Augustin,
To talk about 'preferred food', as if N. lowii can make some sort of conscious decision about what it does or doesn't like to eat is anthropomorphism at its worst. CPs have evolved strategies which enable them to advantageously compete for niche space by supplementing their nutrient intake by the means capturing prey. They don't choose their food or express any preferences, but they can evolve means (trap shape, colour, attractants etc) which tend to attract a particular type of prey over others.
The fact is that large quantities of bird poo has been found in the pitchers of this species and it seems reasonable to me that the plants may have evolved the strategy of attracting birds for this purpose. Until someone does the laborious long-term field studies, in a remote and hostile environment (at great cost too! ), it remains a hypothesis, quite a good one too, in my opinion. If you don't like it, get a botany degree and some post-graduate experience in plant ecology, find some funding and trek off to Borneo for a few months hard work. Then when you have disproved the hypothesis, publish it and convince others of your findings. There is nothing wrong with the hypothesis, its just a reasonable idea, waiting to be investigated.
Vic