The interesting case of blue tits in France

I’ve recently been reading about a several decade long study centred around blue tits in France and on the island of Corsica. The work is being carried out by a group in the evolutionary ecology unit at the Centre D’Ecologie Fonctionnelle & Evolutive and it crosses over all kinds of interesting areas of ecology and evolution.

The mainland study area is dominated by deciduous habitat with some patches of evergreen, whereas the Corsican landscape is the reverse with mostly evergreen habitat and a few patches of deciduous. On the mainland, blue tits in the rare evergreen habitats display a phenotype intermediate between that optimal for deciduous and evergreen patches (Blondel et al., 1993). Compared to blue tits in evergreen Corsican patches, the blue tits in evergreen mainland patches lay too early to match the evergreen caterpillar phenology and have clutch sizes that are larger than their Corsican counterparts.

The mismatch between blue tit phenotype and the evergreen habitat was attributed to gene flow from the dominant deciduous habitats, modified somewhat by phenotypic plasticity. The situation was attributed to source-sink dynamics between the deciduous and evergreen patches, leading to maladaptation in the less common habitats (Dias and Blondel, 1996). The presence of a source-sink dynamic was confirmed with genetic analysis (Dias et al., 1996).

Early papers e.g. Dias et al. (1996) state that there is a similar converse situation on Corsica:

Similar results are available from Corsica where tits in deciduous habitat patches start to lay later than would be expected based on the leafing process of the trees (Lambrechts and Dias, 1993; Dias and Blondel, 1996) so that they also miss the local peak of food abundance

The Dias and Blondel (1996) result is in Table 1, which also shows reduced breeding success in deciduous habitat on island.

(They identified the study location as an isolated valley near Calvi (Fango valley), and described the deciduous patch as being 12 km away from that. It’s not clear to me exactly where the study site was and how it corresponds to the study sites in their later studies. The Fango valley is actually closer to Galeria than Calvi, suggesting that the study sites are those later denoted CD1 and CE1 ”Pirio”. The Blondel et al. (2001) is the best paper for identifying their study sites, as it has a map as well as a figure comparing the lay date and clutch size phenotypes at each location).

In later work on the Coriscan system, the research group emphasised the curious difference between two sites named Muro and Pirio (Lambrechts et al., 1997, Blondel et al., 1999). The former is a deciduous habitat on Corsica, and the latter an evergreen habitat. They should be within the dispersal range of the blue tit species, yet each group is well-adapted to the patch type that they are in. Further, individuals from the two sites have quite different reaction norms, which is only possible under reduced gene flow. Earlier work proposed that gene flow between the two sites might be reduced by mountain barriers (Lambrechts et al., 1997), however many papers identify ‘insular syndrome’ as a likely major determinant. (Blondel et al., 1999).

Insular syndrome refers to a hypothesis by Diamond (1981) that reduced dispersal, a fear of flying over water, and flightlessness are more likely to evolve in island birds. Blondel and colleagues believe that the blue tits on Corsica have insular syndrome and thus lower dispersal. As far as I can tell, the lower dispersal has not been demonstrated directly, but the Corsican tits due demonstrate some of the characteristics typical of insular syndrome. The reasons why they might have insular syndrome are best explained in Blondel (2000), where things like change in body size, population density, clutch size, territory size, etc. (see Table 1) associated with insular syndrome are also observed for Corsican tits. A much earlier study, Blondel et al. (1988), is also cited as evidence for the insular syndrome in blue tits. Interestingly the group found low genetic differentiation between Muro and Pirio quite early on, suggesting a divergence within gene-flow situation. Furthermore it is possible that the genetic markers that were used were not the markers that were diverging (Blondel et al., 2001). Thus the insular syndrome continued to be favoured as a major determinant, perhaps in in conjunction with mechanisms that might reinforce differences such as assortative mating (Blondel, 2007).

One interesting phenotypic difference between Corsican tits is that individuals from deciduous habitats tend to have a larger body size Blondel et al. (1999). Blue tits can remain in the breeding grounds over winter where they join winter flocks. These flocks have a dominance hierarchy such that individuals at the top of the hierarchy occupy safer domiciles and have priority when feeding (Hegner, 1985). (Braillet et al., 2002) investigated the role of social dominance and found that the larger individuals from the deciduous patches were socially dominant over those from evergreen patches. (It is noted that the methodology used in this study controlled for prior occupancy.) This suggests a social-structural reason for why blue tits from the dominant evergreen habitats have not successfully invaded Pirio and genetically swamped local adaptation there.

A recent work from the group analysed the landscape genetic structure on Corisica (Porlier et al., 2012). It was found that the genetic structure was due to habitat type rather than distance or mountain barriers. As possible causes they hypothesise “lower fitness of maladapted migrants, nonrandom gene flow due to habitat choice, and/or nonrandom mate choice”. The question of disentangling these effects is certainly an interesting one and I’m looking forward to seeing what they find.

Blondel, J. (2000). Evolution and ecology of birds on islands: trends and prospects, Vie et Milieu 50(4): 205–220.

Blondel, J. (2007). Coping with habitat heterogeneity: the story of Mediterranean blue tits, Journal of Ornithology 148(1): 3–15.

Blondel, J., Chessel, D. and Frochot, B. (1988). Bird species impoverishment, niche expansion, and density inflation in Mediterranean island habitats, Ecology 69(6): 1899–1917.

Blondel, J., Dias, P. C., Maistre, M. and Perret, P. (1993). Habitat heterogeneity and life-history variation of mediterranean blue tits (Parus caeruleus), The Auk pp. 511–520.

Blondel, J., Dias, P. C., Perret, P., Maistre, M. and Lambrechts, M. M. (1999). Selection-based biodiversity at a small spatial scale in a low-dispersing insular bird, Science 285(5432): 1399-1402.

Blondel, J., Perret, P., Dias, P. C. and Lambrechts, M. M. (2001). Is phenotypic variation of blue tits (Parus caeruleus l.) in Mediterranean mainland and insular landscapes adaptive?, Genetics selection evolution 33: S121–S139.

Braillet, C., Charmantier, A., Archaux, F., Dos Santos, A., Perret, P. and Lambrechts, M. M. (2002). Two blue tit Parus caeruleus populations from Corsica differ in social dominance, Journal of Avian Biology 33(4): 446–450.

Diamond, J. M. (1981). Flightlessness and fear of flying in island species, Nature 293: 507–508.

Dias, P. C. and Blondel, J. (1996). Local specialization and maladaptation in the Mediterranean blue tit (Parus caeruleus), Oecologia 107(1): 79–86.

Dias, P., Verheyen, G. and Raymond, M. (1996). Source-sink populations in Mediterranean blue tits: evidence using single-locus minisatellite probes, Journal of Evolutionary Biology 9(6): 965–978.

Hegner, R. E. (1985). Dominance and anti-predator behaviour in blue tits (Parus caeruleus), Animal Behaviour 33(3): 762–768.

Lambrechts, M. M., Blondel, J., Hurtrez-Bousses, S., Maistre, M. and Perret, P. (1997). Adaptive inter-population differences in blue tit life-history traits on Corsica, Evolutionary Ecology 11(5): 599–612.

Porlier, M., Garant, D., Perret, P. and Charmantier, A. (2012). Habitat-linked population genetic differentiation in the blue tit Cyanistes caeruleus, Journal of Heredity 103(6): 781–791.

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