44 research outputs found
Partial migration in European blackbirds : a study on alternative phenotypes
No full textIn this dissertation, I describe my work on migratory behavior in a partially migratory population of European blackbirds, Turdus merula. Partial migration, when a population consists of migrants and year-round residents, is a common behavior across the animal kingdom and is an intermediate stage between fixed migratory and sedentary life histories at the population and species level. Therefore, partial migration is commonly used as a model for understanding the evolution of migration. However, the question remains: why do individuals in partially migratory populations migrate?Using a combination of year-round radio tracking and geolocators I observed two distinct migratory periods during the study; one in early autumn, and another during the midst of winter. While blackbirds that migrated in autumn were never observed overwintering within 300 km of the study site, blackbirds that departed in winter were often observed within 40 km. Just prior to autumn migration, migrant blackbirds had higher fat scores than non-migrants and tended to have higher levels of baseline corticosterone, suggestive of anticipatory obligate migration. Sex related differences in migratory behavior are common and are often attributed to differences in reproductive strategies. I show that female blackbirds, independent of age and body size, are more likely to migrate in the autumn than male blackbirds. Unlike autumn migrants, I found no differences in the tendency of males and females to migrate in winter, nor did I find any difference in body size or age of winter migrants.Selection shaping annual cycles requires variation in annual organization among individuals. Despite the significance of partial migration in our understanding of the evolution of migration, little is known about the consistency of individual behavior in the wild. Although individual strategies and timing of migratory movements were consistent over the course of the current study, changes in behavior did occur from autumn migration or residency to the third less defined strategy- “winter escape”. Given this novel result, I propose the ‘Two-Step Model of Partial Migration’ for understanding the proximate control of individual strategies.Variations in the external flight apparatus of birds are beneficial for different behaviors. Long distance flight is less costly with more pointed wings and shorter tails; however these traits decrease maneuverability at low speeds. In order to identify the degree of morphological adaptation by migrants within the population to migration, I compared the wing pointedness and tail length of migrant and resident blackbirds. Contrary to a prediction of adaptive migratory phenotypes, I found no differences between migrants and residents in either measure.The results of my dissertation provide a more complex picture of partial migration in passerines than was previously known. By observing individuals throughout the year, I identified the co-occurrence of sex biased anticipatory migration in the autumn and what appears to be facultative migration in the winter. These findings provide support for different selective pressures on males and females during the evolutionary transition between migratory and sedentary behavior. Consistency in autumn migration behavior indicates potential contribution of endogenous mechanisms of autumn migrants. However, similarity in the flight apparatus of migrants and residents could indicate that morphological characteristics are under similar selective pressures.publishe
The flight apparatus of migratory and sedentary individuals of a partially migratory songbird species.
No full textVariations in the geometry of the external flight apparatus of birds are beneficial for different behaviors. Long-distance flight is less costly with more pointed wings and shorter tails; however these traits decrease maneuverability at low speeds. Selection has led to interspecific differences in these and other flight apparatuses in relation to migration distance. If these principles are general, how are the external flight apparatus within a partially migratory bird species shaped in which individuals either migrate or stay at their breeding grounds? We resolved this question by comparing the wing pointedness and tail length (relative to wing length) of migrant and resident European blackbirds (Turdus merula) breeding in the same population. We predicted that migrant blackbirds would have more pointed wings and shorter tails than residents. Contrary to our predictions, there were no differences between migrants and residents in either measure. Our results indicate that morphological differences between migrants and residents in this partially migratory population may be constrained
Rapid evolutionary divergence of a songbird population following recent colonization of an urban area
