See also Winters Are Good For Your Genes: Lynn Book Finds World Average IQ 90, Declining From North To South

Birds are generally selected for lightness, which makes having a big, heavy brain extra expensive. Nonetheless, some birds are notably clever, such as the crow family in higher latitudes and the parrot family in lower latitudes. Other birds, such as the chicken family, are pretty dumb. The grouse is a relative of the chicken that is found in cold climates and it has an especially small brain relative to its weight (up to 14 pounds).

Here’s an interesting scientific article from Nature Communications about the impact of highly seasonal climates on brain size in birds. It finds that in sub-arctic climates, birds tend to have either relatively large (e.g., ravens) or small brains (e.g., grouse) with few in-between.

Like most scientific papers, this one abjures providing readers with easily comprehensible examples until well into the paper. So I just provided you with some.

Published: 23 August 2019
Alternative ecological strategies lead to avian brain size bimodality in variable habitats
Trevor S. Fristoe & Carlos A. Botero

Abstract
The ecological contexts that promote larger brains have received considerable attention, but those that result in smaller-than-expected brains have been largely overlooked. Here, we use a global sample of 2062 species to provide evidence that metabolic and life history tradeoffs govern the evolution of brain size in birds and play an important role in defining the ecological strategies capable of persisting in Earth’s most thermally variable and unpredictable habitats. While some birds cope with extreme winter conditions by investing in large brains (e.g., greater capacity for planning, innovation, and behavioral flexibility), others have small brains and invest instead in traits that allow them to withstand or recover from potentially deadly events. Specifically, these species are restricted to large body sizes, diets consisting of difficult-to-digest but readily available foods, and high reproductive output. Overall, our findings highlight the importance of considering strategic tradeoffs when investigating potential drivers of brain size evolution.

Introduction
Coping with environmental variability can be challenging1, particularly if climatic conditions oscillate between opposing extremes (e.g., hot summers and cold winters); resources vary in type, abundance, or accessibility; and habitat structure changes dramatically within a lifetime. These ecological challenges are further amplified when environmental change is unpredictable because the ability to anticipate it through phenological shifts in physiology, morphology, or behavior is compromised. In these situations, individuals may partially buffer the negative effects of novel, extreme, or unexpected conditions through the rapid deployment of flexible or innovative behavioral responses. Accordingly, the cognitive buffer hypothesis posits that variable and unpredictable environments should favor enhanced encephalization (i.e., larger brains relative to body size) despite the high energetic and developmental costs associated with investment in neural tissue.

Tropical climates tend to provide more predictable weather, which allows greater specialization of species. The Creator’s notoriously inordinate fondness for beetles is manifested mostly in the tropics where bugs become remarkably specialized — e.g., one species will live on horizontal branches of a mahogany tree and another species will live on the vertical trunk. The software for dealing with horizontal environments and the software for dealing with vertical environments get hardcoded in the separate genes. In contrast, if you have to live in a climate that varies between hot in summer and very cold in winter, you probably can’t afford to evolve to be that specialized.

On the other hand, tropical parrots are clever birds. It’s not clear to me how limited they are by latitude. Feral parrots have become common in Los Angeles since the 1970s. More surprisingly large green South American monk parakeets have been spreading in Chicago since the 1960s.

Recent comparative studies on birds have lent support to these ideas by showing that highly encephalized lineages are overrepresented in cold, seasonal, and unpredictable high-latitude habitats and are better able to maintain more stable populations when conditions vary11 as compared to their small-brained counterparts. Nevertheless, not all birds that reside in these habitats year-round

Grouse are too heavy to fly south for the winter.

have relatively large brains. For example, the grouse (subfamily Tetraoninae) occur widely throughout highly seasonal and thermally unpredictable North temperate and arctic habitats but possess some of the smallest relative brain sizes known among birds. Perhaps more surprising is the recent suggestion that, among Galliformes (the larger taxonomic unit to which the grouse belong), there may be a trend toward smaller-than-expected brain sizes in increasingly variable environments. These conspicuous exceptions to the patterns predicted by the cognitive buffer hypothesis have received little attention in the scientific literature, reflecting a possible anthropocentric bias toward investigating the evolutionary forces directly relevant to our own encephalization. However, a better understanding of the evolution of brain size and cognition is likely to result from increased clarity on the ecological contexts that promote both relative brain enhancements and reductions.

… in contrast to birds living in more climatically stable regions of the world, resident birds in thermally variable habitats exhibit an overrepresentation of both very small and very large brains and a nearly complete absence of intermediately sized brains. We then provide initial evidence supporting the idea that this striking morphological pattern can be linked to the presence of alternative eco-morphological strategies that cope with the inherent challenges of extreme winter conditions by emphasizing different aspects of ecological and life history trade-offs imposed by the high metabolic demands of brain tissue.

… Our data indicate that the relative brain size of resident birds is approximately normally distributed throughout the world except in high-latitude regions characterized by harsh winters and highly variable and unpredictable temperatures. In those regions, large and small brain sizes are significantly overrepresented, and intermediate brain sizes are conspicuously absent (Fig. 1).

… In contrast to residents, migratory species that breed at high latitudes exhibit a fairly Gaussian brain size distribution that does not significantly deviate from null expectations (Fig. 2; total sample of n = 623 species). This finding suggests that the alternative brain size strategies found among resident birds in these regions are likely to be a consequence of the challenges experienced during the resource-poor and thermally extreme winter months. Such a possibility is consistent with the observation that well-known cases of alternative life history strategies have evolved from trade-offs imposed by limiting resources. Brain tissue is both metabolically and developmentally costly, so it is likely that investments in larger brains will limit the deployment or use of alternative solutions to the challenges of harsh winters. Thus small-brained species in variable habitats may allocate a greater proportion of their limited budget to other costly morphological and life history traits that facilitate persistence in these difficult environments. In the following two sections, we evaluate support for possible trade-offs between brain size and additional traits, specifically diet and reproductive output, and quantify the extent to which these trade-offs are further constrained by the extreme winter conditions experienced at high latitudes.

One possible way to minimize the impact of environmental fluctuations and winter scarcity is to specialize on resources that show little variation in availability as a function of climate. Buds, twigs, and conifer needles tend to be abundant year-round in many high-latitude habitats and are typically easily accessible even when snow is plentiful. However, these readily available food items are potentially incompatible with the high metabolic demands of large brains because they are fibrous and require a large, energetically costly gut to digest. Indeed, a trade-off between diet and brain size is evidenced in our global sample of resident birds. … Specifically, while small-brained species can include any amount of vegetative plant material in their diet (even to the point of being able to subsist almost exclusively on these food items), large-brained ones cannot.

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