Winter is one of the most challenging seasons for many animals. Cold temperatures, bad weather, short days, long nights and a shortage of food can impose a deadly threat. To avoid these inhospitable conditions, some animals migrate to warmer climes during the winter. These animals include many songbirds, which return to the same habitat in the following spring because it offers abundant resources that are thought to help them to breed more successfully. Yet, migration itself can be risky, and there is little empirical data on the survival benefits of migration in songbirds.

Zúñiga et al. tested whether songbirds that migrate are actually more likely to survive the winter than those that do not migrate. The study focused on a population of European blackbirds over a period of seven years. Some of these birds migrated from the breeding grounds in Germany to their wintering sites in southern Europe, whereas others remained all year at the breeding grounds.

Zúñiga et al. found that migrant blackbirds were 16% more likely to survive the winter than the residents. Yet during the summer, there was no difference in survival between the two groups. This raised the question, if migration confers survival benefits, why do some birds do not migrate at all?

Theory predicts that those birds that do not migrate should have some reproductive benefit instead. This makes sense given that birds which remain at the breeding grounds would have access to prime breeding sites which are limited. Using mathematical modelling, Zúñiga et al. estimated how much of reproductive benefits the residents would need to outweigh their greater risk of not surviving the winter. The model predicted that residents should have at least 61.25% higher breeding success than migrants.

The results provide empirical evidence to help scientist understand how migration evolves and becomes maintained in animal population. Future studies are now needed to confirm the estimated breeding success of both groups. Also, because many songbirds are threatened by human activity during migration and at their overwintering sites, future studies to understand how, where and why migratory songbirds die will be important to direct the conservation efforts to protect migratory species.

The adaptive function of migration has often been hypothesized to be a selective advantage to escape adverse situations caused by seasonal fluctuations of food resources or environmental conditions. This seasonality may impose considerable constraints to life, particularly during the winter season. Seasonal migration allows animals to cope with temporal environmental fluctuations by moving between geographically distant habitats (Fryxell and Sinclair, 1988). Given that much of our planet offers seasonally varying resources, it is not surprising that migration has evolved repeatedly in many taxa (Chapman et al., 2011). Theoretical research on the evolution of migration (Lundberg, 1987; 1988; Taylor and Norris, 2007; Griswold et al., 2010; Kokko, 2011; Shaw and Levin, 2011; Shaw and Couzin, 2013) has yielded a key prediction: migration should offer either survival or breeding benefits compared to residency. In anadromous fish, for example, individuals migrate between freshwater and ocean habitats. Recent comparisons of migrant and resident steelheads (Oncorhynchus mykiss) found that female migrants have higher fecundity than females that remain in fresh water streams (Satterthwaite et al., 2009; Hodge et al., 2014, 2016). Similarly, the noctuid moth (Autographa gamma) performs a multi-generational migration which confer substantial reproductive benefits by allowing a lineage to spread to multiple sites (Chapman et al., 2012). Regarding survival benefits, individuals of a fresh water fish (Rutilus rutilus), increase their survival during the winter by migrating from lakes to streams to avoid predation risks (Skov et al., 2013).

In birds, seasonal migration has often been argued to bring about survival benefits, as it allows individuals to avoid inhospitable conditions during the non-breeding season, while the same region can offer abundant resources during the breeding season (Lack, 1954). Species exhibiting polymorphisms in migratory behavior provide an excellent opportunity to test predictions of fitness components. In partially migratory species, some individuals migrate while others remain as year-round residents, thereby allowing for between-group comparisons within the same population. Theory predicts that if residency enhances breeding success in territorial birds, then there should be a corresponding benefit to migrants; higher survival over non-breeding season is a clear, but empirically understudied, possibility (Lundberg, 1987; Kokko, 2011). Despite the extensive research done on bird migration, there is limited empirical evidence regarding its fitness benefits, as data on fitness-related variation in migratory strategies are logistically difficult to collect in the field. Despite logistical challenges, studies on European robins (Erithacus rubecula) and American dippers (Cinclus mexicanus) report that migrants have lower survival and reproductive success than residents (Adriaensen and Dhondt, 1990; Gillis et al., 2008; Green et al., 2015). Further, a recent study comparing fitness measures of resident and migrant cormorants (Phalacrocorax aristotelis) reported higher reproductive success in residents compared to migrants (Grist et al., 2017).