No full textColonization of a novel environment by a few individuals can lead to rapid evolutionary change, yet there is scarce evidence of the relative contributions of neutral and selective factors in promoting divergence during the early stages of colonization. Here we explore the role of neutral and selective forces in the divergence of a unique urban population of the dark-eyed junco (Junco hyemalis), which became established on the campus of the University of California at San Diego (UCSD) in the early 1980s. Previous studies based on microsatellite loci documented significant genetic differ-entiation of the urban population as well as divergence in phenotypic traits relative to nearby montane populations, yet the geographical origin of the colonization and the contributing factors remained uncertain. Our genome-wide single nucleotide polymorphism data set confirmed the marked genetic differentiation of the UCSD population, and we identified the coastal subspecies pinosus from central California as its sister group instead of the neighbouring mountain population. Demographic infer-ence recovered a separation from pinosus as recent as 20–32 generations ago after a strong bottleneck, suggesting a role for drift in genetic differentiation. However, we also found significant associations between habitat variables and genome-wide variants linked to functional genes, some of which have been reported as potentially adaptive in birds inhabiting modified environments. These results suggest that the interplay between founder events and selection may result in rapid shifts in neutral and adaptive loci across the genome, and reveal the UCSD junco population as a case of contemporary evolutionary divergence in an anthropogenic environment.Funding was provided by grants CGL-2011-25866 and CGL-2015-66381 from Spain’s Ministerio de Ciencia e Innovación to B.M., and USA National Science Foundation IOS-1257527 to T.J.G.Peer reviewe
A physiological perspective on the ecology and evolution of partial migration
No full textBillions of animals migrate between breeding and non-breeding areas worldwide. Partial migration, where both migrants and residents coexist within a population, occurs in most animal taxa, including fish, insects, birds and mammals. Partial migration has been hypothesised to be the most common form of migration and to be an evolutionary precursor to full migration. Despite extensive theoretical models about partial migration and its potential to provide insight into the ecology and evolution of migration, the physiological mechanisms that shape partial migration remain poorly understood. Here, we review current knowledge on how physiological processes mediate the causes and consequences of avian partial migration, and how they may help us understand why some individuals migrate and others remain resident. When information from birds is missing, we highlight examples from other taxa. In particular, we focus on temperature regulation, metabolic rate, immune function, oxidative stress, telomeres, and neuroendocrine and endocrine systems. We argue that these traits provide physiological pathways that regulate the ecological and behavioural causes and/or consequences of partial migration, and may provide insight into the mechanistic basis of wintering decisions. They may, thus, also help us to explain why individuals switch strategies among winters. We also highlight current gaps in our knowledge and suggest promising future research opportunities. A deeper understanding of the physiological mechanisms mediating the causes and consequences of partial migration will not only provide novel insights into the ecology and evolution of migration in general, but will also be vital to precisely modelling population trends and predicting range shifts under global change
Comparison of the flight apparatus of migrant and sedentary blackbirds.
No full text<p>Migrant and sedentary birds did not differ in either wing pointedness (A) or tail to wing ratio (B). Boxplots show the 5<sup>th</sup> and 95<sup>th</sup> percentiles. Circles indicate observations beyond the 5<sup>th</sup> and 95<sup>th</sup> percentiles.</p
Tracking migratory songbirds: accuracy of light-level loggers (geolocators) in forest habitats
No full textGenomes to space stations: the need for the integrative study of migration for avian conservation
No full textOngoing changes to global weather patterns and human modifications of the environment have altered the breeding and non-breeding ranges of migratory species, the timing of their migrations, and even whether they continue to migrate at all. Animal movements are arguably one of the most difficult behaviours to study, particularly in smaller birds that migrate tens to thousands of kilometres seasonally, often moving hundreds of kilometres each day. The recent miniaturization of tracking and logging devices has led to a radical transformation in our understanding of avian migratory behaviour and migratory connectivity. While advances in technology have altered the way researchers study migratory behaviour in the field, advances in techniques related to the study of physiological and genetic mechanisms underlying migratory behaviour have rarely been integrated into field studies of tracking. To predict the capacity of migrants to adjust to a changing planet, it is essential that we combine avian migration data with physiological and genetic measurements taken at key time points prior to, during and after migration.</jats:p
Numbers of migrants and residents in each age and sex category.
No full text<p>Numbers of migrants and residents in each age and sex category.</p
Author response: Migration confers winter survival benefits in a partially migratory songbird
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