Updated: Dec 5, 2020
We wish we could give you a simple answer, but there are multiple factors involved that must be considered. If someone does give you a simple answer, you might rethink trusting that source. (Looking at you, "Birding Deport.")
Birds are often quite hardy, being able to survive in some of the harshest conditions on earth. However, some species struggle under conditions they have not readily adapted to. Typically, when a bird recognizes its food source is running low, it can simply move away from a habitat low on resources. But if we use a hypothetical situation derived from the 'snowmageddon' event from Labor Day 2020 when conditions were 70+ mph winds, a 60° F temperature drop, and several inches of heavy snow, we begin to see a situation that can cause migrating birds to suffer.
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How can some birds survive without eating regularly?
I am not one to be flocking around. Or am I always flocking around? Regardless, giving the runaround is not something we do here on this site. However, before an answer can be provided for our main question, we need to address the variables that are part of this equation. So, what determines how long a bird can survive without eating?
Size - Larger birds are able to survive a longer period without food. I have rescued a plethora of eagles, hawks, and owls that have been reported on the ground for DAYS to WEEKS. However, you will not hear of an instance like this for smaller birds.
Fat stores - If a bird has been eating aplenty for a significant amount of time, it will build up fat stores. These fat stores allow for the bird to burn through this excess storage when energy needs are high and food availability is low.
Species - Different species of birds have different capabilities for handling food shortages and low ambient temperatures. Hummingbirds can enter torpor, an abbreviated version of 'hibernation,' to help them survive cold weather and food shortages. For hummingbirds, this only works for a short amount of time. However, in the Common Poorwill, torpor can last an extended period of time, allowing these goatsuckers to survive longer periods of time without food.
Previous meal - If a bird has recently filled its crop with an ample supply of food, it may not need to feed for a while. Larger birds, like raptors, often eat more calorie-rich food items, allowing for an even more extensive fasting period.
Temperature - Birds can go longer without food when temperatures are not at the low end of the spectrum. Why? Fewer calories are used for keeping internal body temperatures up.
These five factors affect how long a bird can survive without an abundant food supply. Of course, we do not want a complex answer but survival is rarely a simple answer. If you keep reading, I will attempt to simplify this answer.
How long can a bird survive without food?
If we toss out most of the factors listed above and only consider fat stores and temperature for small to medium-sized songbirds, we can estimate a reasonable answer using research performed in the 1950s and 1960s. I will warn you, thinking about how these studies were performed can be upsetting. If you do not want the details, here is a simplified answer for how long a bird can survive without food: a medium-sized songbird can survive 1 - 3 days without food under optimal conditions. However, in adverse conditions, a typical songbird cannot survive more than a day. With low to no fat stores, and at low temperatures, a medium-sized songbird will likely survive less than 24 hours. A small songbird without body fat will probably survive a far shorter amount of time in cold conditions, likely around 12 - 18 hours.
Bird survival without food while resting
If we use a bird similar in size to the Red-winged Blackbird, we can estimate that a bird can survive 3 to 5 days if the bird is in a lowered state of metabolism. You can think of this as roosting or resting. During this time, the bird will lose 30% of its body weight, with approximately 10% of the body weight lost being from fat. If a bird has no fat, its survival time is much shorter.
Bird survival without food while active
Using the same example above of a medium-sized songbird, we can estimate that a typical bird, with 10% of its bodyweight being composed of fat, will survive 1 to 3 days. However, if that bird has no body fat, its survival time could be less than a single day.
Bird survival without food at normal temperatures
In a separate study on European Starlings, researchers were able to simulate the survival of a medium-sized songbird during various ranges of 'typical' temperatures (75°F - 85°F). Roosting birds survived 1 - 2 days when offered no food at normal temperatures. However, this study did not provide the composition of bodyweight from fat, so we are left with some questions about how fat stores would affect a bird's survival.
Bird survival without food in cold temperatures
This study also examined European Starlings surviving in near-freezing temperatures (35°F - 39°F). This part of the study was conducted with lone birds and birds in small groups. In lone birds, death was often a result of hypothermia. Without enough energy to maintain their body temperatures, the individuals in the study lasted from 6 to 24 hours. When more birds were added and studied in a flock, survival times went over 3 days. The birds at lower temperatures were likely staying in a 'rest' state, indicating that migrating birds experiencing a sudden change in temperature would probably survive an even less amount of time.
How can I help starving birds?
During migration and winter seasons, birds require an excessive amount of food to build fat stores, prevent hypothermia, and fly to areas with ample food supplies. However, these seasons can also be the most energy-deficient seasons when it comes to finding food. Birds that feed upon insects are especially afflicted by winter weather, as cold temperatures and snow can prevent their food supply from being readily available.
If you see a starving insectivorous bird, you are likely unable to do much to help that bird. Capturing the bird and getting it to a rehabilitation clinic may help, but you may also make the situation far worse. Some backyard bird lovers will try to put out dried mealworms (Amazon) to help insectivorous birds that are struggling. But not all species will utilize this crunchy 'treat.' Of course, if you feed birds, you are likely using black oil sunflower seed, which is a great source of protein and fat for seed-eating birds.
I hope this long-winded post answered your question. If you 'googlized' this question, you likely got a variety of answers without evidence supporting their claims. However, the information in this article was built upon studies published in the Wilson Journal of Field Ornithology.