Frugivorous birds are found in large numbers in tropical forests mainly because of

Frugivorous birds are found in large numbers in tropical forests mainly because of the availability of fruits throughout the year.  Tropical forests typically have a diverse array of fruit-bearing plants that produce
fruits at different times of the year, providing a continuous food supply for frugivorous birds. This constant availability of fruits supports a higher population of frugivorous birds compared to other habitats where
fruits may be seasonal or less abundant. 
(1) Lack of niche specialisation is not the primary reason for their abundance. In fact, frugivorous birds are specialised to feed on fruits, which is a specific niche within the avian community. 
(2) Though tropical forests do receive higher annual rainfall compared to some other ecosystems, the rainfall does not directly determine the abundance of frugivorous birds. 
(4) Temperature can influence the breeding patterns and behaviour of birds, but it is not the primary factor that determines the abundance of frugivorous birds in tropical forests.