Short answer: not their whole brain, and not as a proven annual rule. Research on black-capped chickadees (Poecile atricapillus) has found seasonal differences in the hippocampal formation, a memory-related region involved in remembering food-cache locations. A 1995 field study found that this region was relatively larger in October, when hoarding was most intense. Other studies found seasonal changes in cell counts or neuron recruitment, while several found no significant seasonal change in hippocampal size. The defensible conclusion is that chickadees can show seasonal plasticity in a particular brain region, but “the brain grows in fall and shrinks in spring” is too broad and too certain.
What part of a chickadee’s brain changes?
The hippocampal formation, not the entire brain
The seasonal finding concerns the hippocampal formation, a structure deep in the bird brain associated with spatial memory. For a food-storing chickadee, that memory can include where individual seeds were hidden and how to navigate back to them. The studies did not demonstrate that the entire brain expands every autumn or contracts every spring.
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Most measurements were also relative or regional. A report that the hippocampal formation occupied a larger share of the brain is not the same as a measured increase in total brain mass. Likewise, a change in cell number, cell density, neuron production or neuron survival is not interchangeable with a change in the region’s volume.
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Why autumn is an important test
Black-capped chickadees cache food during periods when future supplies are less predictable. The 1995 field study found that hoarding intensity in its population peaked in October, making that month a useful point for asking whether the memory system used for caching also varies seasonally. The proposed relationship is biologically plausible, but an association between hoarding and hippocampal size does not prove that caching alone caused the anatomical difference.
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What the major studies actually found
| Study | Birds and setting | Sampling or treatment | Primary outcome | Key result |
|---|---|---|---|---|
| Smulders, Sasson and DeVoogd (1995) | Wild black-capped chickadees in upstate New York | Six times of year | Hippocampal-formation volume relative to the rest of the brain | Relative volume was highest in October, coinciding with peak hoarding intensity. |
| Smulders, Shiflett, Sperling and DeVoogd (2000) | Juveniles and adults | Seasonal comparisons | Cell density and total cell number | Fall volume variation was associated with increases in small and large cells, principally neurons; rostral cell density rose in August and later declined toward October. |
| Hoshooley and Sherry (2004) | Wild birds | Groups captured from October through March | Neuron production, apoptosis, volume and neuron number | No sampled groups differed significantly on these measures; the authors interpreted earlier recruitment differences as more consistent with improved survival of new neurons than with increased production. |
| Hoshooley and colleagues (2007) | Adult male birds | October, January, April and July | Hippocampal size and neuronal recruitment | No significant annual hippocampal-size change; telencephalon volume was larger in October than July, while recruitment peaked in January. |
| Photoperiod experiment (2003) | Captive chickadees | Different day-length treatments | Food-storing behavior and hippocampal volume | Day length altered storing behavior, but groups did not differ in hippocampal volume. |
| Captivity comparison (2009) | Adult birds held captive versus birds living in the wild | Six weeks in fall | Hippocampal size | The captive birds’ hippocampi were reported as 23% smaller than those of wild birds; this was a captivity comparison, not a measure of spring shrinkage. |
How the original “fall growth” finding was obtained
The 1995 field comparison
Researchers sampled a natural upstate New York population at six points across the year. They measured the hippocampal formation and compared its size with the rest of the brain. October birds had the largest relative hippocampal formation among the sampled periods, and October was also when the population cached the most food.
This design shows a seasonal correlation in free-living birds. It does not establish a mechanical chain in which every extra cache causes tissue growth, nor does it show that the region must become smaller in spring. Temperature, food availability, social conditions, movement, stress and other seasonal factors could vary at the same time as hoarding.
The 2000 cell-count study
A later study examined juveniles and adults and estimated both cell density and total cell number. Seasonal volume differences were accompanied by increases in small and large cells, principally neurons, during fall. In the rostral hippocampus, cell density rose in August and then declined toward October. The authors suggested that net cell addition may begin in August.
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“May begin” matters here: the timing was an interpretation of the observed pattern, not proof of a fixed annual program that operates identically in every chickadee population. Cell density can fall even while total cell number rises if the tissue volume changes, so density and total counts must be read together.
Why later research made the story less simple
Neuron production is not the same as neuron survival
New neurons can be generated, incorporated into circuits, or lost. Those are separate biological events. A cell-birth measurement addresses production; a survival analysis asks how many of those cells remain; neuronal recruitment refers to addition or incorporation into the population being studied. Regional volume is another outcome again.
In the 2004 study, wild chickadees captured between October and March did not differ in neuron production, apoptosis, hippocampal volume or neuron number. The authors concluded that the higher fall neuronal recruitment reported in earlier work was not explained by a surge in neuron production and was instead more compatible with enhanced survival of new neurons. Because the sampled groups were similar on volume and neuron number, this result does not establish a general October-to-spring shrinkage cycle.
