Seasonal movements in the Australian honeyeaters (Meliphagidae) and their ecological significance

Abstract

(1) The Meliphagidae (honeyeaters), the largest Australian bird family (69 species), have diverse and complicated patterns of seasonal movements. These are defined and their evolutionary, ecological and competitive significance discussed.

(2) A study of regional faunas shows that there is a broad correlation between the amount, and reliability, of rainfall and the proportion of species undertaking seasonal movements. Where the rainfall exceeds 50 inches per year and is well distributed, 70–85% of species may be resident (Cairns–Atherton area of north Queensland). At 40 inches (Sydney area) 65% of the species are resident. In the north and north-west of the continent (rainfall 15–36 inches but all falling in the hot summer, the winters being dry) only 37–66% are resident, depending on locality. In the central desert (rainfall 8–11 inches, erratic in distribution) similar percentages apply, the resident species commonly being less than 50%.

(3) Where, within a regional area, there is a steep transition in rainfall zones these broad correlations do not necessarily apply, vide in south-western Australia. Here, apparently, the birds arc influenced by the conditions, especially flowering, in adjacent areas.

(4) At the vegetation level rain forests have the greatest proportion of resident species, sclerophyll forests somewhat less, the central mulga and spinifex areas fewest.

(5) There is a good correlation between the number of meliphagids inhabiting an area, the percentage that breed locally, and the percentage that are resident. Climatically stable areas with good rainfall and that are botanically diverse permit the coexistence of the largest number of species (e.g. 30 in the Cairns–Atherton rain forest tract), the highest proportion of residents (70–85% here) and the highest proportion of breeders (95%).

(6) Temperature, per se, is of little, or no, importance as a factor causing seasonal movements in the Australian Meliphagidae.

(7) Flowering of the major nectar-bearing trees and shrubs is the all important factor. In any area the blossoming of a relatively few species accounts for most of the seasonal movements. Most movements take place in summer, autumn, or winter, in southern Australia, that is outside of the breeding season. In the interior and north they tend to be less regular, particularly in dry years when breeding is inhibited. The erratic nature of many movements is mostly accounted for by irregular blossoming, varying nectar flows from year to year, and by the major nectar-bearing trees flowering at different times in different places.

(8) There is a broad correlation between the degree to which movements are developed in a species and its dependence on nectar as food: contrast the markedly nectarivorous Anthochaera carunculata, Lichmera indistincta, Zanthomiza phrygia, Myzomela sanguinolenta and M. pectoralis, and the various Philemon, with Manorina, Myzantha, Stomiopera, and Meliphaga melanops. The long-billed Acanthorhynchus tenulrostris is an exception; this nectar-feeder is possibly resident because of its ability to utilize a wide-range of blossoms, including the tubular Epacris unavailable to other species.

(9) Other factors initiating seasonal movements include the need to obtain minimal breeding requirements (vide interior species like Myzomela nigra, and Certhionyx variegatus concentrate for breeding wherever the right plants are in flower and there is an abundance of insects, essential for feeding the young). In contrast with certain other families there are only few authenticated cases of honeyeaters concentrating in large numbers outside of the breeding season where there are insect plagues. These mostly concern predominantly insectivorous genera like Melithreptus. The movements of berry-feeding species may be correlated with times of ripening, especially Grantiella picta. Droughts may cause erratic occurrences of honeyeaters outside of the normal range and marked shifts in abundance.

(10) Seven categories of seasonal movements may be recognized in the Australian Meliphagidae at the species level:

(i) Latitudinal (south-north) migration – one or two species, plus regional populations of two others.
(ii) Altitudinal migration – regional populations of three species.
(iii) Resident species that do not undertake seasonal movements – 24 species (35% of the total).
(iv) Resident species that undertake local movements – 10 species (15%).
(v) Species in which nomadic habits are moderately well developed – 12 species (17%).
(vi) Blossom nomads undertaking random movements of wide amplitude – 10 species (15%).
(vii) True desert nomads – 3 species (4-5%).

Some of the categories overlap (especially (iii) and (iv)), and it is uncertain to which category a few species should belong.

(11) Usually the kind of seasonal movement undertaken is sufficiently constant to be regarded as a species character, although minor shifts are not uncommon towards the periphery of range. About eight species (15%), however, have basically different patterns of movement in one or more parts of their range, hence they can be said to vary geographically in this character as races do morphologically. Several of these instances concern populations in the isolated forest segment of south-western Australia, a faunistically impoverished area where competitive interrelationships are different from the east.

(12) Different patterns of seasonal movement have a function in reducing interspecific competition for food in the Australian Meliphagidae.

(13) Wing morphology varies with extent of seasonal movements, a long pointed wing characterizing Myzomela nigra, Certhionyx variegatus, and Grantiella picta, whose movements extend the length of the continent. Within Myzomela there are relative differences in wing-length between species undertaking extensive, moderate, and no, seasonal movements. In other genera, and in the case of regional populations within species, slight to moderate differences are not, however, reflected in changed wing-form. This could be because the meliphagid wing is already proportioned for considerable mobility.