(a) Study site and species

This study took place from April 2015 to October 2016 on the island of Saipan (115 km²) in the Mariana Archipelago (15°11′ N, 145°44′ E). The native limestone karst-forests of Saipan contain roughly 40 species of tree and shrub, with 10 species making up approximately 90% of tree stems. This study focused on the dispersal of 15 woody shrub and tree species (figure 1; electronic supplementary material, table S1) by five native forest bird species. These plant species produce fleshy fruits that are known components of frugivore diets, make up 94% of bird-dispersed tree stems in this forest type, and range in seed mass from 0.0001 g (Pipturus argenteus) to 0.62 g (Aglaia mariannensis; electronic supplementary material, table S1). Additional tree species were excluded as they were rare within the study area, did not produce sufficient fruit during the study, or were not bird-dispersed. Tree species included 15 native and two naturalized species (Carica papaya and Triphasia trifolia; electronic supplementary material, table S1).

Posterior predictive distribution of gut passage times for 39 plant–bird interactions on the island of Saipan. Points represent median values and bars represent 95% credible intervals. Note the fourfold increasing scale on the y-axis.

The five extant avian frugivores on Saipan are: Bridled White-eye (Zosterops conspicillatus), Golden White-eye (Cleptornis marchei), Mariana Fruit Dove (Ptilinopus roseicapilla), Micronesian Starling (Aplonis opaca), and White-throated Ground Dove (Alopecoenas xanthonura). All bird species were known to consume fruits and therefore considered potential seed dispersers. Species varied in the amount of fruit included in the diet from purely frugivorous (e.g. Mariana Fruit Dove) to omnivorous (e.g. Bridled White-eye) and in body size (5–150 g). For additional life-history traits such as home range size and habitat selection see Rehm et al. [26]. The Mariana fruit bat (Pteropus mariannus) is an additional frugivore on Saipan and likely influences dispersal patterns, especially for large-seeded plant species. However, the bat is functionally extinct on Saipan and was therefore not included in the current study. In addition, other dispersers have been extirpated within the Mariana Islands prior to or shortly following human settlement but their roles as seed dispersers are unknown [27].