Rapid adaptive adjustment of parental care coincident with altered migratory behaviour

The optimal duration of parental care is shaped by the trade-off between investment in current and expected future reproductive success. A change in migratory behaviour is expected to affect the optimal duration of parental care, because migration and non-migration differ in expectations of future reproductive success as a result of differential adult and/or offspring mortality. Here we studied how a recent emergence of non-migratory behaviour has affected the duration of parental care in the previously (until the 1980s) strictly migratory Russian breeding population of the barnacle geese Branta leucopsis. As a measure of parental care, we compared the vigilance behaviour of parents and non-parents in both migratory and non-migratory barnacle geese throughout the season. We estimated the duration of parental care at 233 days for migratory and 183 days for non-migratory barnacle geese. This constitutes a shortening of the duration of parental care of 21% in 25 years. Barnacle geese are thus able to rapidly adapt their parental care behaviour to ecological conditions associated with altered migratory behaviour. Our study demonstrates that a termination of migratory behaviour resulted in a drastic reduction in parental care and highlights the importance of studying the ecological and behavioural consequences of changes in migratory behaviour and the consequences of these changes for life-history evolution.     

Effects of predation danger on migration strategies of sandpipers

We examine the potential selective importance of predation danger on the evolution of migration strategies of arctic‐breeding calidrid sandpipers. Adult calidrids truncate parental care for reasons not obviously related to levels of food abundance on the breeding areas or at migratory stopover sites, suggesting that a different trade‐off occurs between providing additional care and adult survivorship. The southward migrations of adult western sandpipers precede those of migratory peregrine falcons by almost a month. By moving early and quickly, adults remain ahead of migrant falcons all the way to their non‐breeding areas, where they rapidly moult flight feathers. They complete the moult just as falcons arrive in late September–October. By migrating early, they avoid exposure to falcons when they are unusually vulnerable, due to the requirements for fuelling migratory flight and of wing feather moult. Juvenile western sandpipers migrate south just as falcon numbers start to increase, but do not moult flight feathers in their first winter. Pacific dunlin use an alternative strategy of remaining and moulting in Alaska after falcons depart, and migrating to their overwintering sites after migrants have passed. East of the Rocky Mountains, the southbound migration of falcons begins 4–6 weeks later. Southbound semipalmated sandpipers make extended migratory stopovers, but their lengths of stay shorten prior to falcon migration to the sites in September. Predation danger also may affect the evolution of migration routes. Southbound western sandpipers fly directly from Alaska to southern British Columbia, in contrast to the multi‐stage journey northward along the Alaska panhandle. We estimate that a direct flight would be more economical on northward migration, but may be avoided because it would expose sandpipers to higher mass‐dependent predation danger from migratory falcons, which travel north with sandpipers. By contrast, few raptors are present in Alaska during preparation for the southward flight. A temporal and spatial window of safety may also permit semipalmated sandpipers to become extremely vulnerable while preparing for trans‐Atlantic southward flights. Danger management may account for the these previously enigmatic features of calidrid migration strategies, and aspects of those of other birds.     

Date and parental quality effects in the seasonal decline in reproductive performance of the Tree Swallow Tachycineta bicolor: interpreting results in light of potential experimental bias

In many bird species early breeders have higher reproductive performance than late breeders from the same population. This could be caused by a reduction in environmental factors related to date per se (Date Hypothesis), or because poorer performers nest later (Parent Quality Hypothesis). We manipulated hatch date of Tree Swallows Tachycineta bicolor by switching clutches with different lay dates, generating broods with advanced or delayed timing, and assessed the impact of the experiment on nestling mass. The Date Hypothesis better explained the decline in nestling mass in the first half of the season, while the Parent Quality Hypothesis was supported in the second half. We also found that female mass loss was unintentionally reduced in advanced females and suggest that such impacts of the experiment on parent quality, or correlations between nestlings and their actual parents via heritability or maternal effects, could bias hatch-date manipulation experiments towards supporting the Date Hypothesis. Differential costs of incubation, either due to naturally low temperatures early in the season, or due to the unintentional manipulation of female incubation costs, appear to have driven support for the Date Hypothesis early in the season.     

