The Predation-Starvation Tradeoff

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Many casual bird watchers, particularly those maintaining bird feeders, appear to assume that birds participate in a bimodal pattern of feeding, at least at bird feeders by seed eaters. That is, from early to late morning there is a feeding binge which drops off for the rest of the day and resumes in the early evening and lasts until dusk or slightly after dark. So birds eat in the morning to make up for what they metabolically used up overnight and they fill up just before dark to make it through the night. That’s what this paper on foraging patterns begins to discuss but then goes on to undermine, or at least refine, that idea.

By concentrating feeding times to a few hours in the morning and evening, birds allow predators to figure out the best times to visit the feeders as well. A Cooper’s Hawk is likely to visit at the times when the most prey are available as it would be advantageous to the hawk for the prey to restrict themselves to a few hours of feeding time. So, (and I’m simplifying the results of the research here) as would be expected, the researchers actually found that the seed eating birds spread their feeding over the day, making it less likely that a predator will learn when it is best to pursue its prey. An informal survey indicates that bird feeder birds spend only a little more time (about 7%) feeding in the morning than the afternoon, although I’m sure that’s not statistically significant and you will find all sorts of anecdotal studies on the web that produce inconsistent information.

Immature Cooper’s Hawk

Looking at seed-eating birds in wild environments, the story is much the same but there is more variability in times and locations as seeds are depleted in wild situations while food is replaced regularly at bird feeders. So birds in the wild will move around to find food sources but again feed throughout the day.

Insectivorous bird foraging behavior is much more complex because the prey choices vary so much in size, are mobile, and may only be accessible certain times of the day (cold insects don’t move much). Invertebrates also often have defense or escape mechanisms that make it difficult for predators to catch and eat them. Many fly or burrow, are camouflaged, or are distasteful. And their life cycle with many larval or pupal stages complicate predatory behavior. Plus the habitats, such as tropical vs temperate forests, make insectivory more complicated as noted in this old but interesting 1988 paper by Thiollay.

There are too many foraging behaviors to discuss here, or in any one scientific paper for that matter. Think about the differences between cliff-dwelling seabirds that venture out on the ocean for food, soaring hawks, fish-eaters like kingfishers and osprey, and nocturnal predators like owls. They all have competitors but their fundamental goal is finding sustenance while avoiding becoming the sustenance of some other creature. The strategy then, is for a bird to make its foraging forays as unpredictable as possible. But, as I graphically showed in a previous week’s blog, you never know where danger lurks.

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