We studied a partially migratory population of European blackbirds (Turdus merula) (Figure 1) to test whether migration confers survival benefits during the winter. The migrants of our population overwinter, on average, 800 km west-southwest from the breeding grounds (Fudickar and Partecke, 2012) (Figure 2a and b) and the majority of migrants are females (Fudickar et al., 2013). We fitted multi-event survival models using presence-absence data obtained by capture-mark-recapture and radio-telemetry of 192 resident and 70 migrant free-living blackbirds over the course of seven years. These models account for variation in re-encounter probabilities in relation to space, time and behaviour of the birds.

Figure 1

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Figure 2

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Figure 2—source data 1

Zip file contains five files: ‘locations_data.csv’; ‘kud2009.

Rdata’; ‘kud2010.Rdata’; ‘kud2011.Rdata’; ‘kud2012.Rdata’; ‘kud2013.Rdata’. ‘locations_data.csv’ contains the mean overwintering estimated locations (lat, long) of 22 blackbirds derived from Geolocators during the years 2009–2014 and plotted in Figure 2 panel a (red symbols). Estimated locations were calculated using the ‘geolight’ function from the Geolight R package. Files: ‘kud2009.Rdata’; ‘kud2010.Rdata’; ‘kud2011.Rdata’; ‘kud2012.Rdata’; ‘kud2013.Rdata’: 25% kernel utilization distribution representing the error of each estimated overwintering locations for the different years in R data format and plotted in Figure 2 panel a (red circles).

https://doi.org/10.7554/eLife.28123.005

Download elife-28123-fig2-data1-v1.zip

Figure 2—source data 2

Lat and long and distance to the breeding grounds (km) to the 29 overwintering locations of European blackbirds (2009–2014) used to generate histogram of Figure 2 panel b.

https://doi.org/10.7554/eLife.28123.006

Download elife-28123-fig2-data2-v1.csv

We compared the survival probabilities between residents and migrants during two different seasons: summer (mean start date: March 2 ± 14.5 days - mean end date: November 2 ± 7.4 days) and winter (mean start date: November 3 ± 7.4 days - mean end date: March 1 ± 14.5 days). Based on theoretical models of partial migration in birds (Kokko, 2011), which assume that residency offers reproductive benefits (access to better breeding territories) and that migration should confer survival benefits for at least some individuals if the winter conditions at the breeding ground are harsh, we predicted that migrants should have higher survival probabilities during the winter period, whereas summer survival might not differ between migrants and residents.

We found that winter mortality is an important determinant of lifespan, as blackbirds had lower probability to survive the winter (Φ = 0.60; 95% CI = 0.55–0.66) than the summer season (Φ = 0.89; 95% CI = 0.82–0.94) (Table 1, model 3) despite the shorter duration of the former season. This result strongly supports the hypothesis that migration confers survival benefits compared with residency as an alternative strategy.

There was no difference between juveniles and adults in survival probability within a season. Juveniles (Φ = 0.89; 95% CI = 0.80–0.94; model 4 Table 1) have similar probability to survive the summer as adults (Φ = 0.90; 95% CI = 0.83–0.94; model 4 Table 1). During winter, juveniles also have a comparable probability (Φ = 0.59; 95% CI = 0.49–0.68; model 4 Table 1) to survive as adults (Φ = 0.61; 95% CI = 0.55–0.67; model 4 Table 1).

In line with our predictions, migratory European blackbirds had higher winter survival rates than resident blackbirds. The best model (model 1, Table 1) estimated markedly higher winter survival for migrants (Φ = 0.73; 95% confidence intervals (CI) = 0.62–0.81, Figure 3) than for residents (Φ = 0.57; 95% CI = 0.50–0.63, Figure 3), taking into account the lower detection probability for migrants (P=0.19; 95% CI = 0.13–0.26, Figure 3) compared to residents (P=0.74; 95% CI = 0.69–0.78). Our second model, which included sex and had modest support (model 2, delta AICc = 0.95, Table 1), predicted that migrants have higher winter survival probability than residents, which was also predicted by model 1. Sex differences were not substantial (during summer: male residents Φ = 0.90; 95% CI = 0.83–0.95; female residents Φ = 0.89; 95% CI = 0.89–0.94; male migrants Φ = 0.95; 95% CI = 0.89–0.98, female migrants Φ = 0.94; 95% CI = 0.87–0.97; during winter: male residents Φ = 0.58; 95% CI = 0.51–0.65; female residents Φ = 0.53; 95% CI = 0.44–0.62; male migrants Φ = 0.75; 95% CI = 0.63–0.84, and female migrants Φ = 0.71; 95% CI = 0.60–0.81; detection probability was lower for migrants (P=0.19; 95% CI = 0.13–0.26) than for residents (P=0.74; 95% CI = 0.69–0.78)). It is reassuring that both models 1 and 2 agree on the importance of residency vs. migration in winter, while we refrain from making strong statements regarding the effect of sex, given that Burnham and Anderson, 2002 advise against considering inferior models competitive in cases like our model 2 (delta AIC within about 0–2 units of the best model, the difference being caused by one parameter added to the best model and the log-likelihood essentially unchanged).

