Cues, strategies, and outcomes:How migrating vertebrates track environmental change

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Early spring sex differences in luteinizing hormone response to gonadotropin releasing hormone in co-occurring resident and migrant dark-eyed juncos (Junco hyemalis)

AbstractTo optimally time reproduction, animals must coordinate changes in the hypothalamo-pituitary-gonadal (HPG) axis. The extent of intra-species variation in seasonal timing of reproductive function is considerable, both within and among populations. Dark-eyed junco (Junco hyemalis) populations are known to differ in their reproductive timing response to cues experienced in the same habitat in late winter/early spring. Specifically in juncos cohabitating on shared wintering grounds, residents initiate breeding and reproductive activity but migrants delay reproductive development and prepare to migrate before breeding. Here, we test the hypothesis that the pituitary gland acts as a ‘control point’ to modulate differential HPG axis activity across populations. We sampled free-living resident and migrant juncos on their shared over-wintering grounds in March, thus all individuals were experiencing the same environmental cues, including photoperiod. We predicted that during this critical time of transition, residents would more readily respond to repeated gonadotropin releasing hormone (GnRH) stimulation with increases in luteinizing hormone (LH), in contrast to migrants, which should delay full reproductive activity. Our data indicate that migrant females, while still on the overwintering grounds, have a reduced LH response to repeated GnRH injections compared to resident females. Male migrant and resident birds did not differ in their responsiveness to repeated GnRH. Our results suggest a sex difference in the costs of mistimed activation of the HPG axis, with female migrants being less responsive than residents females and males to repeated stimulation. Further, our data implicate a key role for the pituitary in regulating appropriate reproductive timing responses

Partial migration in European blackbirds : a study on alternative phenotypes

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

The Flight Apparatus of Migratory and Sedentary Individuals of a Partially Migratory Songbird Species

Variations 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

A physiological perspective on the ecology and evolution of partial migration

Billions 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.

<p>Migrant and sedentary birds did not differ in either wing pointedness (A) or tail to wing ratio (B). Boxplots show the 5^th and 95^th percentiles. Circles indicate observations beyond the 5^th and 95^th percentiles.</p

Numbers of migrants and residents in each age and sex category.

<p>Numbers of migrants and residents in each age and sex category.</p

Perceived wintering latitude determines timing of song output in a migratory bird

Abstract Migratory bird populations frequently consist of individuals that overwinter variable distances from the breeding site. Seasonal changes in photoperiod, which varies with latitude, underlie seasonal changes in singing frequency in birds. Therefore, migratory populations that consist of individuals that overwinter at different latitudes with large overwintering ranges could experience within‐population variation in seasonal production of song. To test the influence of overwintering latitude on intrapopulation variance in song production in the spring, we subjected two groups of Eastern Song Sparrows (Melospiza melodia melodia) from the same partially migratory breeding population to different photoperiodic schedules associated with a 1,300‐km difference in overwintering location. One group remained on the natural photoperiodic schedule of the breeding site (resident group) while the other group experienced a nonbreeding photoperiod that mimicked a southern migration in the fall followed by a northern migration back to the breeding site in the spring (migratory group). We compared song output between the two groups in three different stages (nonbreeding, prebreeding, and breeding). Little singing occurred during nonbreeding stage sample dates (20 November, 6 December) for the resident group, and no singing occurred for the migrant group. During the prebreeding stage (27 January, 7 February), significantly more singing occurred in the resident group than in the migrant group. During the breeding stage (21 March, 4 April), after a simulated migration for the migrants, song output was similar in both groups. These results suggest that within‐population variation in wintering latitude may contribute to variation in seasonal changes in singing behavior, which may covary with readiness to breed. Studies utilizing confirmed migrants and residents, rather than merely simulated migrants and residents, are also needed to better understand these processes