Fluff-thieving birds sabotage seed dispersal

Abstract

Characterizing many species interactions as mutualisms can be misleading because some members of the interaction derive greater fitness benefits at the expense of other members. We provide detailed natural history data on a suspected bird-plant mutualism in South Africa where many species of birds use fluffy Eriocephalus seed material to construct their nests, potentially dispersing seeds for the plant. We focus on a common bird, Prinia maculosa, which invests heavily in gathering Eriocephalus material. Prinias spent 5 of their median 6-day nest construction period adding seed material to their nests and frequently travelled outside their territory boundary to gather Eriocephalus material. Yet, prinias gathered primarily Eriocephalus fluff and actively avoided gathering seeds. The average prinia nest contained only 6.6 seeds, but contained fluff from 579 seeds. These data suggest that prinias provide limited dispersal benefits to Eriocephalus plants. By contrast, the large amounts of Eriocephalus fluff in prinia nests, and the effort that prinias invest in gathering it, suggest that prinias benefit from constructing their nests with Eriocephalus material. We end by outlining hypotheses for possible fitness benefits that Eriocephalus material could provide prinias and other birds.

Keywords: Eriocephalus; Karoo prinia; mutualism; parasite; seed dispersal.

Figure 1.

Eriocephalus material in Karoo prinia nests. Karoo prinia nests constructed without (a) and with an abundance (b) of Eriocephalus material. Eriocephalus material may help conceal nests—photo (c) is taken from the ground looking up at the base of a Karoo prinia nest (nest is in centre of the photo). (d) Eriocephalus bush. (e) Eriocephalus seed head with fluff (left), half of fluff missing (plucked by authors) (middle), and fluff removed from seed head (right). (f) Five Karoo prinia nests cut in half (nest entrance is in the upper right in each photo), with a range of Eriocephalus material on the nest interior. The left four nests all have Eriocephalus material; the right most nest has none and is lined primarily with Trichocephalus stipularis fluff and Helichrysum spp. leaves (all photographs: V.G.R.).

Figure 2.

Development of Eriocephalus seed material and the prinia breeding season. (a) Progression of Eriocephalus seed material for three patches of plants at the Koeberg Nature Reserve; y-axis ranks plant development from least (1) to most (8) developed (see electronic supplementary material, table S1, for descriptions of phenological categories). Grey dotted line represents the earliest phenological stage of plants from which we observed prinias gathering fluff. Numbers above boxplots represent the number of plants surveyed, and boxplots show medians (thick lines), 25th and 75th percentiles (boxes), 1.5 times the interquartile range (whiskers), and outliers (points outside 1.5 times the interquartile range). (b) Time series plot of Karoo prinia first egg dates using 139 nests found during the 2013 breeding season at the Koeberg Nature Reserve.

Figure 3.

Distance travelled and time invested in gathering Eriocephalus material. (a) Summary of 83 observations from 33 different Karoo prinia nests showing the distance prinias travelled to gather Eriocephalus fluff regressed on the distance to the closest Eriocephalus plant that was in the same phenological stage as the plant from which the focal bird gathered material; multiple observations for a single nest are shown with standard deviations. Dotted lines at 83.5 m illustrate average diameter of a prinia territory. (b) Boxplot of the distance to the closest Eriocephalus bush for 100 prinia nests that contained Eriocephalus material. (c) Picture of Karoo prinia gathering Eriocephalus fluff from seeds held between toes. (d) Number of days that prinias allocated to nest building and lining the nests with Eriocephalus material. (e) The number of picks that prinias made during a single bout of material gathering regressed on the time spent perched in Eriocephalus bushes while gathering fluff.

Figure 4.

Comparison of Eriocephalus seed heads in bird nests from Koeberg Nature Reserve. (a) Nests of Karoo prinias (Prinia maculosa) contained fewer seeds than nests of other bird species (Wilcoxon rank sum test, p < 0.0001). (b) The number of Eriocephalus seed heads in the nests of the seven bird species grouped in the category ‘other’ in part (a) of this figure.