Are birds less fecund in Australia than in southern Africa?, part 1

@ludwig_muller @alexanderr @rion_c @surfinbird @subirshakya @lukedowney @karimhaddad @colin25 @bushboy @tyroneping @calebcam @joshuagsmith @george_seagull @jadonald @gumnut @ratite @ptexis

It seems reasonable to suspect that birds in Australia are in some sense less fecund than those in southern Africa.

The main reasons are as follows:

  • the peculiar incidence of extremely adapted snakes (Dasypeltis, https://www.inaturalist.org/observations?place_id=any&taxon_id=26716&view=species) that specialise on birds' eggs in southern Africa,
  • the incidence of various, partly bird-eating, carnivorous mammals (e.g. felids, herpestids) and birds in southern Africa, with no counterparts in Australia, and
  • the generally lesser fecundity of mammals (whether metatherian or eutherian) in Australia than in southern Africa.

Reproduction and growth are generally relatively slow in marsupials. Furthermore, even in rodents the Australian forms tend to be less fecund than their southern African counterparts.

Do birds in Australia generally lay fewer eggs, or smaller eggs (or both), than those in southern Africa - given a relevant comparative basis?

To make intercontinental comparison as rigorous as possible, I chose study areas carefully matched in climates, landforms and soils: Fitzgerald River National Park and its environs in Western Australia and Agulhas National Park and its environs in South Africa (see https://www.inaturalist.org/journal/milewski/62510-no-evolutionary-convergence-between-australia-and-southern-africa-in-snakes-and-legless-lizards-part-1#).

First, let us examine those birds that nest at or below ground-level (see part 2 for faunal lists plus information on number of eggs per clutch, and size of eggs).

The ground-nesting avifaunas in the Australian study area (about 30 spp. with eggs of diameter less than 40 mm) are phylogenetically closer to those in the southern African study area (about 37 spp.) than is true for the faunas of mammals or reptiles. At least ten genera of ground-nesting birds are shared intercontinentally within this study.

The only ground-nesting birds present in the Australian study area but with no counterparts in the southern African study area are one species of parrot and one species of pardalote.

Approximate counterparts on the two continents (ducks, quails, bustards, oystercatchers, plovers, nightjars, bee-eaters, pipits, 'wrens' and 'chats) generally lay similar numbers of eggs, of similar size, per clutch.

However, one reason why there are fewer ground-nesting species in the Australian than in the southern African study area is that the several genera of larks (Alaudidae), common in the southwestern Cape of South Africa, have no counterparts in winter-rainfall Western Australia. The species, present in or near Agulhas National Par, lay 2-3 eggs, diameter 15-17 mm, per clutch (see part 2).

Anthus novaeseelandiae (usually 3-4 eggs, diameter 16-17 mm) is closely related to, and reproductively indistinguishable from, Anthus cinnamomeus in South Africa. However, there is an additional, common motacillid (Macronyx, egg diameter 18 mm) in the southern African study area.

Chats that forage and nest mainly on the ground (e.g. Saxicola) in the southern African study area lay about 4 eggs, of diameter about 15 mm. In the Australian study area the only possible counterpart to the larks and chats is the meliphagid 'chat' Epthianura, which lays about 3 eggs, of diameter about 14 mm.

Quails and button-quails lay eggs on the ground in both study areas (Coturnix 7-10 eggs, diameter 22-23 mm; Turnix 4 eggs, diameter 19-23 mm). Coturnix is fecund on both continents but it is rarer in the Australian than in the southern African study area.

Two additional phasianids (Pternistis, Scleroptila) lay large clutches (more than 5 eggs of diameter less than 38 mm). Their only counterpart in the Australian study area is Leipoa ocellata, which differs from all African birds in burying its eggs - which are unusually large (diameter 61 mm).

'Wrens' and nightjars lay slightly fewer eggs per clutch in the Australian than in the southern African study area. For example:

  • Drymodes 1 vs Chaetops, 2, Cercotrichas 2-3,
  • Dasyornis 2 vs Sphenoeacus 2-3,
  • Sericornis and Calamanthus sometimes as few as 2 vs Cisticola never 2 but sometimes as many as 5,
  • the caprimulgid in the Australian study area (1 egg, diameter 25 mm) vs that in the southern African study area (2 eggs, diameter 20 mm).

Focussing further on the shared clades of ground-nesting birds:

In both study areas there are the following plovers:

  • larger-bodied (Vanellus, 3-4 eggs, diameter 29-32, although now rare in the Australian study area), and
  • several smaller-bodied (Charadrius and allied genera, usually 2 eggs, diameter 20-24 mm).

Bee-eaters (Merops): about 4-5 eggs, diameter 18 mm in the Australian vs 22 mm in the southern African study area.

A guild of ground-nesting birds for which comparisons are complicated phylogenetically is the 'wrens': Drymodes, Dasyornis, Malurus, and Sericornis in the Australian vs Erythropygia, Sphenoeacus, Cisticola, and Chaetops in the southern African study area. Drymodes spp. are larger-bodied (body length without tail 12 cm) than Cercotrichas (body length without tail 8.5-10.1 cm; 9.7 cm in Cercotrichas coryphoeus), yet lay fewer eggs (1 and up to 2, vs 2-3 and up to 4).

Let us turn now to birds other than those nesting at or below ground-level.

Among 'warblers', Acanthiza and Stipiturus lay perhaps one egg fewer (3-4; 3 in the species most common in and near Fitzgerald River National Park) than does Prinia (2-5, usually 4) in or near Agulhas National Park. However, the difference is not borne out by Apalis (3 and up to 4).

Do nectar-, fruit- and insect-eating passerines in the Australian study area lay fewer eggs than those in the southern African study area? The answer is no.