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The four-season annual-cycle study
The 2007 study sampled adult males in October, January, April and July. More birds stored food in October, but hippocampal size did not show a significant annual change. Telencephalon volume—the larger forebrain region that contains the hippocampus—was larger in October than in July, while hippocampal neuronal recruitment reached its peak in January.
Those results separate behavior, recruitment and gross regional size. A bird can show a seasonal shift in the addition or retention of neurons without producing a statistically detectable change in the measured size of the hippocampal formation. The study’s abstract states: “Previous research presents a mixed picture of seasonal variation in the hippocampus of food-storing black-capped chickadees.” That is a better summary of the evidence than a universal growth-and-shrink slogan.
What laboratory and captivity results add
Changing day length did not change measured hippocampal volume
In a 2003 captive photoperiod experiment, day-length treatments changed food-storing behavior but did not produce significant differences in hippocampal volume among groups. This shows that a detectable behavioral response does not automatically produce a detectable volume response in a short or controlled experiment. It does not rule out effects of experience, environment or other seasonal cues in wild birds.
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Captivity can alter the comparison
The 2009 study compared adult chickadees held in captivity for six weeks in fall with birds living in the wild. The captive birds had hippocampi reported as 23% smaller than those of wild birds. That percentage describes the difference between those two living conditions in that study. It is not the percentage by which a wild chickadee’s hippocampus shrinks between autumn and spring.
Captivity can change activity, stress, diet, movement, social contact and opportunities to cache. For that reason, a captive-versus-wild result cannot be substituted for a repeated seasonal measurement in the same wild population. The study also described limits on interpreting its measures of newly generated cells, so the size difference should not be used as proof of one specific cellular mechanism.
How to interpret the evidence without mixing unlike measurements
- Regional volume: the physical size of the hippocampal formation, sometimes expressed relative to the rest of the brain.
- Total cell number: how many cells are estimated in the region; this can change independently of density.
- Cell density: cells per unit of tissue volume. A density change does not by itself reveal whether cells were added or the tissue expanded.
- Neuron production: generation of new neurons.
- Neuron survival: persistence of those cells after they are generated.
- Neuronal recruitment: the net appearance or incorporation of neurons in the measured population; it reflects production and survival together.
When a headline turns any one of these outcomes into “the brain grows,” it collapses distinct measurements into one claim. The studies also differed in whether birds were wild or captive, which months were sampled, whether juveniles or adults were included, whether only males were studied, and whether caching was observed or experimentally manipulated. Those design differences can produce apparently conflicting results without making any one measurement meaningless.
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Does a chickadee’s brain shrink in spring?
No study in this body of work establishes a routine, measured spring shrinkage of the whole brain. The 1995 result identified an October relative peak in hippocampal formation volume. The 2004 October-to-March comparison found no group differences in hippocampal volume, and the 2007 four-season study found no significant annual change in hippocampal size. No reliable percentage for normal fall growth or spring shrinkage has been established from these studies.
The most accurate wording is that some chickadees show seasonal plasticity in hippocampal structure or cellular measures, with an October volume peak reported in one field study and different patterns reported elsewhere. Whether a particular population shows a size change depends on the birds, environment, months sampled and measurement used.
Why this matters beyond one bird species
Chickadees became an important model for adult brain plasticity because their ecology poses a clear memory challenge: thousands of food items may be hidden and later recovered. The research demonstrates that an adult vertebrate brain can be responsive to season and behavior, but it also illustrates why behavioral neuroscience needs precise endpoints. A change in performance, neuron survival or regional volume may each be real while reflecting different processes.
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For readers, the practical takeaway is twofold. First, the hippocampal formation is a plausible neural substrate for seasonal demands of food caching. Second, the evidence does not justify picturing a chickadee’s entire brain inflating in autumn and deflating in spring. The strongest scientific answer keeps the intriguing observation—the October hippocampal peak in the foundational field study—while acknowledging that subsequent experiments and annual surveys produced a mixed picture.
Quick Recap
Study records
- Smulders, Sasson and DeVoogd, Journal of Neurobiology (1995), DOI 10.1002/neu.480270103.
- Smulders, Shiflett, Sperling and DeVoogd, Journal of Neurobiology (2000), DOI 10.1002/1097-4695(20000915)44:4<414::AID-NEU4>3.0.CO;2-I.
- Hoshooley and Sherry, Behavioral Neuroscience (2004), DOI 10.1037/0735-7044.118.2.345.
- Hoshooley and colleagues, Brain, Behavior and Evolution (2007), DOI 10.1159/000096984.
- Photoperiod experiment, Animal Behaviour (2003), DOI 10.1006/anbe.2003.2113.
- Captivity comparison, PLOS ONE (2009), PMCID PMC4597778.
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