Fruit properties and the location of urban holly cultivars – do they matter to frugivorous birds?

Birds are without doubt the principal means of seed dispersal by animals, and trees that produce fleshy fruits are typically reliant upon such dispersal means to expand their geographical ranges. Ornamental trees that produce fleshy fruits grown within urban locations, which are not necessarily native to the area in which they grow, may therefore also benefit from such a dispersal mechanism and begin to succeed into the surrounding environment. Granted, frugivorous birds within urban environments may behave differently to birds within woodland stands (or other ‘natural’ setting), and not much is known about the foraging preferences of such birds in the urban setting. Despite this, it is considered that the colour of tree fruit is one driver influencing upon attracting birds, because of the good vision of birds (to make up for their poor sense of smell).

In this study, the authors investigate how the preference of birds to the fruits of Ilex aquifolium cultivars (and the standard Ilex aquifolium) growing within urban environments has impacted upon the dispersal of seed, and in turn enabled the species to advance into previously uncharted territory within northern Europe. Because the main dispersal mechanism of the species’ seed is via birds, understanding what fruit characteristics (colour, size, etc) influence avifaunal frugivory is important. By the same token, do birds preferentially select cultivated Ilex aquifolium, or instead opt to consume the fruits of the native non-cultivated species? Similarly, understanding what site qualities attract birds can help to improve understanding of the dissemination of Ilex aquifolium into the surrounding landscape.

Because the natural range of Ilex aquifolium reaches its north-eastern climax in Denmark, the study site was located around the current climax zone (within Greater Copenhagen). Within this area, six sites were selected, of which three were cemeteries and three botanic gardens – all within urban areas. Over the past few decades, this climax zone around had been relatively stable, until more recently (since the turn of the millennia) when it was observed to expand its range by a distance of 100-200km up into Greater Copenhagen. Such a shift, the authors remark, would be down to the more favourable climatic conditions, though would be facilitated by seed dispersal courtesy of frugivorous birds.

To investigate if birds had different preferences with regards to fruit from different cultivars, the following cultivars were used within the study: ‘Bacciflava’ (strong yellow fruits, in clusters of 3-5, and 8mm in diameter), ‘Crinkle Green’ (vividly red fruits, in clusters of 1-3, and of 7-9mm in diameter), and ‘Pyramidalis’ (vividly red fruits, in clusters of 1-4 and of 8-10mm in diameter). The standard Ilex aquifolium fruits also featured, in order to compare cultivars to the standard (and native) holly species. To understand the fruit properties of all four hollies used, samples were taken from the individuals from which branches were sourced for the study, and the maximum diameter of the fruits, the fresh mass of the fruits (pulp:seed ratio), and water content were measured.

Ilex aquifolium ‘Bacciflava’. Source: University of Richmond.

From the three cultivars and the native species, branches containing fruits were sourced (within Denmark) and placed at the six locations (from December to February). All branches had similar qualities, possessing 10-20 leaves and fruits, and were fixed to identical wooden boards atop a 1.6m wooden stick to expose the fruits to birds exclusively. Five of these boards were set up in open areas at least 5m away from trees over 3m in height, and five were set up under trees and tall shrubs over 3m in height. Over the course of the survey period, the branches were all checked 15 times, and records gathered as to how many of the fruits had been taken from each sample.

Once the survey was completed, it was found that birds (blackbirds and robins) had eaten 2,655 of the 3,404 (78%) fruits across all four branch types. However, the rate at which the fruits were removed varied between branches, with ‘Crinkle Green’ having its fruits removed most abundantly. In terms of the total number of fruits removed from each branch type, the cultivar ‘Bacciflava’ massively reduced the average by having only 35% of its fruits removed. ‘Crinkle Green’ had 94% of its fruits removed, ‘Pyramidalis’ 92%, and the native holly 91%. Therefore, it is evident that birds had a strong preference for red-coloured fruits (fruit of ‘Bacciflava’ was a green-yellow in colour), though there is little evidence to suggest there is any preference as to what fruits were eaten beyond mere red colouration. There was also a marked difference in the rate at which fruit was removed from the different feeding station locations, with branches under trees having their fruit removed at a much higher rate than those in exposed settings.

