There's usually a grain of truth somewhere, or some aspect that chimes with the reader's experience; the trouble is, it gets distorted and twisted as it develops, like Chinese whispers. In this case it seems that things that may be true of crested ducks have been (erroneously) assumed to be true of crested chickens. Disregard for breeds, or consequences of tiny gene pools, or other potentially significant factors are other common sources of confusion.
If you are interested, you might find this useful
https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2024.1389382/full
It discusses the Polish, Houdan, and especially Padovana breeds, with specific reference to where and when physical cranial issues may arise. Swedish Flowers are not mentioned, and the details demonstrate that what may be the case in one crested breed may not be the case in others or another, so generalizations about crested chicken breeds per se should be resisted.
In any case, a key point, and the main one for the OP's question, is that it does *not* have a negative health impact on birds so affected. "Of course, feather crests can obstruct the view (
Figure 2A), and it cannot be excluded that the pain-sensitive dura mater, which is more exposed under cranial foramina, can lead to impaired welfare. However, so far, there are no studies on this, and no studies have shown that the crest or cerebral hernia has a negative impact on brain size or cognitive abilities. The crest and the accompanying enlargement of the skull lead to a larger brain compared to other chicken breeds. However, the consequences of this larger brain for the animals are not yet known. Behavioral studies have not been able to show an advantage in cognitive or learning behavior (
Tiemann and Rehkämper, 2007).... The suitability of the term “cerebral hernia/herniation”, as used by several authors in this context (e.g.,
Yoshimura et al., 2012;
Verdiglione and Rizzi, 2018;
Watanabe et al., 2023), is certainly questionable, as cerebral herniation in the medical sense is usually understood to be a dislocation of parts of the brain in pre-existing foramina of the skull (especially foramen magnum) or under dural duplications. This dislocation usually causes acutely severe neurological symptoms and damage. This pathogenesis initially has nothing to do with the phenomenon of the morphological differences observed here between chickens with “cerebral hernia” and those without. These morphological differences are rather seen as anatomical variants or breed characteristics. We therefore believe that the term cerebral hernia/herniation should not be used in this context, as there are no clearly pathological clinical conditions associated with it... That the abnormal development of cerebral elongation leads to pre- and post-natal mortalities (
Tao et al., 2020) cannot be confirmed by our study. A hatching rate of almost 82% is quite acceptable and not worse than in other fancy poultry breeds (
Anderson Brown, 1988).
Tao et al. (2020) also postulated that the membranous skull is prone to mechanical pressure and injury, resulting in unconsciousness and large economic losses in the poultry industry. They assumed that all their results indicate that the brain is injured with dysfunction in chickens with cerebral hernia. In our opinion, there is no evidence that the brains of cerebral hernia-type chickens are damaged in general."
I am grateful for the alternate perspective that had me recheck my own view on this subject!
