In the northern hemisphere, birds have been heading south for the past few weeks, or even longer, depending on the species and where they are starting from. Some birds have not begun their southward trek yet. Wherever they begin and whatever their destination, it’s a perilous journey.
Just before I sat down to pen this blog, my wife pointed out a dead bird on our doorstep. Birds almost never hit our windows – maybe twice in 30 years- so it was a surprise to see the deceased Mourning Dove. I picked it up and examined it. It had been dead for a few hours as its eyelids were drying and the body was cold. I ran my fingers alongside the keel (carina) the extended part of the sternum on which the major flight muscles attach and could tell that the muscles had shrunk. In a healthy bird, you can run a finger from one side of the keel to another and barely feel the keel in the middle. In a starving bird, the keel protrudes from the chest like a dull knife blade. This bird was half starved.
Another way to evaluate a (small to medium) bird’s condition is to hold it in your hand and blow the feathers away from the neck area where you will detect subcutaneous fat around the furcula. (In this paper is a detailed description of the method.)
So the bird on my porch probably died of starvation or something related to it, certainly a possibility during a long migratory voyage. But of course, there are many other dangers such as cats, estimated to kill 2.5 billion cats a year in the U.S. Another 599 million die by flying into building glass and 215 million meet their demise in collisions with vehicles.
But take a typical songbird, the American Robin. The chances of an egg hatching, growing up, migrating south and back north to become a breeding adult are 15%. For that same bird, the chances of becoming a two year old breeding bird are 6%. It’s definitely a tough life. So you shouldn’t be surprised at finding a deceased bird this time of year, there are many.
While inhabiting breeding or wintering areas, birds do not move around too much, especially if they establish territories. Becoming familiar with the hazards of the area (cats, electrical wires, buildings, etc.), they can avoid them to some degree. But while on migration, the areas they pass through are less familiar; to a bird hatched that year, totally unfamiliar. So the threats are greater. Excluding human-caused threats, birds have to navigate, find food, avoid bad weather, and perhaps cross large areas of water, mountains, or desert. It’s amazing that as many of them make it as they do.