When this episode was made there were only eight red tailed hawk nests in all the Five Boroughs of New York City. Manhattan is the smallest and most urbanized of those boroughs.
In order for "Light Bulb" Len to have changed approx. 1.2-million light bulbs, he would have to change around 150 bulbs a day, 365 days a year, during the 22 years he worked on the subway.
When Beckett and Castle were at the subway, there is an ad for Castle's book.
By 2021, the Twitter account @birdcentralpark routinely features photos of red tailed hawks in and around Central Park.
Unstated but implied TV trope. Ashley's albino Winstar rat that looks just like every other albino Winstar is 5 years old, even though the lifespan for a rat is stated as 2 to 3 years. This beloved pet rat has already been lost/killed and replaced several times without Ashley knowing.