Merging subspecies distribution: potential Swedish hybrid zone and unknown migratory patterns of common chiffchaff

Abstract

Hybrid zones represent crucial areas where two populations meet, mate, and produce hybrid offspring, serving as natural laboratories for studying genetic mechanisms underlying ecological traits and reproductive isolation. In Sweden, the common chiffchaff (Phylloscopus collybita) is represented by two subspecies: P. c. abietinus, which colonised northern Sweden through Finland at least 200 years ago, and P. c. collybita, which colonised southern Sweden via Denmark in the 1970s. These subspecies exhibit differences in morphology, migratory patterns, and mitochondrial DNA. This study focuses on a distribution gap in south-central Sweden, where the two subspecies are expected to come into contact. My aim is to assess the composition of birds within this gap by comparing the northern abietinus population with the southern collybita population, by examining differences in migratory phenotype, morphology, and genetics between the two populations. This study reveals distinct migratory routes, with abietinus birds migrating SSE and farther south to East Africa compared to collybita birds wintering in southwestern Europe. Morphologically, abietinus birds reveal to be larger with longer wings, and exhibit whiter underparts and warmer upperparts compared to collybita birds. Genetic analyses show no fixed differences between the subspecies, but a nuclear SNP located on the Z-chromosome, CC34-1, seems promising for subspecies identification. Birds in the distribution gap exhibit intermediate morphology and genetics, although they resemble collybita birds more. They, however, exhibit a higher frequency of the abietinus allele, suggesting gene flow from the north. While it is too early to refer to south-central Sweden as a hybrid zone, these observations indicate the northward expansion of southern birds. In anticipation of the eventual contact between the collybita and abietinus ranges, further studies are required to fully characterize the emerging hybrid zone. In seeking to identify south-central Sweden as a hybrid zone, I have explored the chiffchaff populations on the opposite side of the Baltic Sea, focusing on another contact zone proposed to be located in central Poland. Morphological and genetic analyses of this region revealed a predominance of birds resembling the abietinus genotype and phenotype. This suggests the possibility that the contact zone may be situated farther west, potentially in Germany. However, further sampling across western regions is necessary to accurately locate the hybrid zone.

Popular Abstract

Long time no see! Unravelling chiffchaff subspecies dynamics in Sweden

Hybrid zones are intriguing regions where distinct populations come together, mate, and produce hybrid offspring. These areas serve as natural laboratories, shedding light on how genetics influence traits related to the environment and reproductive isolation. The common chiffchaff (Phylloscopus collybita), a widely distributed insect-eating songbird in the western Palaearctic, primarily inhabits forests and woodlands in boreal and temperate regions. Historically, only the subspecies P. c. abietinus populated Sweden, ranging from northern Lapland to 60°N, likely arriving via Finland post-glaciation and establishing itself over 200 years ago. Conversely, P. c. collybita colonized southern Sweden from Denmark in the 1970s and has since become prevalent in the region. These subspecies exhibit distinct characteristics, with abietinus birds typically having longer wings and greater body mass than their collybita counterparts. Despite estimated wintering grounds based on ringing recoveries, precise locations remain unconfirmed.

In the early 2000s, a north-southerly area spanning nearly 500 kilometres in south-central Sweden remained largely uninhabited by chiffchaffs, despite the expectation of a northward spread and colonization by P. c. collybita. In this study, my objective was to assess the subspecies composition within this distribution gap in south-central Sweden —whether it involves the northward expansion of southern birds, the southward expansion of northern birds, or the emergence of hybrids — by comparing the northern abietinus population with the southern collybita population. Migratory patterns of these birds were tracked using data from ringing recoveries and geolocators. Morphological characteristics of the populations were investigated through biometric measurements collected between 1998 and 2023, and plumage coloration differences were analysed using digital imaging techniques. Genetic differences were explored by examining single nucleotide polymorphisms (SNPs), representing variation at a single position in a DNA sequence, between populations, utilizing blood samples collected between 1998 and 2023.

The results revealed that collybita birds migrate southwest to winter in southwestern Europe, while abietinus birds head south-southeast to winter in East Africa, which is much farther south than expected. Indeed, based on ringing recoveries, abietinus birds were thought to winter in the eastern Mediterranean region. Morphologically, abietinus birds exhibited larger size and longer wings compared to collybita birds. Digital imaging analyses revealed subtle colour differences between the two subspecies, although these distinctions were not readily apparent to the naked eye. Genetic analyses uncovered differences in allele frequencies between the two subspecies, potentially facilitating subspecies identification.
Although it is premature to designate south-central Sweden as a hybrid zone, the observation of the northward expansion of southern birds is notable. Further investigations are necessary to fully understand the dynamics of this potential hybrid zone. Exploring a second contact zone in central Poland, considering the species' colonization history, yielded additional insights, suggesting that the contact zone south of the Baltic Sea may extend farther west than previously assumed. However, more research is essential to accurately pinpoint and characterize this contact zone.





Michaëla BERDOUGO
M.Sc. in Animal Ecology – 60 credits
Department of Ecology, Lund University
[e-mail]
Supervisor: Staffan Bensch (Lund University)
[e-mail]

• author: Berdougo, Michaëla
• supervisor: Staffan Bensch
• organization: Degree Projects in Biology
• course: BION03 20222
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9150552
• date added to LUP: 2024-04-04 16:13:35
• date last changed: 2024-04-04 16:13:35