Figure 3

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Figure 3—source data 1

Results of the best ranked model (Model 1).

Survival estimates and confidence interval values predicted by model 1 of migrants and residents during winter and summer. Values are plotted in Figure 3.

https://doi.org/10.7554/eLife.28123.009

Download elife-28123-fig3-data1-v1.csv

Our findings support the theoretical predictions that migration yields survival benefits during the winter. In addition, our results provide an explanation for the maintenance of the migrant phenotype in the partially migratory population of European blackbirds that we studied. Residency is predicted to provide reproductive benefits given that year-round occupancy provides, for example, advantages in establishing breeding territories (Kokko, 2011). The two phenotypes can persist as evolutionary stable strategies (ESS) due to frequency dependent selection if the overall fitness of migrants and residents is equal (Lundberg, 1987). Given the lack of data on the reproductive performance of migrants and residents in our present study, we estimated how much the reproductive performance of residents should be to compensate the survival benefits of migration. If we assume migrant and resident winter survival to be 0.73 and 0.57 respectively, and summer survival of 0.89 for both strategies, then we can estimate the expected number of reproductive attempts for a migrant as 0.73 + $\frac{0.73\times 0.89}{1-0.73\times 0.89}$ = 2.58, and 0.57 + $\frac{0.57\times 0.89}{1-0.57\times 0.89}$ = 1.60 for residents. Therefore, the expected lifetime number of reproductive seasons is 61.25% higher for migrants than for residents due to higher survival of the former. This calculation assumes that the first breeding season requires one overwintering to be completed successfully, while all other events require an additional surviving sequence of summer followed by winter before a new breeding event can happen. The format for this assumption is s₁s₂ / (1– s₁s₂) which is the solution for the series s₁s₂ + (s₁s₂)²+(s₁s₂)³+…, (s₁ corresponds to winter survival probability and s₂ to summer survival probability), each subsequent term requiring one sequential survival event through one summer and one winter season.

We conclude from this calculation that the reproductive performance of residents would have to be 61.25% higher than in migrants to achieve equal fitness of the alternative strategies. Such benefits could come about from prior residency effects (either occupying a better territory or avoiding floater status), combined with a longer time spent in the breeding habitat which can make multiple nesting or re-nesting (in case of failure) more likely. Considering that blackbirds are a multi-brood species (2–3 broods a year), it could be possible that residents gain a 61.25% higher breeding success compared to migrants. Future studies need to confirm these calculations. If resident breeding success is higher than 61.25%, then the fitness of migrants will be lower than the fitness of residents and migration would be a conditional strategy operating under frequency-dependent selection. For conditional migration strategies, some intrinsic phenotypic characteristics (sex, age, dominance) result in a need to adopt a strategy that might yield overall lower fitness than what residents on average achieve, but it is the better choice to optimize individual fitness (Lundberg, 1987). To distinguish between these two alternatives, data of reproductive success for this species are needed (note that comparisons within existing studies, such as Grist et al., 2017 on cormorants, do not incorporate all the processes we have envisaged above).

It is also plausible that year-to-year variation of winter environmental conditions at the breeding grounds play a role shaping the incidence of migration versus residency over time. For instance, during a harsh and long winter, the survival of residents might be lower compared to a mild and short winter. If fewer residents survive an unusually harsh winter and establish breeding territories during the subsequent breeding season, many high-quality territories would remain vacant for migrants to take advantage of after arrival in the spring. Furthermore, if residents that do survive harsh winters begin the breeding season in poor condition, then physically dominant migrants could successfully take-over prime territories from residents. Under this scenario, the prior residency effect would not be acting at full strength (Drent et al., 2003; Jahn et al., 2010; Kokko, 2011) and migrants would gain breeding benefits.

