Abstract
The phenomenon of bird migration has been closely observed and has intrigued humans throughout history, yet the mechanisms which underlie bird movements over vast geographical scales are still poorly understood. Long-range migratory birds embark on a bi-annual feat of endurance, undergo rapid morphological changes, and have adapted to fuel high-intensity exercise and transition through multiple metabolically expensive phases (moult, migration and reproduction) of their life cycle. This work approaches this field from a physiological perspective, to elucidate some of the functional traits that govern spatiotemporal distributions of highly mobile species through the annual cycle, from drivers of migratory routes to the orientation apparatus in passerines. The scope of the thesis is scalar: the work builds upon population wide modelling studies of movement and fitness, to the individual level where tracking and sampling of birds in the natural environment with the aim to better understand the physiological determinates of migratory decision making.
Simulations of migratory tracks within the Afro-Palearctic flyway were used to demonstrate that intrinsic and extrinsic factors can explain long distance migration into sub-Saharan Africa and complex routing where the non-breeding periods are defined by a series of discrete wintering locations. Results indicated that avian migration may be a result of functional physiological limit of minimum air temperature or that as a cue, minimum temperature could be used to derive resource gains during the poorest periods of the annual cycle. Furthermore, at a high temporal scale, seasonal spatiotemporal variation in the resource metrics, NDVI and surplus NDVI, can explain the patterns of complex migratory routes, observed in nature. Free-flying snow buntings were tracked to better understand how an obligate migrant, that is well adapted to breed in extreme environment of the high Arctic, utilised the majority of the annual cycle: the non-breeding period. This population demonstrated a flexible migratory strategy, enduring the extreme cold of the Siberian Steppe to utilise vast crop- and grasslands: physiologically well adapted to low ambient temperatures and revealed consistencies between physiological traits beneficial for told tolerance and endurance flight. Additionally it could be shown that birds gained a tailwind and risk advantaged by undertaking a substantially longer overwater flight across the Arctic Ocean.
Survival modelling was used to explore the mortality associated with negotiating an ecological barrier, utilising the immense dataset of ringing data generated at three bird observatories on the Baltic Sea. This data reveals a consistent high mortality probability of adults robins following autumn migration compared to young during the same period. This pattern was not observed in the larger species the song thrush and indicative of the two age classes responding to ecological barrier differently depending on condition, and that this is reflected on the ultimate physiological metric of survival. This was explored at the individual levels by quantifying a suite of physiological parameters of long- and short- term condition in wild birds equipped with radio tags to determine subsequence migratory decisions and stopover behaviours. While there was no evidence that, as a group, adult robins were physiologically compromised compared to young, tracked individuals revealed that multiple intrinsic parameters: circulatory corticosterone, size, fuel load and haematocrit, were responsible for migratory decisions in a medium distance migrant. The integration of orientation mechanisms, endurance flight and stress physiology was further investigated on the Faroe Islands were vagrant and naturally displaced birds arriving on a remote island after a 600 km open water crossing were sampled for baseline corticosterone. Despite expectations that being far outside their
normal range would elicit a stress response, which may trigger corrective behaviour observed in wind displaced birds, depleted fat stores only, was found to promote elevated levels of corticosterone. However empirical evidence was obtained to suggest that obligate endurance flight supressed adrenocorticoid activity, even with exhausted fuel loads, as an apparent life preservation response.
The flexibility of navigational processes and orientation capacity was tested in the experimental displacement of small long-distance migrants equipped with transmitting devices to obtain high resolution spatial data throughout the individual’s autumn migration. Methodology was developed to optimise positional data from these tags, and despite low sample size, data apparently confirmed true navigational capacity in adult birds but not young, but that temporal segregation of age classes is not maintained throughout the migration schedule. In the final study, experimental manipulation of sensory apparatus was used to investigate the environmental cues required for orientation and navigation in free-flying wild birds. This fundamental mechanism which enables billions of birds to undertake precise intra-continental scale movements was tested, and it was found that even young passerines may incorporate olfactory cues into their putative orientation sense. Collectively this thesis extends our understanding of the drivers complex spatiotemporal distribution of life on earth, offers insights in the role of environmental factors and adaptations to them in migration, and attempts to elucidate behaviour of an individual from its biochemistry and how that may be extrapolated to functional level.
