During the emergence of West Nile virus in the United States from 1999-2006 (which year depended on what part of the country you are in) provided biosurveillance operators with a couple of easy indicators for impending transmission: wild bird die-offs (i.e. corvids) and equine illness / mortality.
This year, however, in the current surge of West Nile transmission (our local example may be found here), we did NOT see pre-event bird die-offs reported. In our part of the country there are plenty of corvids to monitor, but presumably thanks to herd immunity we were not provided the benefit of a month or two "heads up". The fallback position was the early and abrupt WNV-positive mosquito pools noted 6 weeks in advance of the earliest known date of positivity.
Although our part of the country hosts a substantial equine population, the West Nile vaccine has mitigated much of the illness we would have normally seen. Indeed, this year it was almost as if human cases were sentinels for equine cases.
This year has indeed been associated with a warmer than usual winter, spring, and summer. Temperature is a crucial driver for mosquito transmission competency, dramatically shortening the time it takes for a mosquito to go from a viremic avian blood meal to being able to infect another host (less than 24 hrs if incubated from 26-30 degrees Centrigrade in the lab, for instance). However, as noted above, the loss of the expected signature pattern may present a challenge now for those US locales that have not yet seen a resurgence of West Nile- especially those cities who have deactivated or toned down their mosquito surveillance programs thanks to the poor economy.