The behavioural ecology of provisioning in birds

Ydenberg, R. 2000. The behavioural ecology of provisioning in birds. Ostrich 71 (1): 361.

In this talk I outline the idea of provisioning, stressing the difference between feeding and delivery. Foraging studies have just begun to recognise this distinction, and to explore some of its implications for behaviour. I outline a basic model of provisioning, based on central place foraging, in which the forager must spend some extra time on each excursion to gather enough energy to cover the excursion's energetic costs. I describe studies of provisioning pertinent to this framework, and go on to consider risk-sensitive provisioning, the role of predation danger in provisioning, and the question of how hard provisioners should work.     

Vegetation growth and a seasonal habitat shift of the barnacle goose (Branta leucopsis)

Summary Barnacle geese (Branta leucopsis) wintering on the island of Schiermonnikoog in the Netherlands abruptly switch all their foraging activities from a dairy pasture (a polder) to an adjacent salt-marsh during the early spring. We present evidence to show that this shift is related to changes in the quality of the diet available in these different habitats. Barnacle geese shift from polder to salt-marsh at the precise time that these are equal in dietary protein availability, which occurs as the food plants on the salt-marsh undergo a sudden spring growth. The dairy pasture undergoes its own spring growth shortly afterwards, and more dietary protein is available there for the rest of the year. We suggest that the salt-marsh is a more preferred habitat, but that low dietary protein during the winter prevents its use by barnacle geese. We hypothesize that the salt-marsh may be more preferred due to a lower level of disturbance which permits geese to graze more slowly, improving the utilization of food plants.     

Timing games in the reproductive phenology of female pacific salmon (Oncorhynchus spp.)

We use a game-theoretic framework to investigate the reproductive phenology of female kokanee (Oncorhynchus nerka). As in the other semelparous species of Pacific salmon, females construct nests in gravel, spawn with males, bury their fertilized eggs, and defend their nest sites until they die several days later. Later-breeding females may reuse previous nest sites, and their digging behavior is thought to subject previously buried eggs to mortality. Using game-theoretic models, we show that females can reduce this risk by allocating resources to longevity (the period between arrival and death) as opposed to eggs. Waiting before territory settlement is also expected if it allows females to conserve energy and delay senescence. The models demonstrate how these costs and benefits interact to select for a seasonal decline in longevity, a well-known phenomenon in the salmonid literature, and a seasonal decline in wait duration. Both of these predictions were supported in a field study of kokanee. Female state of reproductive maturity was the most important proximate factor causing variation in longevity and wait duration. With more than 30% of territories being reused, dig-up is likely an important selective force in this population.     

Time and energy constraints and the relationships between currencies in foraging theory

Measured foraging strategies often cluster around values thatmaximize the ratio of energy gained over energy spent whileforaging (efficiency), rather than values that would maximizethe long-term net rate of energy gain (rate). The reasons forthis are not understood. This paper focuses on time and energyconstraints while foraging to illustrate the relationship betweenefficiency and rate-maximizing strategies and develops modelsthat provide a simple framework to analyze foraging strategiesin two distinct foraging contexts. We assume that while capturingand ingesting food for their own use (which we term feeding),foragers behave so as to maximize the total net daily energeticgain. When gathering food for others or for storage (which weterm provisioning), we assume that foragers behave so as tomaximize the total daily delivery, subject to meeting theirown energetic requirements. In feeding contexts, the behaviormaximizing total net daily gain also maximizes efficiency whendaily intake is limited by the assimilation capacity. In contrast,when time available to forage sets the limit to gross intake,the behavior maximizing total net daily gain also maximizesrate. In provisioning contexts, when daily delivery is constrainedby the energy needed to power self-feeding, maximizing efficiencyensures the highest total daily delivery. When time needed torecoup energetic expenditure limits total delivery, a low self-feedingrate relative to the rate of energy expenditure favors efficientstrategies. However, as the rate of self-feeding increases,foraging behavior deviates from efficiency maximization in thedirection predicted by rate maximization. Experimental manipulationsof the rate of self-feeding in provisioning contexts could bea powerful tool to explore the relationship between rate andefficiency-maximizing behavior.     

