The mysterious attractions of black berries

Thursday's Guardian Country Diary focuses on bird cherry Prunus padus, which has produced an exceptionally fine crop of its beautiful black fruits in Weardale this year. This is probably the result of excellent flowering conditions in late spring, fewer small ermine moths that often defoliate the trees and fewer instances of pocket plum disease which can destroy the fruits before they ripen.

Most wild fruits that are dispersed by birds are red (hawthorn, rowan, guelder rose and honeysuckle, to name but a few), so how come bird cherries (and also blackberries, buckthorn, deadly nightshade and elder berries) are black? Is fruit colour of any great significance for fruit-eating birds?

Over the last few decades there have been several scientific investigations into the ways in which fruit colours attract birds, addressing the question  'why are some ripe fruits red and others black?'

So here's a selective summary of some findings to date:

1983.
A study in the US using black cherry Prunus serotina concludes that it's not the final black colour of the ripe fruit that's important in attracting birds, but the mixture of intermediate red-coloured ripening fruits and black fruits that attract birds' attention. They observe the same results with pokeweed Phytolacca americana, whose fruits change from green to pink to black, and the conclusion is that the bicoloured phase of fruit presentation is important, rather than the final ripe colour. 

1999.
Unlike humans, birds can perceive ultra-violet light so the colours that they see don't correspond to our perception. Blue, violet and black berries reflect ultra-violet light, which may make them more conspicuous to birds and indicate their stage of ripeness. The bloom on the surface of bilberries (which is caused by a surface layer of yeast) reflects UV. When redwings were allowed to choose between bilberries with the bloom on the surface and others where the bloom had been polished away adult redwings preferred berries with the UV reflecting bloom, but inexperienced juveniles didn't distinguish between the two, suggesting that the birds learn to associate UV reflectance with fruit ripeness. 

2003
Another study revisits the bicoloured fruit hypothesis from 1983 and finds evidence to support it. 

2004.
Another study with juvenile redwings, which initially prefer red fruits but learn to associate black/UV reflective fruits with palatability, suggests that learning plays a key role in fruit colour choice. The study suggests that the colour of the fruits, not the contrast between their colour and the background, is particularly most important. 

  
2004
Another study that examines the relative significance of fruit colour and the contrast between fruit colour and background colour concludes that red and black fruits contrast more strongly with background foliage colours than any other fruit colours, which may be why they predominate. Tests on four bird species indicate that it's conspicuousness relative to background colours, not fruit colour itself, that's most important to foraging fruit-eating birds.

2006
Tests with crows in a flight cage conclude that artificial  red fruits are more conspicuous to them from a distance than artificial black fruits. When the crows were offered blueberries with or without their UV reflecting bloom, they were more successful in finding the UV reflecting ones - but that also depended on how UV-reflecting the background was.

2009.
A study concludes that blackcaps preferentially eat black or UV-reflecting fruits, choosing those with the highest intensity of colour, and that these darker fruits have high purple anthocyanin pigment antioxidant levels (good for their health) and also higher energy content. 

2009
A  study shows that birds can preferentially select ripe elder berries that have red stalks (indicative of high anthocyanin content) or ripe elder berries with green fruit stalks (that make the fruits more conspicuous and indicate a higher sugar content) - all suggesting that birds don't just gobble fruit down, they can be picky eaters and make dietary choices based on fruit quality.

2011
Studies using captive blackbirds and redwings that are offered artificial fruits show that blackbirds preferentially choose red fruits whereas redwings prefer black ones, but that these preferences can change depending on previous experience - they tended to prefer fruits of a colour that they had previously fed on, so learning was important.

2011
A review of the literature on the subject concludes (a) there is evidence that fruit colour is an important signal that birds react to; (b) that black fruits high in antioxidants (purple anthocyanin pigments), which tend to be formed in the fruits in bright light under cold conditions, enhance the immune systems of blackcaps; (c) that high levels of antioxidants reduce the fruits' proneness to fungal attack, benefiting the plant and suggesting that black fruits might have originally evolved for this reason, rather than to attract birds. 


So, back to the original question :'why are some ripe fruits red and others black?'. In summary, it's all to do with - 

- the fact that these are the two colours that birds find most attractive

- that the mixture of reds and blacks in a branch of ripening fruit (think of blackberries ripening from pink to black) may be important in attracting birds' attention

-  UV reflectance (that we can't see) is very important especially in fruits like blueberry, bilberry and sloe that have a UV-reflecting surface 'bloom'.

- that the contrast between the fruit colour and background colour is important

- bird species may have innate preferences but these can change when they learn to associate colour with fruit quality

- some birds can select fruit, on the basis of colour signals, that has higher nutritional value

. black colour, with high antioxidant anthocyanin pigment content, might have evolved in plants as a defence mechanism against fungal disease rather than to attract birds.

As is so often the case, there are no easy answers; scientific research rarely delivers simple explanations but always produces a whole raft of more interesting, more sophisticated questions. 

Which is why science is so addictive - the natural world is always far more complex than anyone can possibly imagine.

As for the birds in my garden:

- the blackbirds and thrushes never touch yellow-berried holly but eat all the red berried holly as soon as it ripens.

- waxwings and bullfinches only eat the yellow fruit of 'Golden hornet' crab apple when it goes brown and begins to rot. If you want to attract birds to a garden in autumn, don't plant yellow-fruited trees and shrubs

- blackbirds go mad for the blue, bloom-covered UV reflecting, high anthocyanin containing berries of red flowering currant and Berberis - there's purple-stained bird poo all over our garden right now!


Click here for two more long-standing botanical mysteries

Sources:

Willson,M.F. and Melampy, M.N. (1983) The effect of bicolored fruit displays on fruit removal by avian frugivores. Oikos 41: 27-31.

Siitari, H; Honkavaara, J; Viitala, J (1999). Ultraviolet reflection of berries attracts foraging birds. A laboratory study with redwings (Turdus iliacus) and bilberries (Vaccinium myrtillus). Proc.Roy. Soc (B) 266,2125-2129

Jennifer M. Cramer, Maria L. Cloud, Nathan C. Muchhala, Anastasia E. Ware, Brent H. Smith and G. Bruce Williamson (2003) A test of the bicolored fruit display hypothesis: Berry removal with artificial fruit flags. Journal of the Torrey Botanical Society 130, 30-33

Honkavaara, J; Siitari, H; Viitala, J (2004) Fruit colour preferences of redwings (Turdus iliacus): Experiments with hand-raised juveniles and wild-caught adults. Ethology, 110, 445-457

Schmidt, V; Schaefer, HM; Winkler, H (2004) Conspicuousness, not colour as foraging cue in plant-animal signalling. Oikos 106, 551-557

Schaefer, H. Martin; Levey, Douglas J.; Schaefer, Veronika; et al. (2006) The role of chromatic and achromatic signals for fruit detection by birds. Behavioural Ecology 17,784-789

Schaefer, H. M.; McGraw, K.; Catoni, C. (2008) Birds use fruit colour as honest signal of dietary antioxidant rewards. Functional Ecology 2, 303-310


Schaefer, H.M. and Braun, J . (2009)  Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra). Ecology 90, 1564-1573.


Larrinaga, Asier (2011) Inter-specific and intra-specific variability in fruit color preference in two species of Turdus. Integrative Zoology 6, 244-258


Schaefer, H.M. (2011) Why fruits go to the dark side. Acta Oecologica 37, 604-610.