Extrinsic and intrinsic factors influence fitness in an avian hybrid zone

Abstract

The effects of hybridization on evolutionary processes are primarily determined by the differential between hybrid and parental species fitness. Assessing the impacts of hybridization can be challenging, however, as determining the relationship between individual fitness and the extent of introgression in wild populations is difficult. We evaluated the fitness consequences of hybridization for pure and hybrid females in a hybrid zone between two tidal marsh birds, the saltmarsh sparrow (Ammodramus caudacutus), a salt marsh obligate, and Nelson's sparrow (A. nelsoni), which has a broader ecological niche and a much younger evolutionary association with salt marshes. Biotic stressors associated with nesting in tidal environments suggest an important role for differential adaptation in shaping hybrid zone dynamics, with saltmarsh sparrows predicted to be better adapted to nesting in salt marshes. We collected DNA samples from adults (n = 394) and nestlings (n = 431) to determine the extent of introgression using 12 microsatellite loci and tested for the influence of extrinsic (nest placement) and intrinsic (genotype) factors on female reproductive success. We monitored nests (n = 228), collected data on reproductive output, and estimated daily nest survival rates using female genotype and nest characteristics as covariates. To test for reduced survival of hybrid females, we also used capture data to assess the distribution of admixed male and female individuals across age classes. Reproductive success of females varied by genotypic class, but hybrids did not have intermediate success as predicted. Instead, we found that pure Nelson's sparrows had, on average, 33% lower hatching success than any other genotype, whereas F1/F2 hybrids, backcrossed Nelson's sparrows, and backcrossed and pure saltmarsh sparrows all had similar hatching success. We found no effect of genotype or nest placement on daily nest survival probabilities. However, hybrid individuals with a higher proportion of saltmarsh sparrow alleles exhibit nesting behaviours better suited to nesting successfully in tidal marshes. Further, while the proportion of F1/F2 individuals was similar between nestling and adult males, we found that the proportion of F1/F2 individuals was 2.3 times greater in nestling females compared with adult females, indicating reduced survival of F1 females. We conclude that differences in reproductive success among pure and admixed individuals coupled with intrinsic mechanisms (reduced survival in F1 females) shape hybrid zone dynamics in this system.

Bidirectional adaptive introgression between two ecologically divergent sparrow species

“…Additional field data are required to fully evaluate this hypothesis; however, saltmarsh sparrow nestlings may exhibit a faster growth rate (muscle development) in salt marshes, a trait that is being transferred to admixed individuals compared to slower muscle development in Nelson's sparrows. Consistent with this expectation, previous work in this system has identified higher reproductive success in F1 females compared to Nelson's sparrow females (Walsh et al 2016c), supporting a role for adaptive introgression in bolstering reproductive success in this system.…”

“…Consistent with this expectation, previous work in this system has identified higher reproductive success in F1 females compared to Nelson's sparrow females (Walsh et al. ), supporting a role for adaptive introgression in bolstering reproductive success in this system.…”

Plasticity in nesting adaptations of a tidal marsh endemic bird

“…Females that experienced nest flooding also had a slightly lower proportion of high marsh vegetation in their subsequent nests, and successful nests had ~5% lower high marsh composition than flooded nests, although these trends were not statistically significant. High marsh vegetation is relatively simple in structure; nests constructed with a mixture of S. patens and S. alterniflora may have greater structural support, may have better withstand flooding, and may be more able to retain overall nest shape during and following flooding events than nests comprised of primarily S. patens (Walsh et al., ). The effect of vegetation composition, however, must be balanced with surface elevational differences in high vs. low marsh, which likely explains the slight differences that were observed (~6% change in vegetation composition, co‐occurring with a slight increase in surface elevation).…”

“…Tidal height has strong effects on flooding probability, and successful nests withstand higher tide heights than those that fail due to flooding (Bayard & Elphick, 2011). It is intuitive that nests that are placed higher in the vegetation will be less impacted by tidal flooding than those placed lower, and damage from multiple flooding events may have cumulative impacts (Bayard & Elphick, 2011;Walsh et al, 2016). The height of the nest in relation to tidal height, however, is build nests in areas of higher elevation compared to random locations on the marsh.…”

Quantifying the importance of geographic replication and representativeness when estimating demographic rates, using a coastal species as a case study

“…Our models also do not predict lower growth rates for the hybrid zone compared to the core of the range, even though our survival estimates include some hybrid individuals at those sites. Previous research suggests that survival for hybrid females may be lower (Walsh et al 2016), but if this mortality happens before these individuals are caught as adults, they would not be represented in our dataset. Our results are qualitatively similar to existing studies of demographic rates across a species range, although there are few such studies, making comparison with similar taxa difficult.…”