Owing to native species declines, mutualisms; including avian dispersal of fleshy-fruited plants, may become dependent on substituted service by functionally equivalent introduced species. There is, however, an emergent pattern in which native and exotic frugivores are observed to consume larger proportions of native and exotic fruiting plants, respectively, while seemingly avoiding novel partnerships.

How much this is influenced by particular bird species preference versus state dependence of resources or bird populations is unknown. It is difficult to form concrete ideas about the relative contribution of different birds to disperser mutualisms across space and time, in which abundance and distribution of fruiting plants vary. It is also unknown if differences in patterns of fruit usage and selective preference are reflective of features of the fruit themselves.

Drawing from 21 studies including over 19,000 observations of frugivory, I have established baseline data about fleshy-fruit within the diet of native and exotic frugivorous birds across New Zealand. This includes species such as the kereru (Hemiphaga novaeseelandiae), bellbird (Anthornis melanura), tui (Prosthemadera novaeseelandiae), silvereye (Zosterops lateralis), blackbird (Turdus merula), song thrush (Turdus philomelos), and starling (Sturnus vulgaris). Fruit features including colour, size, flesh to seed ratio as well as moisture, nutrients and energy content were explored in relation to the presence of fruit in the diet across bird species. Lastly, fruit preference was measured by a pairwise feeding choice experiment which controlled for differences in availability prevalent in wild flora.

It was found that exotic frugivores have a substantially larger average proportion of their diet made up of adventive fruit, 23.6%, than in their native counterparts, in which adventive fruit make up only 2% of their diet. Furthermore the composition of fruit in the diet was substantially different between the groups; with 44 native fruiting plants not consumed at all by exotic birds, while consumption of weed species was predominantly chosen by these species.

There were substantial differences between the features of native and adventive fruit; including higher levels of carbohydrates, gross energy, width and whole wet mass in the native fruit. All of these features were correlated to the presence of fruit in the diet of wild frugivores, as well as the differences between fruit preference in native vs introduced frugivores.

Lastly avian frugivores showed pronounced differences in fruit preference within the choice feeding experiments, a tendency only partially explained by fruit features. These preferences varied considerably from patterns of consumption recorded in the wild; which may highlight the role of availability in determining resource usage. Alternately, choice may be due to the constraints of this type of feeding experiment; in which the loss of contextual cues mean it is impossible to fully replicate behaviours of wild frugivores.

Foraging behaviour; including preference or aversion to particular fruits, may facilitate mutualism shifts and/or confer mutualism resistance by maintaining strong or weak relationships between dispersers and plants. Understanding such patterns allows for a better prediction of outcomes related to fluctuating resource availability and long-term demographic trends between native and exotic species.