I have a huge Cascabella thevetia by my house and I know that its sap and fruits are extremely poisonous.However I have seen red vented bulbuls and wasps feed on its nectar.Also this confirms it but doesnt specify nectar infact it says plant parts...The same source says the poison affects vertebrates and the birds are definitely vertebrates.So why it doesnt affect them?
The toxicity of Cascabela thevetia is due to the presence of cardiac glycosides such as thevetins A and B. Although these compounds are highly toxic for humans and several domestic animals, other vertebrates are much less sensitive to them.
This review article discusses various aspects of oleander toxicity, and the authors draw attention to species-specific differences in toxicity.
In addition to dogs and cats, Szabuniewicz et al., (1972) also examined oleander toxicity in monkeys, goats, mice, rats and chickens. Dogs, cats, goats and monkeys were found to be very sensitive to the cardioactive glycosides extracted from dried oleander leaves. In contrast, the rodent and avian species were observed to be insensitive to oleander cardioactive glycosides.
They go on to suggest that differences in the toxin target molecule, the cardiac Na⁺,K⁺ ATPase, is the most likely explanation for this:
Akera et al., (1979) postulated that the low affinity between cardiac glycosides and rat cardiac Na⁺,K⁺ ATPases could be due to stearic constraints in substrate (glycoside) binding. In this scenario, quaternary conformation of the OL-subunit glycoside binding sites physically precludes the simultaneous binding of all three glycoside structural components. Species-specific quaternary conformation of Na⁺,K⁺ ATPases coupled with subtle differences in glycoside composition (e.g. various sugar moieties attached to the steroid backbone) could, thus, conceivably account for the observed species-specific responses to cardiac glycosides, including those found in oleander.
We have a tendency to assume that susceptibility to the toxic effects of plant materials is universal, but the toxicity of onions and chocolate for domestic animals are clear examples that this is wrong. The variation in sensitivity to the toxins in oleander is a good example coming from the other direction.
Shannon D. Langford & Paul J. Boor (1996). "Oleander toxicity: an examination of human and animal toxic exposures". Toxicology. 109 (1): 1–13. PMID 8619248. doi:10.1016/0300-483X(95)03296-R.