiarity.

1. The concept that animals benefit from gaining familiarity with physical spaces is widespread among ecologists and constitutes a theoretical pillar in studies of territory defence, philopatry and habitat selection. Yet proximate causes and fitness benefits of site familiarity are poorly known. 2. We used data from marked common loons Gavia immer breeding on 98 territories over 14 years to investigate the 'win-stay, lose-switch rule' for nestsite placement (if eggs hatch, reuse nestsite; if predator takes eggs, move nestsite). Males controlled nest placement in this species: pairs used the rule if both members remained the same from the previous nesting attempt or if only the male remained the same but not if only the female remained the same. 3. By means of the nesting rule, male common loons benefited from site familiarity, increasing nesting success by 41% between their first and third years on a territory. In contrast, females exhibited no increase in nesting success with increased territorial tenure. 4. Owing to site familiarity, a male loon competing for a breeding territory faces a considerable 'familiarity deficit' compared with the male breeder already established there. The familiarity deficit probably explains why resident animals often fight hard to retain familiar territories, when challenged, and why animals of many species tend to remain on familiar territories rather than moving when territories of higher intrinsic quality become available nearby.

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.

Multi-modal Orientation Cues in Homing Pigeons.

How homing pigeons displaced into unfamiliar territory find their way home has been the subject of extensive experimentation and debate. One reason for the controversy is that pigeons seem to use multiple cues. Clock-shifting experiments show that experienced pigeons use the sun as a preferred compass; when it is not available they rely on magnetic cues. That pigeons can home successfully while wearing frosted lenses suggests that landmarks, while not an essential navigational cue, are important in the final stages. The sensory basis of the "map" or position finding system is probably equally or even more complicated. When conditions around the loft are suitable, pigeons may use olfactory cues to find their way or might use some feature of the earth's magnetic field for their navigation. The Wiltschkos (1989) showed that pigeons raised without free access to ambient odors are not disoriented when anosmic while their siblings raised with free access to the prevailing wind were disoriented. Similarly, sibling pigeons from two lofts in Lincoln, Massachusetts. were well oriented or totally disoriented when released at magnetic anomalies under sunny skies depending upon which of the two lofts they had been reared in. All of these experiments and many more suggest that pigeons use multiple and redundant cues to find their way home. Further, there is the suggestion that which cues they adopt may well be influenced by the characteristics of the area around the home loft in which they were reared.