But to answer your question: wild dogs. But in India they spread rabies and other diseases, attack and kill people. And their excrements are highly toxic, and kill plants (unlike vultures).
For an animal to evolve to fill this niche, the niche would have to convey some advantage that favours their survival and reproduction (i.e. natural selection). Aside from the fact that this isn’t actually a terribly efficient process and is subject to lots of random effects, the niche in its current form as influenced by human activity is clearly not conveying a survival benefit, or the vultures wouldn’t have declined. A member of another bird species deciding to take to scavenging trash heaps whilst happening to have a mutation that protects it from diclofenac (which is toxic to lesser degrees in other non-vulture birds) is not impossible, but unlikely, and could very well be a process that occurs at a timescale that is incompatible with the rate of continued change by humans.
Other (especially non-avian) scavengers could have better reactions to the cattle drug that the article blames for poisioning the vultures and thus thrive when their competition is gone.
efficient at what? what's the measure, utility function, what are nature's KPIs? :)
utilizing energy gradients? maybe, but there's plenty of that on Mars yet it's barren. so probably certain ecosystems can be more or less efficient at this, right?
But to answer your question: wild dogs. But in India they spread rabies and other diseases, attack and kill people. And their excrements are highly toxic, and kill plants (unlike vultures).
Nature is not always efficient.