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...continued from https://www.inaturalist.org/journal/milewski/95204-a-new-interpretation-of-the-evolutionary-and-ecological-strategy-of-whiteyes-silvereyes-zosterops-part-2-a-nectarivorous-species-z-chloronothos-as-an-exception-proving-a-rule#
INTRODUCTION
Zosterops is among the more encephalised (https://dictionary.apa.org/encephalization) of birds.
This is based on brain mass relative to body mass (corrected allometrically, https://en.wikipedia.org/wiki/Allometry).
This braininess may help to explain how Zosterops has adapted to various biomes, on many islands as well as on mainlands, with minimal morphological modification.
Adaptation in this genus seems to be mainly a matter of behavioural versatility, i.e. via 'software' rather than 'hardware'.
The cognitive capacity of Zosterops may be evident, for example, in
- vocal mimicry,
- discrimination among subspecies, allowing these coexist for part of the year without loss of subspecific integrity, and
- ability to stitch a cup-shaped nest between adjacent flimsy stems in the crowns of various trees, despite various stem-configurations according to the type of tree.
There is a correlation between vocal mimicry and encephalisation (https://cdnsciencepub.com/doi/10.1139/z03-190 and https://www.perplexity.ai/search/Is-there-a-8dJObx_JS0CR6jhS3.Dl4g and https://www.animalecologylab.org/the-mimics-among-us.html).
Birds capable of mimicking the calls of other birds tend to be relatively brainy.
A noteworthy phenomenon, in Zosterops lateralis (https://www.inaturalist.org/taxa/202505-Zosterops-lateralis) in southeastern mainland Australia, is that several subspecies overlap in geographical distribution.
In the normal concept of subspeciation, geographical separation is necessary to maintain the distinctiveness of subspp. In Z. lateralis, the nomadic/migratory movements in the non-breeding season mix several subspp. (https://avithera.blogspot.com/2014/04/silvereyes.html). However, this has not compromised the subspecific distinctions.
This ability to coexist without interbreeding seems to reflect cognitive capacity, possibly extending to 'culture'.
All of the above raise the question:
How brainy is Zosterops, compared to other like-size, small birds?
AIMS
In this Post, I compare Zosterops with other birds in terms of encephalisation, i.e. braininess.
SOURCE OF DATA AND BASIS OF COMPARISON
My reference for body mass and brain mass in various spp. of birds is the data-set compiled by Andrew N Iwaniuk (https://www.ulethbridge.ca/artsci/neuroscience/dr-andrew-iwaniuk). The data refer to adults of both sexes, in all spp.
METHODS
I searched the entire data-set for bird spp. with body mass similar to that of Zosterops, viz. about 10.4 g. I then compared the brain masses on the basis of body masses similar to that in Zosterops.
RESULTS
The mean data for Zosterops are
- body mass 10.2 g, brain mass 0.54 g in Zosterops japonicus (n=10), and
- body mass 10.6 g, brain mass 0.47 g in Zosterops lateralis.
This shows that Zosterops has brain mass 0.50 g, at body mass 10.4 g.
Let us now compare this brain mass with those of various other birds with similar body masses.
Families are in alphabetical order.
Apodidae (https://en.wikipedia.org/wiki/Swift_(bird)) :
I found two spp., in two genera, with body masses 9.9-13.6 g. Brain masses were 0.28-0.30 g, indicating that Zosterops clearly exceeds apodids in braininess.
Apodids attain brain mass of 0.5 g only at body masses > 25 g, e.g. in Chaetura pelagica (https://www.inaturalist.org/taxa/6571-Chaetura-pelagica), which has brain mass 0.46 g at body mass 23.6 g. (n=8).
Certhiidae (https://en.wikipedia.org/wiki/Treecreeper) :
I found four spp., in four genera, with body masses 9.8-11.0 g. Brain masses were 0.51-0.53 g, indicating that certhiids exceed Zosterops in braininess.
Cisticolidae (https://en.wikipedia.org/wiki/Cisticolidae) :
I found two spp., in one genus, with body masses 10.0-10.3 g. Brain masses were 0.43-0.51 g, indicating that Zosterops slightly exceeds cisticolids in braininess.
Fringillidae (https://en.wikipedia.org/wiki/Finch) :
I found 15 spp., in 14 genera, with mean body masses 8.9-12.0 g. Brain masses were somewhat variable, with particular braininess evident in Melopyrrha nigra (https://www.inaturalist.org/taxa/10266-Melopyrrha-nigra), which had brain mass 0.84 g at body mass 10.9 g (n=7).
Overall, the data indicate that Zosterops resembles fringillids in braininess.
Hirundinidae (https://en.wikipedia.org/wiki/Swallow) :
I found two spp., in two genera, with mean body masses 9.7-11.2 g. Brain masses were 0.36-0.43 g, indicating that Zosterops exceeds hirundinids in braininess.
Lybiidae (https://en.wikipedia.org/wiki/Lybiidae) :
I found only one comparable species, with body mass 15.5 g and brain mass 0.44 g, indicating that Zosterops exceeds lybiids in braininess.
Maluridae (https://en.wikipedia.org/wiki/Australasian_wren) :
I found three spp., in one genus, with mean body masses 9.8-11.4 g. Brain masses were, on average, about 0.48 g, indicating that Zosterops slightly exceeds malurids in braininess.
Melanocharitidae (https://en.wikipedia.org/wiki/Melanocharitidae) :
The data indicate that Zosterops exceeds melanocharitids in braininess.
Meliphagidae (https://en.wikipedia.org/wiki/Honeyeater) :
I found nine spp., in seven genera, with mean body masses 7.8-11.9 g in Myzomela, and 10.3-11.4 g in other genera. Brain masses were somewhat variable. However, they indicate that, overall, Zosterops exceeds meliphagids in braininess.