Ilex aquifolium ‘Pyramidalis’. Source: Helmers.

In light of the results, it can first be noted that birds did eat the fruits from all four branch types and across both feeding station locations (exposed and sheltered). Therefore, there is potential for many Ilex aquifolium cultivars to enable for seed dispersal and the associated expansion in the native range of the species. Granted, red fruits were eaten far more readily, and this suggests that cultivars with red fruits (and the red fruits of the standard Ilex aquifolium) are far more appealing to birds. It must of course be noted that other fruit colours, such as white and orange (which some Ilex aquifolium cultivars have), were not used within this study, and therefore all that can be ascertained is that red fruits are more desirable to birds than green-yellow fruits.

Looking beyond fruit colour, it can be said that smaller fruits are removed more readily by birds. This is because ‘Crinkle Green’, which had 94% of its fruits removed, possessed the smallest fruits. Such a finding does however conflict with the understanding of larger fruits being more desirable to birds (up to a point, when fruits become too large to fit within a bird’s beak), and particularly earlier in the fruiting season of Ilex aquifolium. The pulp:seed ratio was however shown not to be an influencing factor, as ‘Crinkle Green’ in fact had the lowest pulp:seed ratio, whilst the second highest branch type from ‘Pyramidalis’ had the highest ratio. Building upon this, because the nutritional profile of fruits was not measured, it is difficult to make the assertion that the amount of flesh on a fruit is significant in determining bird frugivory. It may very well be that birds seek nutritional fruits, and these fruits may very well have varying pulp:seed ratios.

A robin perched upon the branch of an Ilex aquifolium. Source: Warren Photographic.

In terms of the amount of fruit per branch, because the number of fruits was relatively similar across all branches (10-20 fruits on each branch), there was little data to support claims that the abundance of fruits influences upon bird frugivory. Of course, if entire specimens were studied, then it may  very likely be found that fruit abundance does influence upon frugivorous birds. In fact, other research has shown exactly this, and the authors remark that Ilex aquifolium cultivars that produce more fruits will hasten the species’ expanding range by attracting birds more readily.

The fact that birds also more routinely ate fruits from branches sheltered by the canopy of trees and tall shrubs is also telling. From an evolutionary and habitual perspective, this is not surprising, because birds will utilise the cover to reduce the risk of predation whilst they are foraging for food. An open environment leaves the bird exposed, and therefore if fruits can be obtained in sheltered settings then that is much preferred. Such a preference is actually quite beneficial for Ilex aquifolium, the authors allege, because it is a shade-tolerant species that can readily exist beneath tree canopies. By this token, the habit of birds eating fruits from sheltered hollies in urban areas may enable the species to expand its range by using urban woodland sites (and other sheltered locations) as vectors.

Therefore, it can be said that, if an Ilex aquifolium is to be planted within an urban setting then it is to be located in an exposed area, and should not have red fruits. Granted, this is assuming that its northward spread is not to be desired. Furthermore, because the survey sites were only in botanic gardens and cemeteries, there is a failure in recognising how small to medium-sized gardens within the urban and sub-urban setting may impact upon bird frugivory and the subsequent dispersal rate of Ilex aquifolium. Nonetheless, the results are interesting, and there is certainly scope for considering what types of cultivar to plant within an urban environment if succession of the species is of concern. This applies not only to Ilex aquifolium, but across the entire botanical spectrum.

Source: Møller, L., Skou, A., & Kollmann, J. (2012) Dispersal limitation at the expanding range margin of an evergreen tree in urban habitats?. Urban Forestry & Urban Greening. 11 (1). p59-64.

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Fruit properties and the location of urban holly cultivars – do they matter to frugivorous birds?

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