Meliphagids lay no fewer eggs per clutch (1-3, usually 2) than promeropids and nectariniids. Zosterops, a shared genus, lays no fewer eggs in the Australian (2-4, usually 3) than in the southern African (2-3) study area.

Partly fruit-eating meliphagids lay scarcely fewer eggs (3 or less than 3, except for Manorina which often lays 4) than do the more specialised fruit-eaters (pycnonotids 2-3, coliids 3-4) in southern Africa. Pomatostomus (https://www.inaturalist.org/taxa/14491-Pomatostomus-superciliosus) lays 2-3 and up to 5, whereas Cossypha (https://www.inaturalist.org/taxa/12786-Cossypha-caffra) lays 2 and up to 3.

In both study areas, certain passerine birds nest colonially. These are omnivorous, aggressive species dependent on the most productive situations: small areas combining relatively rich alluvial soils, perennial moisture, tree growth, and bare ground with fast-growing annual plants.

The only species in this category in the Australian study area is the meliphagid (Manorina flavigula, https://www.inaturalist.org/taxa/12223-Manorina-flavigula). By contrast, several species occur in the southern African study area:

Species additionally relevant here are the ploceid Euplectes (https://www.inaturalist.org/taxa/13964-Euplectes-capensis), and the starling Lamprotornis (https://www.inaturalist.org/taxa/517007-Lamprotornis-bicolor).

Manorina (2-5) lays no fewer eggs than Ploceus (2-5) or Euplectes (2-4, usually 3).

However, the meliphagid is likely to be less productive of eggs than the ploceids. Males outnumber females in the meliphagid, whereas females outnumber breeding males in Ploceus. Furthermore, adolescents of the meliphagid assist a relatively small number of parents to rear offspring, whereas in the ploceids a) the only cooperative breeding is nest-sharing, and b) males alone build the nests, leaving females free to devote all their energy to laying and provisioning.

Lamprotornis bicolor does not breed gregariously. However, it often nests in burrows, accessible to snakes; and it often lays two clutches (each of usually 4 eggs but with a range of 2-6) in a given season (https://en.wikipedia.org/wiki/Pied_starling#Breeding).

The fecundity of phasianids is noteworthy. Members of this family are common in the southern African study area but rare in the Australian study area. They lay eggs small, numerous, exposed, and localised enough to be viable for the dietary specialisation of Dasypeltis scabra (https://www.inaturalist.org/taxa/26717-Dasypeltis-scabra).

Several relevant kinds of birds present in or near Agulhas National Park have no counterparts in or near Fitzgerald River National Park. Since most birds breed at a restricted time of year in winter-rainfall climates, an egg-eating snake depends on large concentrations of eggs to allow it to lay down enough fat to tide it over seasonally.

Thus, the ability of the southern African study area to support a specialised egg-eating snake may be owing to a combination of:

  • the presence of ploceids that breed gregariously,
  • the presence of starlings that nest below ground level, and
  • the commonness of phasianids.

Summarising more generally:
For most families/genera/guilds of birds, any intercontinental differences in the number of eggs per clutch, or the size of eggs, are negligible. However, in a few important cases the Australian study area has no counterparts for particularly fecund birds, such as relatively large-bodied phasianids. This may help to explain the differences between continents in the incidence of bird-eating predators and in particular Dasypeltis.

Please see part 2 for faunal lists of the ground-nesting birds in the study areas...

Publicado el marzo 5, 2022 10:33 TARDE por milewski milewski

Comentarios

Also note the difference in cooperatively breeding birds.

Publicado por milewski hace casi 2 años

@alexanderr Not only is there no predator specialising on birds' eggs in the Australian study area, but even among the snakes with generalised diets there do not seem to be any records of birds' eggs being eaten. Juveniles of Notechis scutatus (see https://nre.tas.gov.au/wildlife-management/fauna-of-tasmania/reptiles-and-frogs/tasmanian-snakes/tiger-snake) and particularly Morelia imbricata (see https://www.aussiepythons.com/threads/do-carpet-pythons-go-after-eggs.202983/) may eat birds' eggs, because both climb woody plants and take various prey. However, the main reptile eating birds' eggs in the Australian study area is likely to be Varanus rosenbergi (https://www.inaturalist.org/taxa/39441-Varanus-rosenbergi).

Publicado por milewski hace casi 2 años

The following pattern is evident in comparison of the avifaunas in the Australian and southern African study areas. Pternistis, Telophorus, Saxicola, and Estrilda all occur in open vegetation in and near Agulhas National Park. The approximate counterparts in the Australian study area, namely Leipoa, Psophodes, Eopsaltria, and Emblema, tend to be restricted to the tallest, densest vegetation. Possible reasons include: a) predation by non-indigenous carnivores has eliminated these forms from open vegetation, and b) under the particularly oligotrophic conditions of Australia, insufficient food is produced unless there is accumulation of litter.

Publicado por milewski hace casi 2 años

Telophorus zeylonus (body mass 65 grams, body length 230cm, https://www.inaturalist.org/taxa/8188-Telophorus-zeylonus) corresponds approximately to an intermediate between Psophodes nigrogularis (24-25.5 cm, https://www.inaturalist.org/taxa/117179-Psophodes-nigrogularis) and Oreoica guttural's (20-23 cm, https://www.inaturalist.org/taxa/8537-Oreoica-gutturalis).

Publicado por milewski hace casi 2 años

That makes sense, African varanids love eggs too, but do you think that they have filled the niche and excluded snakes from taking advantage of it. What about squirrels in Europe, they eat eggs right? Perhaps they filled that niche too and excluded Dasypeltis, or displaced them or similarly adapted species?

Publicado por alexanderr hace casi 2 años

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