Fitness consequences of different migratory strategies in partially migratory populations: A multi‐taxa meta‐analysis

Abstract

Partial migration—wherein migratory and non‐migratory individuals exist within the same population—represents a behavioural dimorphism; for it to persist over time, both strategies should yield equal individual fitness. This balance may be maintained through trade‐offs where migrants gain survival benefits by avoiding unfavourable conditions, while residents gain breeding benefits from early access to resources. There has been little overarching quantitative analysis of the evidence for this fitness balance. As migrants—especially long‐distance migrants—may be particularly vulnerable to environmental change, it is possible that recent anthropogenic impacts could drive shifts in fitness balances within these populations. We tested these predictions using a multi‐taxa meta‐analysis. Of 2,939 reviewed studies, 23 contained suitable information for meta‐analysis, yielding 129 effect sizes. Of these, 73% (n = 94) reported higher resident fitness, 22% (n = 28) reported higher migrant fitness, and 5% (n = 7) reported equal fitness. Once weighted for precision, we found balanced fitness benefits across the entire dataset, but a consistently higher fitness of residents over migrants in birds and herpetofauna (the best‐sampled groups). Residency benefits were generally associated with survival, not breeding success, and increased with the number of years of data over which effect sizes were calculated, suggesting deviations from fitness parity are not due to sampling artefacts. A pervasive survival benefit to residency documented in recent literature could indicate that increased exposure to threats associated with anthropogenic change faced by migrating individuals may be shifting the relative fitness balance between strategies.

How migratory populations become resident

“…Interestingly, this evolutionary inertia due to this pattern of phenotypic expression has hitherto not been considered in phenotypic evolutionary models of partial migration [18]. These models generally assume that for partial migration to persist the fitness of migrants and residents should be approximately equal over time, which is at odds with empirical findings [32]. Our model demonstrates that even in the presence of large fitness differences, partially migratory populations may evolve rapidly and persist during a long period of time, as a consequence of the evolutionary dynamics of semi-continuous threshold traits.…”

“…The best territories, however, are occupied by residents, which have the advantage of prior residency [28,29]. Furthermore, a number of studies have shown that residents have higher fitness than (short-distance) migrants [30][31][32], and that birds with longer migration distance have lower fitness [33,34]. Based on these findings, we designed three continuous fitness functions: one where the decline in fitness with increasing migratory activity is exponential, one where it is linear and one that follows a logistic relationship (figure 1).…”

Strong survival selection on seasonal migration versus residence induced by extreme climatic events

“…Surprisingly few previous studies have tested for differential survival of migrants versus residents, or hence quantified survival selection, under any conditions and in any taxa (Buchan et al., 2019; Reid et al., 2018). Recent studies demonstrated higher survival in migrants than residents in blackbirds Turdus merula (Zúñiga et al., 2017) and roach Rutilus rutilus (Skov et al., 2013), but this was not evident in American dippers Cinclus mexicanus (Green et al., 2015), and residents had higher survival than migrants in elk Cervus canadensis (Hebblewhite & Merrill, 2011) and red‐spotted newts Notophtalmus viridescens (Grayson et al., 2011).…”

Carryover effects of long-distance avian migration are weaker than effects of breeding environment in a partially migratory bird

“…Partial migration, where migrant and non-migrant individuals exist within the same population 14 , 15 , provides a powerful natural experiment to explore these carryover effects, by comparing fitness parameters of migrants and residents 16 . Although parity in fitness under both strategies is necessary for the evolutionary maintenance of partial migration 14 (especially if migratory strategy is heritable 17 ), it is hypothesised that this balance is maintained through trade-offs where migratory costs that reduce fitness in one parameter (survival or breeding success) are compensated by higher fitness in another 14 , 18 .…”

“…Most studies of within-population partial migration in birds are in short-distance systems 16 . Here we examine carryover effects in an unusual example of long-distance partial migration (lesser kestrels, Falco naumanni ), where non-migratory individuals are fully resident in the Spanish breeding grounds throughout the year, while migrants undertake a c. 3000-km trans-Saharan migration, such that individuals may be exposed to very different costs between the two strategies.…”