Figure 1.
Study populations, phenotypic variation of brambling and chaffinch eggs, and geographical distribution of museum clutches.
A: range of egg colors of chaffinch (rows 1-3) and brambling clutches (rows 4-6). Each egg comes from a different clutch. The bottom row gives examples of cuckoo eggs found in brambling nests in Kittilä. B: location of study populations. C: geographic origin of museum clutches. The points are slightly jittered for plotting purpose. The maps were drawn using the maps package for R [88].
Figure 2.
Bar plot showing the relationship between rejection rate and color contrast (ΔTC) between host and parasitic eggs. Superimposed is the host discrimination function obtained from a univariate logistic regression on the probability of rejection (95% confidence interval indicated by dotted lines). For plotting purposes the width of the bars for some groups were adjusted to obtain similar sample sizes across groups. Sample sizes for each group depicted above the bars.
Table 1.
Generalized linear models of the probability of accepting congeneric parasitic eggs in chaffinches and bramblings.
Table 2.
Summary statistics describing different aspects of the clutch color distributions of the brambling and chaffinch.
Figure 3.
Colors of cuckoo eggs in relation to predicted optimum for parasitism of bramblings.
Black triangles (A–D) and black bars (E) indicate the mean color contrast of cuckoo eggs resulting from comparison with all brambling clutches in a population. The leftmost vertical line in each figure indicates the lowest average color contrast that can be achieved, given the color distribution of the host's clutches. The rightmost vertical line in each figure indicates the highest average color contrast a cuckoo egg could have, and still be a perfect match to at least one of the host clutches in the population. Cuckoo eggs in A–D were all found in Kittilä (N = 5), whereas cuckoo eggs in e were measured in museums (N = 72). A: Kittilä (N = 88 host clutches). B: Ammarnäs (N = 97 host clutches). C: Tydal (N = 47 host clutches). D: Kittilä, Ammarnäs and Tydal pooled (N = 232 host clutches). E: museum clutches collected in the period 1841–1979 (N = 343 host clutches). F: a brambling clutch with three host eggs and one cuckoo egg (top).
Table 3.
Mantel and partial Mantel tests.
Figure 4.
Color (hue) distributions of brambling, chaffinch and cuckoo eggs.
(See main text for explanation of Robinson projections). A: the entire projected tetrachromatic hue-space for reference. Red triangles indicate the projections of the ultraviolet (uv), short (s), medium (m) and long wavelength (l) vertices of the tetrahedron. Solid lines indicate the projections of the four edges connecting the different vertices. B–C: the distribution of hues for 232 brambling (pooled clutches from Kittilä, Ammarnäs and Tydal), and 157 chaffinch clutches (Stjørdal), respectively. D–E: the distribution of hues for 343 brambling (Fennoscandia) and 625 chaffinch clutches (Great Britain and Fennoscandia) held in various museum collections. F: the distribution of hues for 72 museum cuckoo eggs classified as Fringilla egg morphs (blue color) and five fresh cuckoo eggs found in brambling nests in Kittilä in the period 2005–2008 (red color). For plotting purposes only, three clutches in E located close to the s-vertice (i.e. bluish hues) were slightly transformed in order to make them appear within the plotted frame. Note the overlap between the hue-spaces occupied by the two hosts (B–E). The circles in B and D have different colors to illustrate the direction of selection on hue under the assumption that cuckoo egg color distributions are given by the five and 72 cuckoo eggs in Figure F, respectively. Different colors indicate differences in the average color distance (ΔTC) between host and cuckoo eggs. Parasitism clearly imposes disruptive selection, and an eventual evolutionary response is therefore predicted to change the distribution towards stronger bimodality, whereupon it would converge with the chaffinch's hue distribution (C, E).
Figure 5.
Forms of selection imposed on cuckoo egg colors by different host egg color distributions.
Forms of selection on the longitudinal (θ) hue component of hypothetical cuckoo eggs imposed by brambling (A) and chaffinch (B–F) egg distributions. The curves are drawn by cubic spline interpolation and are based on 3000 egg colors that were drawn from within the tetrachromatic color space of each of the hosts to obtain a uniform distribution of θ-values. The mean acceptance probability of each cuckoo egg is based on matching with 232 brambling clutches (A), 157 chaffinch clutches (B), and various subsets of the 157 chaffinch clutches (C–F). Insets show the hue distribution (Robinson projection) of the host clutches in each case. C–F indicates the effect of an increase in the bimodality or skew of the chaffinch's hue distribution. C: chaffinch clutches with θ-values between 30th and 50th percentiles excluded. D: clutches with 0-values between 40th and 60th percentiles excluded. E: clutches with -values between 50th and 70th percentiles excluded. F: clutches with θ-values between 30th and 70th percentiles excluded.