Simulations of migratory tracks within the Afro-Palearctic flyway were used to demonstrate that intrinsic and extrinsic factors can explain long distance migration into sub-Saharan Africa and complex routing where the non-breeding periods are defined by a series of discrete wintering locations. Results indicated that avian migration may be a result of functional physiological limit of minimum air temperature or that as a cue, minimum temperature could be used to derive resource gains during the poorest periods of the annual cycle. Furthermore, at a high temporal scale, seasonal spatiotemporal variation in the resource metrics, NDVI and surplus NDVI, can explain the patterns of complex migratory routes, observed in nature. Free-flying snow buntings were tracked to better understand how an obligate migrant, that is well adapted to breed in extreme environment of the high Arctic, utilised the majority of the annual cycle: the non-breeding period. This population demonstrated a flexible migratory strategy, enduring the extreme cold of the Siberian Steppe to utilise vast crop- and grasslands: physiologically well adapted to low ambient temperatures and revealed consistencies between physiological traits beneficial for told tolerance and endurance flight. Additionally it could be shown that birds gained a tailwind and risk advantaged by undertaking a substantially longer overwater flight across the Arctic Ocean.
Survival modelling was used to explore the mortality associated with negotiating an ecological barrier, utilising the immense dataset of ringing data generated at three bird observatories on the Baltic Sea. This data reveals a consistent high mortality probability of adults robins following autumn migration compared to young during the same period. This pattern was not observed in the larger species the song thrush and indicative of the two age classes responding to ecological barrier differently depending on condition, and that this is reflected on the ultimate physiological metric of survival. This was explored at the individual levels by quantifying a suite of physiological parameters of long- and short- term condition in wild birds equipped with radio tags to determine subsequence migratory decisions and stopover behaviours. While there was no evidence that, as a group, adult robins were physiologically compromised compared to young, tracked individuals revealed that multiple intrinsic parameters: circulatory corticosterone, size, fuel load and haematocrit, were responsible for migratory decisions in a medium distance migrant. The integration of orientation mechanisms, endurance flight and stress physiology was further investigated on the Faroe Islands were vagrant and naturally displaced birds arriving on a remote island after a 600 km open water crossing were sampled for baseline corticosterone. Despite expectations that being far outside their
normal range would elicit a stress response, which may trigger corrective behaviour observed in wind displaced birds, depleted fat stores only, was found to promote elevated levels of corticosterone. However empirical evidence was obtained to suggest that obligate endurance flight supressed adrenocorticoid activity, even with exhausted fuel loads, as an apparent life preservation response.
The flexibility of navigational processes and orientation capacity was tested in the experimental displacement of small long-distance migrants equipped with transmitting devices to obtain high resolution spatial data throughout the individual’s autumn migration. Methodology was developed to optimise positional data from these tags, and despite low sample size, data apparently confirmed true navigational capacity in adult birds but not young, but that temporal segregation of age classes is not maintained throughout the migration schedule. In the final study, experimental manipulation of sensory apparatus was used to investigate the environmental cues required for orientation and navigation in free-flying wild birds. This fundamental mechanism which enables billions of birds to undertake precise intra-continental scale movements was tested, and it was found that even young passerines may incorporate olfactory cues into their putative orientation sense. Collectively this thesis extends our understanding of the drivers complex spatiotemporal distribution of life on earth, offers insights in the role of environmental factors and adaptations to them in migration, and attempts to elucidate behaviour of an individual from its biochemistry and how that may be extrapolated to functional level.
| Original language | English |
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| Publisher | Natural History Museum of Denmark, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2018 |