We found no evidence for sex differences in survival (though some ambiguity remains, as a moderately supported model two includes sex as an explanatory variable — note that the best model does not). This raises the question: why are females more likely to migrate than males in the study population (Fudickar et al., 2013)? We can think of two potential reasons for this observation: either there is differential survival, or differential breeding success for each sex. Regarding survival, one line of thinking is to argue that residency is more dangerous for females than for males, because overwintering blackbirds form foraging flocks and an individual’s access to food is related to its position within the flock hierarchy (Lundberg and Schwabl, 1983). Within these flocks, females are subordinate to males (Lundberg and Schwabl, 1983; Lundberg, 1985). Therefore, females would be predicted to suffer higher mortality if they remain as residents during winter, when food is limited, than if they migrate. However, our data do not align perfectly with this interpretation: if overwinter survival during residency was a strong factor driving sex differences in migratory strategy, we ought to have seen lower winter survival in resident females than in males, but this was not the case. The other possible explanation relies on differential breeding success between sexes. It is conceivable that resident males enjoy priority access to prime territories as soon as the breeding season starts. However, it should always be remembered that females, too, may benefit from better territories, thus an early presence may be beneficial for them as well (Creighton, 2001; Kokko et al., 2006; Kokko, 2011; Snow, 1956). It would be important to understand exactly how territory acquisition differs between males and females, especially because earlier data from the same geographical area have shown that reproductive success of migrant and resident blackbirds is sex-dependent (Schwabl, 1983) such that male residents have higher reproductive success than male migrants, while female residents and migrants have similar reproductive success. Understanding the mechanisms of territory acquisition could help explain why fewer males migrate: if frequency-dependency penalizes late-arriving males whereas late breeding females are not severely penalized, then the same magnitude of survival differences will favor a larger migratory population within females than within males.

In our study, we excluded 11 birds that migrated during the winter and 11 that switched strategies between years, as we considered these sample sizes to be too small for detailed inferences. Departures during the winter usually occurred during periods of cold temperatures and snow accumulation (Fudickar et al., 2013). Extreme weather conditions and low food availability might trigger these movements during winter. Future, more extensive studies could conceivably determine lifetime fitness of these strategies. By examining the fitness benefits conferred by migration, our study is able to provide strong support for the hypothesis that migration confers winter survival benefits.

Our methodology, which allows comparisons between classes that differ greatly in detectability, can hopefully also shed light on systems where benefits and risks of migration are shared by all individuals of a population, many of which are threatened by risks along their migratory flyways (Wilcove and Wikelski, 2008). Further understanding of how, where and why migratory animals die will illuminate the path to direct conservation efforts to protect migratory species.

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Author details

1. Daniel Zúñiga

   1. Max Planck Institute for Ornithology, Radolfzell, Germany
   2. Department of Biology, University of Konstanz, Konstanz, Germany

   Contribution

   Conceptualization, Data curation, Formal analysis, Methodology, Writing—original draft, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-7198-7242

2. Yann Gager

   1. Max Planck Institute for Ornithology, Radolfzell, Germany
   2. Department of Biology, University of Konstanz, Konstanz, Germany

   Contribution

   Formal analysis, Writing—review and editing

   Competing interests

   No competing interests declared

3. Hanna Kokko

   Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland

   Contribution

   Formal analysis, Writing—review and editing, Conceptualization of the model to estimate breeding success

   Competing interests

   No competing interests declared

4. Adam Michael Fudickar

   1. Max Planck Institute for Ornithology, Radolfzell, Germany
   2. Department of Biology, University of Konstanz, Konstanz, Germany
   3. Environmental Resilience Institute, Indiana University, Bloomington, United States

   Contribution

   Conceptualization, Data curation, Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

5. Andreas Schmidt

   Max Planck Institute for Ornithology, Radolfzell, Germany

   Contribution

   Methodology, Capture and tagging of birds. Collection of a substantial amount of the presence absence radio telemetry data. Management of the automatic radio telemetry system

   Competing interests

   No competing interests declared

6. Beat Naef-Daenzer

   Swiss Ornithological Institute, Sempach, Switzerland

   Contribution

   Methodology, Writing—review and editing, developement of radio transmitters used in 2013

   Competing interests

   No competing interests declared

7. Martin Wikelski

   1. Max Planck Institute for Ornithology, Radolfzell, Germany
   2. Department of Biology, University of Konstanz, Konstanz, Germany

   Contribution

   Supervision, Funding acquisition, Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

8. Jesko Partecke

   1. Max Planck Institute for Ornithology, Radolfzell, Germany
   2. Department of Biology, University of Konstanz, Konstanz, Germany

   Contribution

   Conceptualization, Supervision, Methodology, Project administration, Writing—review and editing

   For correspondence

   partecke@orn.mpg.de

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-9526-8514

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