Pigeon Homing: The Navigational Map Developed in Adulthood is Based on Olfactory Information

Efferent and afferent connections of the olfactory bulb and prepiriform cortex in the pigeon (Columba livia)

Although olfaction in birds is known to be involved in a variety of behaviors, there is comparatively little detailed information on the olfactory brain. In the pigeon brain, the olfactory bulb (OB) is known to project to the prepiriform cortex (CPP), piriform cortex (CPi), and dorsolateral corticoid area (CDL), which together are called the olfactory pallium, but centrifugal pathways to the OB have not been fully explored. Fiber connections of CPi and CDL have been reported, but those of other olfactory pallial nuclei remain unknown. The present study examines the fiber connections of OB and CPP in pigeons to provide a more detailed picture of their connections using tract-tracing methods. When anterograde and retrograde tracers were injected in OB, projections to a more extensive olfactory pallium were revealed, including the anterior olfactory nucleus, CPP, densocellular part of the hyperpallium, tenia tecta, hippocampal continuation, CPi, and CDL. OB projected commissural fibers to the contralateral OB but did not receive afferents from the contralateral olfactory pallium. When tracers were injected in CPP, reciprocal ipsilateral connections with OB and nuclei of the olfactory pallium were observed, and CPP projected to the caudolateral nidopallium and the limbic system, including the hippocampal formation, septum, lateral hypothalamic nucleus, and lateral mammillary nucleus. These results show that the connections of OB have a wider distribution throughout the olfactory pallium than previously thought and that CPP provides a centrifugal projection to the OB and acts as a relay station to the limbic system.

Navigational abilities of adult and experienced homing pigeons deprived of olfactory or trigeminally mediated magnetic information

Anatomical evidence and conditioning experiments have suggested that magnetoreceptors innervated by the ophthalmic branch of the trigeminal nerve are located in the upper beak of homing pigeons. Following these findings it has been proposed that the trigeminally-mediated magnetorececeptors are able to detect magnetic field intensity, which might be useful for a position finding mechanism for pigeons homing from unfamiliar locations. Recent data have shown that, in inexperienced pigeons, section of the ophthalmic branch of the trigeminal nerve does not impair navigational abilities. Similarly, no impairment was observed if the trigeminal section was performed on young pigeons, before they have had the opportunity to learn a navigational map. By contrast, section of the olfactory nerve either in adult inexperienced pigeons or in young birds before map learning, disrupted their homing performance. Nevertheless, because a magnetic map mechanism requires training flights for learning the magnetic gradient of the territory around the loft, the question remains as to whether the navigational performance of adult experienced pigeons can be affected by lack of magnetic information. To answer this question we extensively group-trained adult pigeons and then surgically deprived them of either olfactory or trigeminally mediated magnetic information, prior to testing their navigational abilities. The birds deprived of trigeminally mediated magnetic information displayed similar navigational abilities as intact control pigeons, whereas the olfactory-deprived pigeons were dramatically impaired in homing. Our data show that even in trained adult pigeons, olfactory cues are needed for homing from unfamiliar locations and that the lack of magnetic information does not affect navigational abilities of experienced adult homing pigeons.

Neurobiology of the homing pigeon--a review

Homing pigeons are well known as good homers, and the knowledge of principal parameters determining their homing behaviour and the neurological basis for this have been elucidated in the last decades. Several orientation mechanisms and parameters-sun compass, earth's magnetic field, olfactory cues, visual cues-are known to be involved in homing behaviour, whereas there are still controversial discussions about their detailed function and their importance. This paper attempts to review and summarise the present knowledge about pigeon homing by describing the known orientation mechanisms and factors, including their pros and cons. Additionally, behavioural features like motivation, experience, and track preferences are discussed. All behaviour has its origin in the brain and the neuronal basis of homing and the neuroanatomical particularities of homing pigeons are a main topic of this review. Homing pigeons have larger brains in comparison to other non-homing pigeon breeds and particularly show increased size of the hippocampus. This underlines our hypothesis that there is a relationship between hippocampus size and spatial ability. The role of the hippocampus in homing and its plasticity in response to navigational experience are discussed in support of this hypothesis.

Navigational abilities of homing pigeons deprived of olfactory or trigeminally mediated magnetic information when young

Anatomical evidence and conditioning experiments have recently suggested that magnetoreceptors are located in the upper beak of homing pigeons, where they are innervated by the ophthalmic branch of the trigeminal nerve. These findings have raised the issue of whether the trigeminally mediated magnetoreception is involved in the navigational mechanisms of homing pigeons. Recent data have shown that, in inexperienced pigeons, section of the ophthalmic branch of the trigeminal nerve does not impair navigational abilities, whereas the navigational performance of inexperienced pigeons is disrupted after section of the olfactory nerve. Nevertheless, the issue of whether the stimuli available during development of the navigational mechanism can influence the types of cues used in determining the direction of displacement remains unresolved. To address this issue, we surgically deprived young pigeons of either olfactory or trigeminally mediated magnetic information, and then later tested their navigational abilities subsequent to an intensive training flight program of up to 10 km in different directions. The birds deprived of trigeminally mediated magnetic information when young developed navigational abilities at the same level as intact control pigeons, whereas the olfactory deprived pigeons displayed randomly scattered initial orientation and poor homing performance. Our data show that olfactory cues are needed for the development of navigational abilities from unfamiliar locations and that the lack of magnetic information does not affect the development of homing abilities.