Tuesday, 23 August 2016

Bird of Paradise Flowers

The striking flower arrangement shown above is by Jane Hass and features two blooms of Strelitzia reginae, commonly known as the Bird of Paradise Plant, or the Crane Flower. These names were given to the plants because they reminded some observers of the two birds, but only an avid Creationist would believe in such mimicry, or that they were devised for our visual pleasure. S. reginae is native to South Africa, where it grows on river banks and in woodland clearings, and it was so attractive to collectors that seeds have been exported widely and the plant grows well in warm climates. It is the official flower of the City of Los Angeles [1].

Each flower consists of bright orange sepals and darker petals and these emerge from a spathe (the pointy section that looks like the beak of the bird in our imagination). At the base of the petals is a nectary containing sugars that change in their composition over time [2], although we do not know the reason for this.

Nectar is produced by almost all flowering plants and provides an attractant for animals that use it as an energy source and then collect pollen that is transferred to another plant to ensure fertilisation. Most commonly, the pollinators are insects and the plant loses sugars (produced abundantly during photosynthesis), and some of the pollen (many bees collect this, for example), but the mutualism between the plant and the pollinating insects is clearly of benefit to both and is a very successful strategy. Insects are not the only pollinators, however, and S. reginae is fertilised by birds, with its pollen formed into threads and aggregates [3] to enhance attachment and transmission. The most common pollinator is the Cape Weaver (Ploceus capensis – see below), that feeds on the pollen aggregates as well as the nectar and transfers pollen on its feet while visiting another flower [4,5], with the rigid spathe providing an ideal initial landing place. 

Visits by weaver birds have described by Hoffman et al. [6]:

Landing of the bird on the blue sheath-forming petals exposes the hidden pollen to the feet of the bird, while the bird probes the corolla tube with its beak and extends its tongue to reach the nectar.. .. Once landed and feeding, these birds have been observed to seldom move their feet, thus keeping self-pollination low.. .. As a consequence, the best place for the bird to feed is also the best position for pollination.

Sunbirds (Cinnyris spp.) also visit S. reginae plants, but it is likely that they play little part in pollination and they are regarded as "nectar thieves" [5]. As Coombs and Peter note [7]:

The nectary of S. reginae is covered by the convoluted bases of the two fused petals forming a barrier to the opening of the corolla tube.. ..The behaviour of sunbirds indicates that they are nectar thieves and can manipulate the nectar barrier with their beaks to gain access to the nectar without causing obvious damage to the flowers of S. reginae.

Pollination in S. reginae is thus an example of the evolution of a strategy that depends on mutualism, with other species taking advantage of the "gifts" provided by the plant. 

So what happens when seeds are taken to other countries? An abundance of imported seeds ensures that plants can be grown without the need for fertilisation, but we now know that S. reginae is pollinated by indigenous birds. The common yellowthroat of southern North America (Geothlypis trichas – see below), a type of warbler, feeds on nectar and, just as with the weaver birds in Africa, picks up pollen on its feet. As Hoffman et al. point out [6] it is unlikely that "adaptive floral changes have started the association" in the very short time during which S. reginae have been grown in the USA, but the behaviour of the warblers allowed them to discover the nectaries and release pollen on to their feet and thus ensure fertilisation. All this mimicking an association that evolved over very long time periods in South Africa where the plant is endemic.

It is interesting to speculate on how yellowthroats developed this behaviour. The bright sepals of S. reginae may be an attractant to many animals, including the insects and other invertebrates on which the warbler feeds. Could it be that the habit of feeding on nectar, and picking up pollen on the feet, happened through successive visits of some birds to feed on insects; a behaviour that became established in populations as a result of learning from other individuals? Whatever its origins, it is another example of the wonder of evolution and of Natural History.


[2] Eva C. Kronestedt-Robards, Maria Greger and Anthony W. Robards (1989) The nectar of the Strelitzia reginae flower. Physiologia Plantarum 77: 341-346.

[3] ] Eva Kronestedt-Robards (1996) Formation of the pollen-aggregating threads in Strelitzia reginae. Annals of Botany 77: 243-250.

[4] Adrian J. F. K. Craig (2014) Nectar feeding by weavers (Ploceidae) and their role as pollinators. Ostrich 84: 25-30.

[5] G. Coombs, S. Mitchell and C. Peter (2007) Pollen as a reward for birds. The unique case of weaver bird pollination in Strelitzia reginae. South African Journal of Botany 73: 283.

[6] F. Hoffmann, F. Daniel, A. Fortier and S.-S. Hoffmann-Tsay (2011) Efficient pollination of Strelitzia reginae outside of South Africa. South African Journal of Botany 77: 503-505

[7] G. Coombs and C. I. Peter (2009) Do floral traits of Strelitzia reginae limit nectar theft by sunbirds? South African Journal of Botany 75: 751-756

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