Should I stay or should I go? Natal dispersal in the Seychelles warbler

Voor dit proefschrift hebben we de patronen van dispersie van jonge Seychellen zangers onderzocht. Deze kleine zangvogels (15g) leven in groepen bestaande uit een dominant broedpaar met eventueel daarnaast jongen die nog door hun ouders gevoerd worden en/of onafhankelijke jongen die hun dispersie uitgesteld hebben. Een deel van de onafhankelijke jongen helpt de ouders bij het voeren van de volgende generatie jongen, het zogenaamde helpen-bij-het-nest. Alle leden van de groep verdedigen gedurende het hele jaar een territorium tegen andere Seychellen zangers. Ons onderzoek vond plaats op Cousin, één van de vier kleine eilandjes (29 ha.) in de Seychellen waar deze soort voorkomt. Met ongeveer 110 territoria is Cousin geheel verzadigd en geen enkel begroeid plekje blijft onbezet. In dit proefschrift hebben we geprobeerd de individuele verschillen in de afstand en de timing van dispersie te verklaren. Waarom vestigt het ene jong zich in een territorium naast dat van zijn ouders, terwijl het andere zich 8 territoria verderop vestigt? Waarom verlaat het ene jong zijn ouderlijk territorium reeds na 6 maanden terwijl het andere meerdere jaren thuis blijft wonen? En waarom vestigt het ene jong zich wel met succes in een eigen territorium en het andere niet? Het beantwoorden van zulke vragen helpt ons de dynamiek en de genetische structuur van populaties te doorgronden en draagt bij aan het begrijpen waarom bepaalde diersoorten in familieverband leven.

Migratory common blackbirds have lower innate immune function during autumn migration than resident conspecifics

Animals need awell-functioning immune systemto protect themselves againstpathogens. The immune system, however, is costlyand resource trade-offs withother demands exist. For migratory animals several (not mutually exclusive)hypotheses exist. First, migrants reduce immune function to be able to allocateresources to migration. Second, migrants boost immune function to cope withmore and/or novel pathogens encountered during migration. Third, migrantsreallocate resources within the immune system.We tested these hypotheses bycomparing baseline immune function in resident and migratory commonblackbirds (Turdus merula), both caught during the autumn migration seasonon the island of Helgoland, Germany. Indices of baseline innate immune function(microbial killing capacity and haptoglobin-like activity) were lower inmigrants than in residents. There was no difference between the groups intotal immunoglobulins, a measure of baseline acquired immune function.Our study on a short-distance avian migrant supports the hypothesis thatinnate immune function is compromised during migration

Data from: Size and accumulation of fuel reserves at stopover predict nocturnal restlessness in a migratory bird

Early arrival at the breeding site positively affects the breeding success of migratory birds. During migration, birds spend most of their time at stopovers. Therefore, determining which factors shape stopover duration is essential to our understanding of avian migration. Because the main purpose of stopover is to accumulate fat as fuel for the next flight bout, fuel reserves at arrival and the accumulation of fuel are both expected to affect stopover departure decisions. Here, we determined whether in northern wheatears (Oenanthe oenanthe), captured and temporarily contained at spring stopover, fuel reserves and fuel accumulation predict a bird’s motivation to depart, quantified by nocturnal migratory restlessness (Zugunruhe). We found that fuel reserves at capture were positively correlated with Zugunruhe, and negatively correlated with fuel accumulation. This indicates that fat birds were motivated to depart, whereas lean birds were set on staying and accumulating fuel. Moreover, the change in fuel reserves was positively correlated with the concurrent change in Zugunruhe, providing the first empirical evidence for a direct link between fuel accumulation and Zugunruhe during stopover. Our study indicates that, together with innate rhythms and weather, the size and accumulation of fuel reserves shape stopover duration, and hence overall migration time

A hidden cost of migration? Innate immune function versus antioxidant defense

Migration is energetically demanding and physiologically challenging. Migrating birds, for example, need to boost their antioxidant defenses to defeat the pro-oxidants produced during high energetic activity. The enhanced antioxidant defense possibly withdraws limited resources (e.g., energy or micronutrients) from other physiological functions, such as immune defense. Such a trade-off might not occur outside the migration seasons or in resident individuals. Here, we investigate whether there is a negative relationship between innate immune function and antioxidant defense by sampling both migrating and resident blackbirds (Turdus merula) at the same location during the same period of the annual cycle. We show that in migrating blackbirds microbial killing capacity (BKA), an integrative measure of baseline innate immune function was negatively correlated with total nonenzymatic antioxidant capacity. In contrast, in resident conspecifics, sampled at the same time and location, these two physiological measures were not correlated. This suggests that migrating birds trade off innate immune function and antioxidant defense. Furthermore, and likely a consequence of this trade-off, in migrant blackbirds BKA was positively correlated with oxidative damage to lipids. In resident blackbirds BKA and degree of lipid oxidation were uncorrelated. The mechanism and currencies of the supposed trade-off are currently unknown, but energetic investments or micronutrients are likely candidates. Future experimental studies could provide more conclusive evidence for this trade-off; yet, our results open up a new level of thinking about the physiological costs of migration

