It was once thought that competition with early bird species might have resulted in the extinction of many of the pterosaurs.
Part of this is due to the fact it used to be thought that by the end of the Cretaceous, only large species of pterosaurs were present (no longer true; see below).
The smaller species were thought to have become extinct, their niche filled by birds.
However, pterosaur decline (if actually present) seems unrelated to bird diversity, as ecological overlap between the two groups appears to be minimal.
In fact, at least some avian niches were reclaimed by pterosaurs prior to the KT event.
At the end of the Cretaceous period, the Cretaceous–Paleogene extinction event, which wiped out all non-avian dinosaurs and most avian dinosaurs as well, and many other animals, seems also to have taken the pterosaurs.

In the early 2010s, several new pterosaur taxa were discovered dating to the Campanian/Maastrichtian, such as the ornithocheirids Piksi and "Ornithocheirus", possible pteranodontids and nyctosaurids, several tapejarids and the indeterminate non-azhdarchid Navajodactylus.
Small azhdarchoid pterosaurs were also present in the Campanian.
This suggests that late Cretaceous pterosaur faunas were far more diverse than previously thought, possibly not even having declined significantly from the early Cretaceous.

Small-sized pterosaur species apparently were present in the Csehbánya Formation, indicating a higher diversity of Late Cretaceous pterosaurs than previously accounted for.
The recent findings of a small cat-sized adult azhdarchid further indicate that small pterosaurs from the Late Cretaceous might actually have simply been rarely preserved in the fossil record, helped by the fact that there is a strong bias against terrestrial small sized vertebrates such as juvenile dinosaurs, and that their diversity might actually have been much larger than previously thought.

At least some non-pterodactyloid pterosaurs survived into the Late Cretaceous, postulating a Lazarus taxa situation for late Cretaceous pterosaur faunas.
It was once thought that competition with early bird species might have resulted in the extinction of many of the pterosaurs. Part of this is due to the fact it used to be thought that by the end of the Cretaceous, only large species of pterosaurs were present (no longer true; see below). The smaller species were thought to have become extinct, their niche filled by birds. However, pterosaur decline (if actually present) seems unrelated to bird diversity, as ecological overlap between the two groups appears to be minimal. In fact, at least some avian niches were reclaimed by pterosaurs prior to the KT event. At the end of the Cretaceous period, the Cretaceous–Paleogene extinction event, which wiped out all non-avian dinosaurs and most avian dinosaurs as well, and many other animals, seems also to have taken the pterosaurs.
In the early 2010s, several new pterosaur taxa were discovered dating to the Campanian/Maastrichtian, such as the ornithocheirids Piksi and "Ornithocheirus", possible pteranodontids and nyctosaurids, several tapejarids and the indeterminate non-azhdarchid Navajodactylus. Small azhdarchoid pterosaurs were also present in the Campanian. This suggests that late Cretaceous pterosaur faunas were far more diverse than previously thought, possibly not even having declined significantly from the early Cretaceous.
Small-sized pterosaur species apparently were present in the Csehbánya Formation, indicating a higher diversity of Late Cretaceous pterosaurs than previously accounted for. The recent findings of a small cat-sized adult azhdarchid further indicate that small pterosaurs from the Late Cretaceous might actually have simply been rarely preserved in the fossil record, helped by the fact that there is a strong bias against terrestrial small sized vertebrates such as juvenile dinosaurs, and that their diversity might actually have been much larger than previously thought.
At least some non-pterodactyloid pterosaurs survived into the Late Cretaceous, postulating a Lazarus taxa situation for late Cretaceous pterosaur faunas.
Pterosaur