HiCustomer and welcome to JA!
The most likely reason that the flamingos are dying at Lake Nakuru is that they are consuming toxic waste that was dumped there (see the first reference below). They may have migrated to sewage ponds to feed, ingesting toxic substances. In addition, flamingos migrate to other lakes (because the lake level gets lower and their food sources get depleted).
Populations have fluctuated wildly over the past 50 years. For example, according to the second reference below, there were about 915,000 birds in 1972 and 1973, but by 1974, there were only 113,000 birds. That study found that these short time flamingo population fluctuations that are correlated with algal densities (which is one of their food sources). According to the third reference below, the lowest population of flamingos was in 1969 (about 68,000 birds), and the highest was in 1995 (about 2,800,000 birds).
Here are some refs for you to look at:
I hope this helps. Let me know if you have any questions,
I saw your feedback. Thanks for the accept!
Did you have a problem accessing those references?
The simple answer to your question about research, is that I use Google Scholar (which searches specifically research papers):
There are many relevant papers about Lake Nakuru and flamingos.
From my research, it appears that the Lake level fluctuates wildly over time.
Also, here is another reference (http://www.webkenya.com/eng/safari/nakuru.php) that says "From 1993 to 1997, a large number of birds died at Lake Nakuru and high concentration of toxins (up to 9 different metals in excess amounts) were found in their body. In 1998, about 137 tons of chemical wastes were dumped into Nakuru town and that is despite the good effect on pollution by El Nino heavy rains." Basically, even though there was lots of rain (and the lake level went up), the 137 tons of toxins could not be diluted.
Some more reading:
Let me know if I can help you further with this,