Early March gave us a gift, or so it seemed. Warm days, active birds, and that unmistakable feeling that spring had arrived ahead of schedule. Bluebirds did what they’ve evolved to do: they started nesting.
And then, on March 18th and 19th, temperatures dropped into the 20s.
What followed was sobering. Many nests failed. Eggs didn’t hatch. A season that began with promise turned into a lesson in timing, and how easily nature can fall out of sync.
But not everywhere.
At Kahite, there were bright spots. In Box 9, one pair managed to fledge three chicks by mid April, even though two eggs never hatched. That raises an obvious question: why did some nests fail completely, while others partially succeeded?
A recent study offers an important clue.
Researchers in Delaware set up cameras at nest boxes and recorded more than 8,000 feeding visits. What they found was remarkably consistent.
About 40% of the diet was caterpillars. The rest was mostly grasshoppers, crickets, and spiders. This pattern held across seasons and between males and females.
In other words, bluebirds do not just eat insects. They depend heavily on one thing: caterpillars.
And for good reason.
Caterpillars are close to a perfect food for growing birds. They are soft, easy to digest, and packed with protein, fats, and key nutrients. They are especially rich in carotenoids, which support immune function and development. For nestlings, caterpillars are not just food. They are survival.
So what went wrong in March?
Here is the key connection.
Caterpillars do not appear randomly. They depend on the timing of leaf out on native plants and on stable spring temperatures.
When March warms early, trees leaf out and birds begin nesting. But if a hard freeze follows, insect activity, especially caterpillars, can stall or collapse. A short cold snap can damage new leaves and interrupt the development of the insects that depend on them.
That creates a problem before eggs even hatch.
During incubation, female bluebirds must keep eggs warm almost constantly. On cold nights, that becomes much harder. If food is scarce, she has to leave the nest more often to feed herself.
That combination can be enough to cause failure. Eggs cool too much. Embryos stop developing. Some, or all, never hatch.
A study of Pied Flycatchers in Spain documented a similar pattern. A brief cold snap was followed weeks later by widespread breeding failure, not because of direct cold exposure, but because the insect food supply had collapsed.
Box 9 is a good example of how small differences matter.
Three eggs hatched. Two did not.
That kind of partial success suggests slightly better protection from cold, better access to early season insects, or simply a stronger, better fed female able to remain on the nest longer. Sometimes a small edge is enough.
Native plants support more caterpillars. More caterpillars support better-fed females, more consistent incubation, and higher hatching success.
Disrupt one part, especially timing, and the effects ripple quickly.
Bluebirds can adjust how often they feed or what size prey they bring. But they do not easily replace caterpillars with something else. That dependence makes them vulnerable when spring does not behave the way it used to.
