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Tesi etd-03182019-151646


Thesis type
Tesi di dottorato di ricerca
Author
CERRITELLI, GIULIA
URN
etd-03182019-151646
Title
The role of environmental factors on the spatial behaviour of three long-distance migratory species
Settore scientifico disciplinare
BIO/05
Corso di studi
BIOLOGIA
Commissione
tutor Prof. Luschi, Paolo
tutor Dott. Giunchi, Dimitri
controrelatore Dott. Cerere, Jacopo
controrelatore Dott.ssa Hochscheid, Sandra
controrelatore Dott.ssa Gagliardo, Anna
Parole chiave
  • animal movement; exogenous cues; tracking
Data inizio appello
29/03/2019;
Consultabilità
parziale
Data di rilascio
29/03/2022
Riassunto analitico
Many vertebrate species embark in short or long migratory movements in order to make
the most of the available resources. The animal’s decision on when to start and where to
orient its movement is driven by an integration of the internal state of the individual with
the external cues perceived: firstly, the migrant has to decide when is the right moment
to start the migration in order to find good conditions along the route and at their final
destination; and then it has to decide where to move, i.e. it has to orient towards the final
goal. Apart from the central role played by the internal status of the animal in starting the
migratory behaviour, the importance of external stimuli in influencing the final movement
and route followed by the migrant, cannot be underestimated. Indeed, once the migration
has started, environmental stimuli continue to have an effect on the animal movement by
affecting the speed, accuracy and timing of migration, helping the individual in scheduling
its arrival to the final destination (again related to the “when to move” issue) and influencing the actual path followed by the migrant (linked to the “where to move” problem).
With the present thesis, I aimed at evaluating the role of environmental factors on the
spatial behaviour of three migratory species: the European teal (Anas crecca), the loggerhead (Caretta caretta) and green sea turtle (Chelonia mydas) that were satellite tracked
during their long-distance migrations. Teal data was used to answer the “when to go”
question by assessing the influence of external cues on the onset of their spring migration.
Conversely, sea turtle data was used to address the “where to go” issue investigating the
turtles’ responses to the effect of sea currents during their migrations.
Due to a general lack of information regarding teal migratory flyways in Europe, I
firstly investigated teal migratory phenology by analysing tracking data of individuals
wintering in three Italian sites. In this way I determined the course, speed and duration of
their migratory movements, together with the number and length of stopovers made along
the route. Most of the tracked teals left the wintering grounds between mid-February
and March following a straight and direct route along the Black Sea-Mediterranean flyway, and reached the breeding sites located in North-Eastern Europe in May. Along their
migratory route most birds stopped for several weeks at stopover sites, especially at the
very beginning of migration, and this led to a slow overall migratory speed (36 km/d on
average). Nevertheless, the active flight segments of the migration were covered at much
higher speeds, up to 872 km/d.
To correctly interpret the birds’ migratory strategy, it is crucial to properly identify
the start of migration. Nevertheless, at present a univocal and objective method to assess
the start of migration is missing, so I performed a comparative analysis between three different methods to determine the onset of migration using teal tracking data as an example.
The starting dates identified by the tested methods were not always comparable, with the
greatest inconsistencies among techniques being evident when the birds started early the
migration and then stopped for a long period (>20 days) in a stopover area. This shows
that caution is needed when choosing one technique over another, as the method used is
likely to strongly affect any following analyses on migratory phenology.
After having defined the migratory phenology of this species, I finally focused on investigating the effect of environmental cues on teal’s decision to start the migration. My analysis aimed at assessing if temperature and winds experienced during teal’s sojourn
in an area could have had an effect on the decision to leave their wintering grounds and
the site of the longest stopover, i.e. a stopover frequented at the beginning of migration
for more than 20 days. I run two distinct Cox proportional models for wintering and the
longest stopover area, including photoperiod as a control to determine if the models interpreted correctly the animal choices. As already noted for some goose species, the start
of migration from the wintering ground was only triggered by the progressive increase of
daylight hours, but, once the migration had started, the photoperiod lost its importance
and teals took into consideration only the experienced temperatures to continue their migration from the longest stopover area. Winds experienced before departure, on the other
hand, did not seem to play a role in teal’s decision to leave or not an area.
The second part of my work addressed the “where to go” issue, specifically investigating the effect of sea currents on turtles during their open-sea migrations. Given that every
swimming animal is affected by the flow of the medium it is moving in, the reconstructed
route obtained from satellite-derived positions may not always be representative of the
actual movement of the turtle and of its swimming speed. To assess the turtle’s active
contribution to the tracked, ground-derived movement, I performed a vector subtraction
between the ground-derived and sea currents velocities of three turtle populations migrating in quite different oceanographic conditions. The effect of sea currents along the
turtle’s migratory routes was not homogeneous among populations, being greater in turtles migrating in the Indian Ocean than in those moving in the South Atlantic Ocean or in
the Mediterranean Sea. These results thus highlight the importance of evaluating the sea
currents encountered en-route by migrating turtles when studying their movements and
inferring the behaviour, speed, etc. from tracking data. Deriving information on turtle’s
migratory behaviour simply from tracking data and considering it representative of the
actual active speed of the migrant, can indeed lead to errors, especially if the turtle is
encountering strong and variable current fields along its route.
The last aspect I investigated is strictly connected with the previous analysis, where it
was observed that turtles nesting at Ascension Island and returning to the Brazilian coast
at the end of their breeding season, mostly encountered weak currents flowing in the same
direction of the turtle heading. This particular condition made me to hypothesise that
this population could rely on a very simple navigational mechanism to reach their foraging
grounds, like following a given direction for a certain amount of time (vector navigation).
To test this hypothesis, I simulated the post- and pre-nesting migrations of turtles relying
on different navigational strategies and, only for the post nesting migration, I compared
the modelled turtle routes with the actual routes of turtles tracked. From the simulations
obtained it was evident that the post nesting migrations towards a large target like the
continental coast could indeed be performed following a simple vector strategy. The pre
nesting migrations towards Ascension, instead, require a more complex system, like true
navigation, given the difficulties in reaching such a small target located in the middle of
the ocean. Thus, the navigation system of Ascension turtles could be another example of
a coexistence of different navigational mechanisms in the same animal, that are used in
different migrations to accomplish different orientation tasks.
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