Size-based fruit selection by a keystone avian frugivore and effects on seed viability

The non-random selection of fruits by frugivores can affect plant demography by changing the probability of seedling recruitment. We investigated whether the New Zealand pigeon (Hemiphaga novaeseelandiae) exhibits size-based preference of fruit in two indigenous tree species, Vitex lucens and Prumnopitys ferruginea, that have large fleshy diaspores containing single seeds. We also assessed if there was an advantage for seeds that were actively dispersed since birds may select fruits that have viable seeds (although the cue for such selection remains unclear) and whether a combined scarification-deinhibition advantage was conferred on germination due to seed passage through the gut. Using seed length as a proxy for fruit size, we compared the length of passively dispersed fruits in traps with the estimated fruit length of actively dispersed seeds in six trees of each species across three areas in one fruiting season. For both species, actively dispersed seeds were significantly longer than seeds from passively dispersed fruit, suggesting frugivore preference for larger fruit. Seeds from passively dispersed P. ferruginea fruit had significantly higher viability than actively dispersed seeds and there was no measurable scarification-deinhibition effect. For V. lucens, there was no significant difference in viability between actively and passively dispersed seeds and there was a total lack of seed germination in our trial. Our results show a significant preference by H. novaeseelandiae for larger fruit in both tree species, which was possibly associated with annual fruit production since P. ferruginea trees that had more fruit produced larger fruit on average. That H. novaeseelandiae selected longer fruit in our study is consistent with the prediction that birds will prefer larger fruit when beak gape limitation is absent. However, active dispersal of larger seeds may not always be advantageous since passively dispersed intact fruit germinated equally well and may even have had a viability advantage.