MAIN FINDINGS

MAIN FINDINGS

Each study, during the course of my doctoral work, was reliant upon the accurate

measure of individual migratory strategies in the population. In order to achieve this goal, in chapter 1 I tested the accuracy of light level loggers (geolocators) for tracking

migratory blackbirds. I collected location estimates from stationary geolocators deployed over the migratory trajectory of blackbirds in Western Europe during the non-breeding season. I also fitted non-migratory blackbirds with geolocators over a 12 month period to compare known locations of individuals with locations estimated by geolocators.

Although I found error most likely due to seasonal changes in habitat and behavior, the

results of chapter 1 indicate that the low degree of error of longitude estimates attained from geolocators makes this technology suitable for identifying relatively short distance movements by birds in longitude.

In chapter 2 I tested three classical hypotheses of partial migration: The ‘Arrival Time’,

‘Thermal Tolerance’, and ‘Dominance’ hypotheses. Over the course of the study, I identified two distinct periods of post breeding departures from the population (autumn and winter). In autumn, as predicted by the ‘Arrival Time’ hypothesis of partial

migration, female blackbirds were significantly more likely to migrate than males,

independent of age or body size. Like obligate migratory birds, autumn migrants prepared for migration by building up fat stores. Autumn migrants also tended to have higher levels of baseline corticosterone, indicative of preparation for migration. In combination, the results of chapter 2 suggest that autumn migration in the study population is a sex-biased anticipatory behavior. Unlike autumn migrants, birds that departed during the winter did not differ in age, sex, or body size.

Although individual strategies and timing of migratory movements were mainly

consistent, suggesting the potential existence of rigid programs, changes in behavior did occur from autumn migration or residency to a third less defined strategy - “winter escape”. These results indicate that partially migratory populations can include individuals with consistent strategies in autumn and individuals with more flexible behavior in winter. Based on the results in chapter 3, I proposed a new model for understanding the proximate control of individual strategies in partially migratory

populations, the ‘Two-Step Model of Partial Migration’. The first step, the decision to stay in the autumn, may be partly genetically determined and is therefore potentially an obligate stage. In the next step during the winter, the facultative stage, an individual departs as a last resort in order to survive extreme conditions (Figure 1).

Two-step model of partial migration

Fig. 1. The ‘Two-Step Model of Partial Migration’. During the autumn, the ‘decision’ to

migrate or remain sedentary is obligate. In the second stage, during the winter, the ‘decision’ to depart the breeding grounds is facultative.

Migratory passerines typically have external flight apparatus which enhance the

efficiency of long distance flight (Thomas 1996; Bowlin and Wikelski 2008). In chapter 4 I tested for differences in the external flight apparatus of migrant and resident blackbirds.

In a comparison of 38 migrants and 93 residents I found no differences in two primary

indices of flight apparatus. Further, I found no correlation between these two indices in the population. The results of chapter 4 indicate that either: 1) migration in the study population is not significantly aided by adaptations to flight apparatus or 2) enough genetic combination occurs between migrants and residents within the population to eliminate any morphological differences. In combination, the results of chapter 4 and body size comparisons in chapter 2 indicate that, morphologically, migrants and residents are similar in the study population.

IMPLICATIONS and FUTURE DIRECTIONS

The current study provides a detailed description of the annual movement behavior of individuals from a partially migratory population of European blackbirds. Using state of the art tracking techniques (chapter 1), this is the first study to detail the movements of free-living partially migrant songbirds over multiple years. By quantifying the phenology of movements of individuals over multiple years and comparing the morphology and physiological measures of individuals, the results of this study suggest the co-occurrence of obligate autumn migrants, facultative migrants in the winter, and year-round residents (chapters 2 and 3). While I show that autumn migration in the population resembles obligate migration in anticipation of declining conditions as reflected by consistency in migratory behavior, consistency in departure date, premigratory fattening, and a tendency for increases in circulating baseline levels of corticosterone before migration, we know nothing about the condition of individuals before winter departure.

The ‘Environmental Threshold Model of Migration’ proposes that migration, in partially migratory populations, is a quantitative genetic trait with the behavior of genetically intermediate individuals (between sedentary and migratory) partly influenced by the environment (Pulido 2011). Given the timing of winter departures from the study population, individuals that depart during the winter could represent the intermediate phenotype described by The Environmental Threshold Model of Migration.

Alternatively, the observation of two modes in annual departure dates (one in autumn and one in winter) could reflect a 2-step process consisting of an initial obligate stage in autumn, followed by a facultative winter stage (Figure 1). Facultative movements by typical migrants after the initial obligate migration phase occur when insufficient

resources are available at over-wintering locations (Terrill 1990). Facultative movements by sedentary individuals during the winter could be the result of similar mechanisms.

Future work is necessary to investigate the underlying mechanisms that shape the three phenotypes observed.

Sex differences in migratory behavior are common. For long distance migratory passerines, males typically arrive at breeding grounds before females (Morbey and Ydenberg 2001; Crick 2004; Rubolini et al. 2004; Saino et al. 2010). In differential migration, sexes are segregated during the winter at different latitudes, with females typically migrating farther than males (Ketterson and Nolan 1976; Prescott and

Middleton 1990; Belthoff and Gauthreaux 1991). In chapter 2, I report a sex bias in the tendency to migrate, with females more likely to migrate than males. Although sex

differences in partial migration have been shown before, they are usually accompanied by

age differences (Schwabl 1983; Lundberg 1985; Nilsson et al. 2008; Jahn et al. 2010).

Observations of combined age and sex differences in the tendency to migrate have led to the conclusion that migration is a condition dependent response to a dominance

hierarchy, with females subordinate to males and juveniles subordinate to adults (Lundberg 1985). The absence of an age difference in the current study indicates that either juvenile blackbirds are not subordinate to adults or the ‘decision’ to migrate involves additional or even different cues.

It is important to note that previous studies on partial migration in passerines could have overestimated the migratory fraction of populations given the limitations of classifying individuals based on recapture or re-sighting in the winter. For example, Schwabl (1983) found that male blackbirds that were migratory during the first year of observation were sedentary > 50% in the second year of observation (the current study site is 5 km from the study site of Schwabl (1983). The results of these former studies have led to the conclusion that migration, in partially migratory populations, is a trait that subsides within an individual’s lifetime. It is my belief that the current study could be the first, on free-living partially migrant passerines, to accurately identify the migratory and sedentary fractions of a population. The year-round tracking techniques employed in this study are unprecedented. Future studies employing year-round tracking in partially migratory systems should benefit greatly from accurate measures of behavior. More importantly, I feel that an increased focus on sex related differences in partial migration could provide new insight into the evolution of life history traits. Males and females differ in relation to the physiological, morphological, and behavioral traits that increase individual fitness

(Ball and Ketterson 2007). Therefore, selective pressures shaping the migratory life histories of males and females are likely to differ.

In summary, it is my hope that the chapters contained in this thesis establish a new

standard for studies on partial migration. By precisely measuring the behavior of partially migrant animals in the wild, we open the door to a new era in migration research.

Summary ___________________________________________

In 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.

Zusammenfassung