If while on a lengthy interstellar voyage you noticed other people on board the spacecraft dying at an accelerating rate, you might reasonably worry that something had gone wrong with the spaceship environment. Likewise if, instead of people, dogs or cats on board started dying off. Similarly, if parakeets were brought along as pets, it would be worrisome if each successive generation consisted of fewer, and smaller, birds.
That is essentially the situation now on our planet, the main difference being that the birds at issue are not parakeets, but shorebirds known as red knots. The details of why they are disappearing and shrinking are of some interest, for they point to the likelihood that our environment is changing in ways that may be fatal, for unanticipated reasons, to many of the planet's inhabitants. Each spring, the red knots migrate 9,300 miles from the tropics to the arctic, where they lay their eggs. The chicks hatch, eat the abundant local insects, and less than two months later fly south. Now with global warming the snow melts earlier, and the insects emerge and reach their peak populations earlier. Since the birds have not adjusted their migration schedule, there is less for the chicks to eat, so they tend to grow less big. Back in Mauritania, where wintering red knots traditionally use their long bills to dig clams from the sand, researchers were surprised to find some instead digging up and consuming less nutritious sea grass roots. The clams lie deeper in the sand than the grass roots, and it appears the smaller birds have bills too short to reach them, so these birds must make do with poorer diets. Thus fewer survive, accounting apparently for a 50% decline in the Mauritanian red knot population. Ornithologist Martin Wikelski commented, "If that continues, they're going to go extinct." Though more research is needed to confirm what is happening to them, we should consider how likely it is that in many ways we might not suspect, various species, perhaps including ourselves, may be endangered by environmental changes.
For more details see Carl Zimmer's Climate Change and the Case of the Shrinking Red Knots, NYT, May 12, 2016.