Taxonomy
Explanation
The Joke
A man is shown excitedly holding up birds and shouting "Dinosaurs! Dinosaurs!" The caption below reads: "Dad spent seven years in that sanitarium before we remembered he was a taxonomist."
The Humor
The joke relies on the fact that, taxonomically speaking, birds ARE dinosaurs. Modern phylogenetic classification places birds within the clade Dinosauria as the only surviving lineage of theropod dinosaurs. So a taxonomist running around pointing at birds and yelling "Dinosaurs!" is technically, scientifically correct. However, to anyone who does not know this, he appears to be a raving lunatic -- hence the family having him committed to a sanitarium for seven years before someone finally remembered his profession and realized he was not insane but simply being very precise about his taxonomy. The comedy comes from the gap between scientific accuracy and everyday perception: being technically right in a way that makes you look completely crazy.
References
The classification of birds as dinosaurs is based on cladistic taxonomy, which groups organisms by shared evolutionary ancestry. Birds are classified as members of the group Maniraptora, which is within Theropoda, which is within Dinosauria. This classification has been widely accepted in paleontology since the 1990s-2000s, building on work by scientists like John Ostrom and others who established the evolutionary link between theropod dinosaurs and modern birds.