Birds and Food Shortage

The term“food shortage”has been used for food that is inadequate in quantity,quality,or availability.Whatever the abundance of food in the environment,if it is for some reason unavailable,bird species in either inland or coastal areas may starve. Food sources like intertidal invertebrates retreat deeper in the mud in cold weather,putting them beyond the reach of shorebirds,while many soil-dwelling invertebrates move deeper into the ground during drought.Such behaviors illustrate the general points that only a proportion of potential food items may be accessible to birds at any one time,and that this proportion may change through time in response to other conditions.Secondly,if food is plentiful but of low nutrient content or digestibility,a bird may be unable to process enough each day to maintain its weight.This is particularly true of herbivores (plant eaters)eating fibrous,or coarse,vegetation,and food shortage may arise partly from the structure of a bird’s inner parts,or gut,which limits the rate at which food can be processed.In general among birds,species differences in gut structure correspond with dietary differences,and in species whose diet changes seasonally,gut structure may change accordingly.The need for change in gut structure can make some species vulnerable at times of rapid shift in diet,as when snowfall forces birds suddenly from one food type to another.

Many bird species cushion the effects of food shortage by storage, either of body fat or of food itself.In small birds,fat stores are accumulated mainly for use overnight,and few species can last more than a day or two without feeding.Larger birds,with their relatively lower metabolic (energy)demands,can store relatively more energy per unit of body weight.As a result,larger birds can rely on internal reserves for longer than small ones,more than two weeks in large waterfowl.Many birds store some body fat and protein in preparation for breeding,extreme examples being Arctic-nesting geese,in which the females produce and incubate their eggs almost entirely on reserves accumulated in wintering or migration areas.For some weeks,then,they are protected against the need to feed on their breeding grounds.Most migratory birds accumulate body fat for the journey,the amount depending on the duration of unbroken flight.

Species that store food items outside the body may retrieve them hours,days,or even months later,depending partly on the type of food.Some meat eaters cache (store)prey for consumption later in the day or in the following days,while some seed eaters may store food in autumn to last through the winter or even year-round.Long- term storage is frequent among various birds,such as tits, nuthatches,and woodpeckers,reaching its extreme in certain jays and nutcrackers.In some such species,stored food can provide more than 80 percent of the winter diet,and can also feed the adults into the next breeding season,influencing the numbers of young raised.In good seed years,huge amounts of seed can be stored.Crucial aspects of hoarding behavior are that individuals should guard their stores or should hide them (to reduce theft)and remember where they are.Some species store items individually,each in a separate place,and in experiments have shown incredible feats of memory. But none of this behavior alters the dependence of such birds on their food supplies;it simply enables them to use their foods more effectively,taking advantage of temporary gluts (oversupply)to lessen later shortages.It does not prevent them being limited by shortages in years of poor seed crops.

Yet other birds can cushion themselves against temporary food shortages by becoming torpid,meaning they achieve an inactive state in which body temperature and heart,respiratory,and metabolic rates are greatly depressed,thereby conserving energy.In some small species,this happens in some degree every night,but it is especially marked in some small hummingbirds,such as Anna’s hummingbird of western North America.Some other species,such as the common swift,can remain torpid for several days at a time when cold weather renders their insect prey inactive,and at least one species,the common poorwill,can remain torpid for several weeks,in a state akin to hibernation in mammals.These adaptations help the species concerned to last through difficult periods,whether hours,days,or weeks,but they cannot eliminate altogether the risk of starvation.

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