Mistletoes - cryptic mimicry

The bird search image hypothesis

There is an alternative hypothesis (see previous page) which explains mimicry in terms of co-adaptation involving the feeding behaviour of mistletoe birds. In mistletoes which have high host specificity, a mechanism which increases the likelihood of seeds being dispersed preferentially to the usual host will reduce wastage of seeds and therefore have a selective advantage. Vegetative resemblance of the mistletoe to its usual host may have forced the bird to use the the host rather than the mistletoe as its feeding search image. This would result in the bird preferentially visiting trees of the correct host species when searching for food, whether mistletoes were present or not, and would increase the chance of seed dispersal to uninfested hosts. This interesting hypothesis receives strong support from some ornithologists.

These hypotheses for mimicry are not mutually exclusive, and both could be involved. Regardless of which is correct, cryptic mimicry by some mistletoes of their hosts is an important example of co-evolution.