Monarchidae (https://en.wikipedia.org/wiki/Monarch_flycatcher) :
I found two spp., in two genera, with mean body masses 10.2-11.0 g. Brain masses were 0.52-0.59 g, indicating that monarchids exceed Zosterops in braininess.
Muscicapidae (https://en.wikipedia.org/wiki/Old_World_flycatcher) :
I found five spp., in five genera, with mean body masses 7.5-15.4 g. Brain masses were 0.31-0.6- g. One species, viz Ficedula albicollis (https://www.inaturalist.org/taxa/13133-Ficedula-albicollis, n=10), closely resembled Zosterops in mean body mass (10.3 g), and had brain mass 0.45 g.
Overall, these data indicate that Zosterops exceeds muscicapids in braininess.
Nectariniidae (https://en.wikipedia.org/wiki/Sunbird) :
I found two spp., in two genera, with mean body masses 8.9-11.7 g. Brain masses were, on average, about 0.45 g, indicating that Zosterops exceeds nectariniids in braininess.
Pardalotidae (https://en.wikipedia.org/wiki/Pardalote) :
I found three spp., in two genera, with mean body masses 9.2-11.6 g. Brain masses were, on average, about 0.45 g, indicating that Zosterops exceeds pardalotids in braininess.
Paridae (https://en.wikipedia.org/wiki/Tit_(bird)) :
I found three spp., in one genus, with mean body masses 10.2-11.3 g. Brain masses were, on average, about 0.65 g, indicating that parids are brainier than Zosterops.
Passeridae sensu lato, including Estrildidae (https://en.wikipedia.org/wiki/Old_World_sparrow and https://en.wikipedia.org/wiki/Estrildidae) :
I found seven spp., in six genera, with mean body masses 9.2-10.9 g. Brain masses were, on average, about 0.45 g, indicating that Zosterops exceeds passerids in braininess.
Petroicidae (https://en.wikipedia.org/wiki/Australasian_robin) :
I found three spp., in two genera, with mean body masses 9.6-11.4 g. Brain masses were, on average, about 0.52 g, indicating that petroicids are slightly brainier than Zosterops.
Picidae (https://en.wikipedia.org/wiki/Woodpecker) :
I found two spp., in two genera, with mean body masses 8.1-11.0 g. Brain mass was 0.58-0.60 g, indicating that picids exceed Zosterops in braininess.
Rhipiduridae (https://en.wikipedia.org/wiki/Rhipiduridae) :
I found one species, with body mass 10.2 g (n=7). Brain mass was 0.36 g, tentatively indicating that Zosterops exceeds rhipidurids in braininess.
Sittidae (https://en.wikipedia.org/wiki/Nuthatch) :
There is only one genus in this family, represented in the data-set by Sitta canadensis (body mass 10.5 g, brain mass 0.57 g, n=6). This indicates that sittids exceed Zosterops in braininess.
Sylviidae (https://en.wikipedia.org/wiki/Sylviidae) :
I found only one species (n=7), with mean body mass 10.8 g. Brain mass was 0.45 g, tentatively indicating that Zosterops exceeds sylviids in braininess.
Thamnophilidae (https://en.wikipedia.org/wiki/Antbird) :
I found four spp., in four genera, with body masses 9.3-11.2 g. Brain masses (means) were 0.45-0.7 g, indicating that thamnophilids are slightly brainier than Zosterops.
Trochilidae (https://en.wikipedia.org/wiki/Hummingbird) :
I found only one species large-bodied enough to be comparable, with mean body mass 10.2 g. Brain mass was 0.32 g, indicating that Zosterops exceeds trochilids in braininess.
Tyrannidae (https://en.wikipedia.org/wiki/Tyrant_flycatcher) :
I found eight spp., in eight genera, with mean body masses 8.6-11.9 g. Brain masses were, on average, about 0.37 g, for six of the spp., indicating that Zosterops exceeds tyrannids in braininess.
The brain mass of Zosterops (about 0.5 g) is attained in these tyrannids only when body mass reaches about 14 g, which is >3.5 g heavier than Zosterops.
However, apparently brainier than other tyrannids are two spp., viz.
DISCUSSION
Brain mass, relative to body mass, varies up to 2.5-fold in small-bodied birds about the size of Zosterops. This can be seen in the following three carefully chosen examples:
The percentage value for Zosterops, viz. 4.8%, falls approximately in the middle of the above range (2.8-7.7%) of values.
However, Zosterops is somewhat brainier than most birds that are comparable on a basis of matched body mass but different familial affinity.
The following summarises the ranking of Zosterops in braininess, relative to like-size, small birds, family by family.
Zosterops is less brainy than
- monarchids,
- parids,
- picids, and
- sittids.
Zosterops is somewhat/slightly less brainy than
- petroicids,
- thamnophilids, and
- one or two genera of tyrannids.
Zosterops is similar in braininess to
Zosterops is somewhat/slightly brainier than
- cisticolids, and
- malurids.
Zosterops is brainier than
- certhiids,
- hirundinids,
- lybiids,
- melanocharitids,
- meliphagids,
- muscicapids,
- nectariniids,
- pardalotids,
- passerids,
- rhipidurids,
- sylviids,
- trochilids, and
- most tyrannids.
Zosterops seems much brainier than
This means that Zosterops is brainier than like-size birds in most avian families.
However, Zosterops is less brainy than like-size birds in perhaps seven families, of which parids (tits) are the most renowned for their cognitive capacity (https://lup.lub.lu.se/search/files/31008096/e_spik_utku.pdf).