Genetic consequences of breaking migratory traditions in barnacle geese Branta leucopsis

Cultural transmission of migratory traditions enables species to deal with their environment based on experiences from earlier generations. Also, it allows a more adequate and rapid response to rapidly changing environments. When individuals break with their migratory traditions, new population structures can emerge that may affect gene flow. Recently, the migratory traditions of the Barnacle Goose Branta leucopsis changed, and new populations differing in migratory distance emerged. Here, we investigate the population genetic structure of the Barnacle Goose to evaluate the consequences of altered migratory traditions. We used a set of 358 single nucleotide polymorphism (SNP) markers to genotype 418 individuals from breeding populations in Greenland, Spitsbergen, Russia, Sweden and the Netherlands, the latter two being newly emerged populations. We used discriminant analysis of principal components, F_ST, linkage disequilibrium and a comparison of geneflow models using migrate ‐n to show that there is significant population structure, but that relatively many pairs of SNPs are in linkage disequilibrium, suggesting recent admixture between these populations. Despite the assumed traditions of migration within populations, we also show that genetic exchange occurs between all populations. The newly established nonmigratory population in the Netherlands is characterized by high emigration into other populations, which suggests more exploratory behaviour, possibly as a result of shortened parental care. These results suggest that migratory traditions in populations are subject to change in geese and that such changes have population genetic consequences. We argue that the emergence of nonmigration probably resulted from developmental plasticity.     

Migration of two calidrid sandpiper species on the predator landscape: how stopover time and hence migration speed vary with geographical proximity to danger

The effects of relative fuel load on migration speed and on vulnerability have been investigated, but the effects of seasonal variation in predation danger on the amount of fuel and duration of stopover have not been considered. We analyzed seasonal patterns of stopover residence times for western and semipalmated sandpipers Calidris mauri and C. pusilla on southward migration in relation to the passage of migratory peregrine falcons Falco peregrinus. We predicted that individuals on stopover far in advance of the seasonal arrival of falcons would adjust stopover length and hence relative fuel load to migrate slowly and cautiously. We predicted that individuals on stopover later in the season would increase migratory speed as the arrival of migratory falcons came closer, while individuals on stopover under or behind the passage of falcons would migrate slowly. Adult and juvenile semipalmated and adult western sandpipers migrated prior to seasonal increases in peregrine abundance, and as predicted, the seasonal patterns of their stopover durations are consistent with an increase in the speed of migration as the date of peregrine arrival approached. Juvenile western sandpipers, in contrast, migrating concurrently with falcons, slowed their speed of migration as predator abundance increased. Stopover patterns differ between species due to different relative fuel loads. The results fit predictions made based on a ‘mortality‐minimizing’ migration strategy.     

Parental effort and reproductive skew in coalitions of brood rearing female common eiders

Members of breeding groups face conflicts over parental effort when balancing antipredatory vigilance and feeding. Empirical evidence has shown disparate responses to manipulations of parental effort. We develop a model in which we determine the evolutionarily stable effort of partners given their body conditions, allowing the benefits of shared care to be unevenly divided, and we test this model's predictions with data on common eiders (Somateria mollissima). Eiders show uniparental female care; females may share brood rearing, or they may tend alone, and their body condition at hatching of the young shows large environmentally induced variation. The model predicts that parental effort (vigilance) in a coalition is lower than when tending alone, controlling for parental condition; this prediction is supported by the data. The parental effort in a coalition should be positively correlated with body condition, and this prediction is also supported. Finally, parental effort should increase when partner condition decreases and vice versa; this prediction is partially supported. The Nash bargaining game may provide promising avenues by which to determine the precise settlement of reproductive skew and effort between coalition partners in the future.