FDR and food intake

FDR of temporarily caged and wild wheatears on Helgond island, Germany

Data from: A hidden cost of migration? Innate immune function versus antioxidant defense

Migration is energetically demanding and physiologically challenging. Migrating birds, for example, need to boost their antioxidant defenses to defeat the pro-oxidants produced during high energetic activity. The enhanced antioxidant defense possibly withdraws limited resources (e.g. energy or micronutrients) from other physiological functions, such as immune defense. Such a trade-off might not occur outside the migration seasons or in resident individuals. Here, we investigate if there is a negative relationship between innate immune function and antioxidant defense by sampling both migrating and resident blackbirds (Turdus merula) at the same location during the same period of the annual cycle. We show that in migrating blackbirds microbial killing capacity (BKA), an integrative measure of baseline innate immune function, was negatively correlated with total non-enzymatic antioxidant capacity. In contrast, in resident conspecifics, sampled at the same time and location, these two physiological measures were not correlated. This suggests that migrating birds trade off innate immune function and antioxidant defense. Furthermore, and likely a consequence of this trade-off, in migrant blackbirds BKA was positively correlated with oxidative damage to lipids. In resident blackbirds BKA and degree of lipid oxidation were uncorrelated. The mechanism and currencies of the supposed trade-off are currently unknown, but energetic investments or micronutrients are likely candidates. Future experimental studies could provide more conclusive evidence for this trade-off; yet, our results open up a new level of thinking about the physiological costs of migration

Migrating birds rapidly increase constitutive immune function during stopover

Migratory flight is physiologically highly demanding and has been shown to negatively affect multiple parameters of constitutive immune function (CIF), an animal’s first line of physiological defence against infections. In between migratory flights, most birds make stopovers, periods during which they accumulate fuel for the next flight(s). Stopovers are also commonly thought of as periods of rest and recovery, but what this encompasses is largely undefined. Here, we show that during stopover, northern wheatears Oenanthe oenanthe, a long-distance migratory bird, can rapidly increase constitutive innate immune function. We caught and temporarily caged birds under ad libitum food conditions at a stopover site in autumn. Within 2 days, most birds significantly increased complement activity and their ability to kill microbes. Changes in immune function were not related to the birds’ food intake or extent of fuel accumulation. Our study suggests that stopovers may not only be important to refuel but also to restore immune function. Additionally, the increase in CIF could help migrating birds to deal with novel pathogens they may encounter at stopover sites

Data from: Faster spring migration in northern wheatears is not explained by an endogenous seasonal difference in refueling rates

A widespread phenomenon in migrant birds is that they travel faster in spring than in autumn. During migration birds spend most time at stopover sites and, correspondingly, the faster spring migration is mainly explained by shorter stopovers in spring than autumn. Because a main purpose of stopovers is to replenish the fuel used in flight, a higher rate of fuel deposition (FDR) in spring is thought to explain the shorter stopovers and hence shorter total duration of migration in spring. Critical migratory processes, including the onset and extent of pre-migratory fueling, are endogenously regulated. It is therefore not unlikely that refueling at stopover sites is, at least partly, also under endogenous control. We here tested whether there is an endogenous seasonal difference in food intake and FDR, which could contribute to shorter stopovers and hence faster migration in spring. We measured daily food intake and daily FDR in two subspecies of the northern wheatear Oenanthe oenanthe, temporarily confined at stopover under identical constant indoor conditions in spring and autumn. The two wheatear subspecies differed markedly in absolute food intake and FDR. Within subspecies, however, food intake and FDR did not differ between spring and autumn, indicating that faster spring migration in northern wheatears is not explained by an endogenously controlled seasonal difference in birds’ motivation to refuel. To further substantiate this claim, similar measurements should be taken at other locations along northern wheatears’ migration routes. Comparable experiments in other species could test the generality of our results