Neue Aspekte zur zugzeitlichen Frugivorie der Gartengrasmücke (Sylvia borin)

Birds see the true colours of fruits to live off the fat of the land

Communication is a characteristic of life, but its reliability and basic definition are hotly debated. Theory predicts that trade among mutualists requires high reliability. Here, we show that moderate reliability already allows mutualists to optimize their rewards. The colours of Mediterranean fleshy-fruits indicate lipid rewards (but not other nutrients) to avian seed dispersers on regional and local scales. On the regional scale, fruits with high lipid content were significantly darker and less chromatic than congeners with lower lipid content. On the local scale, two warbler species (Sylvia atricapilla and Sylvia borin) selected fruit colours that were less chromatic, and thereby maximized their intake of lipids-a critical resource during migration and wintering. Crucially, birds were able to maximize lipid rewards with moderate reliability from visual fruit colours (r(2) = 0.44-0.60). We suggest that mutualisms require only that any association between the quality and sensory aspects of signallers is learned through multiple, repeated interactions. Because these conditions are often fulfilled, also in social communication systems, we contend that selection on reliability is less intense than hitherto assumed. This may contribute to explaining the extraordinary diversity of signals, including that of plant reproductive displays.

Limits of selection against cheaters: birds prioritise visual fruit advertisement over taste

The concept of biological markets aims to explain how organisms interact with each other. Market theory predicts that organisms choose the most rewarding partner in mutualisms. However, partner choice may also be influenced by advertisement which may not be reliable. In seed dispersal mutualism, we analysed whether seed dispersers prioritise taste cues over visual advertisement to select the most rewarding fruits and whether they select against partners with unreliable advertisement. We conducted experiments on black elder (Sambucus nigra), a species of which the colours of the peduncles match the sugar content of their fruits. We created infructescences the colours of which matched or mismatched the sugar content of their fruits. There was no selection against cheaters in the field or by captive blackcaps (Sylvia atricapilla) as seed dispersers. Blackcaps were constrained to select against unreliable advertisement because they swallowed fruits entirely and thus did not obtain an immediate feedback by taste. Instead, blackcaps selected fruits according to the colour variation of red peduncles. Overall, we suggest that the concept of constraints should be incorporated into biological markets. We further contend that biological markets can be more complex than currently acknowledged because a moderate degree of reliability occurred in black elder even in the absence of selection against cheaters.

Characteristics of vertebrate-dispersed fruits in Hong Kong

Hong Kong has a native angiosperm flora of approximately 1800 species, of which 27% (482 spp.) bear fleshy, presumably vertebrate-dispersed fruits, including 76% of the 337 tree and shrub species and 70% of the 103 climber species. Morphological characteristics were determined for 255 species and nutritional characteristics of the fruit pulp for 153 species. Most fruit species were black (45.1%) or red (24.3%) and 85.9% had a mean diameter <13 mm. Nutritional characteristics varied widely between species with ranges and median values as follows: pulp percentage (range 10.0–99.2%, median 69.2%), water content of pulp (11.1–94.0%, 78%), lipid (0–84.0%, 2.0%), soluble carbohydrate (4–88%, 53%), nitrogen (0.2–3.4%, 0.86%), neutral detergent fibre (1–44%, 14.3%). Fruit development time (50–360 d, 156 d) showed a negative correlation with lipid content, but no significant correlation with fruit or seed size. Principal components analysis of fruit characteristics was dominated by a trend from single-seeded fruits with a thin, lipid-rich pulp layer to multiple-seeded fruits with much, watery, carbohydrate-rich pulp. Birddispersed species cover the full range of fruit characteristics except those too large to swallow and too hard to peck bits from. Mammals (bats, civets and/or macaques) are known or suspected to consume most of the fruits too large for birds as well as many bird fruits but none with high-lipid content. Summer fruits (May—September) were significantly larger and had significantly higher seed size and carbohydrate content than winter fruits (November-March). Winter fruits took more than twice as long to develop as summer fruits.