Merri BirdWatch data – scientifically valuable

May 13 2014

Our bird surveys since November 2008 have amassed a significant body of data. Susanna Bryceson has analysed the data from 199 surveys as part of her Biological Sciences studies at La Trobe University. The analyses explored bird diversity along the Merri’s rural to inner-urban corridor, relationship to habitat types and to rainfall, and aspects of seasonal migration. Here are some of her interesting findings.

Overall results:  Over the five years of surveys, 117 species were recorded. The most commonly-recorded species are shown in Table 1. Of the 20 species recorded in more than half the surveys, six are exotic (non-native) species. The main families represented were honeyeaters; pardalotes and thornbills; ducks, geese and swans; parrots; eagles and kites; cockatoos; and cormorants. The average for all surveys was 27 species, with a maximum of 40 species at Galgi Ngarrk in February 2011.

In terms of feeding guilds, 64% of species are insect-feeders, seed eaters comprise 25% including 16% that are also insectivorous, and 22% of species are aquatic feeding. Omnivores make up 15%, and carnivores 9%.

Table 1. Most commonly recorded species in the Merri bird surveys, 2008-2013.

Species No. of surveys (total 199)
Australian Magpie 194
Common Blackbird 180
Little Raven 178
Red Wattlebird 178
Spotted (Turtle) Dove 173
Common Myna 170
Magpie-lark 167
Pacific Black Duck 165
Rainbow Lorikeet 160

Changes from rural to inner urban: The FoMC ten survey sites extend from the rural-urban fringe in the north (Galgi Ngarrk near Craigieburn) to inner-urban Clifton Hill. A number of researchers elsewhere have reported a gradient of declining bird diversity from the rural edge to the central city, with exotic species increasing towards the city centre. Does this occur along the Merri? Well, no; the average species diversity shows no clear trend from urban fringe to inner urban sites. Indeed, the most diverse site is in East Brunswick (31.8 average species), and the Clifton Hill site scores slightly higher than the outer urban Bababi Marning. There is no consistent trend of increasing exotic species towards the inner city; in fact the inner areas recorded a lower proportion of exotic species (22.3%) compared with the outer areas (31.2%). On the other hand, carnivores (birds of prey) are mostly found in the remnant vegetation (outer) sites and are almost absent from the inner urban sites.

Habitat type: The ten survey sites can be classified into three main habitat types: remnant indigenous vegetation (mainly grassland), lake-wetlands, and sites revegetated with indigenous shrubs, trees and ground layer plants since the 1980s, along with areas of mown grass. Sue Bryceson found that bird diversity (i.e. the number of species present) varies with habitat type all along the Merri corridor. She concludes that vegetation type is probably more important than distance to the urban fringe in determining which and how many species occur at any site. Revegetation sites had the highest species diversity. In the grassland sites, many species were only there at particular times of year, reflecting habitat fluctuations over the seasons. The shrub-tree plantings at revegetation sites provide more stable habitat. The range of ecological niches created in the revegetation sites may attract and concentrate a broad range of species that have few other local alternatives.

“That the proportion of exotic birds per survey falls across the 10 sites towards the inner city rather than the other way round….shows the great value of the revegetated Merri Creek landscapes for attracting and retaining native birds, particularly given that exotic bird dominance is expected to be high in the immediate surrounding urban matrix.”

Migration: On the assumption that seasonal and rainfall-related migration patterns are likely to be better revealed with species that occur sporadically, Sue analysed the occurrence of 38 species recorded in less than 20% of surveys, along with another four species. At least 24 of these 42 species are seasonal visitors, and some have strong associations with particular sites each year. “A number of species appeared to strictly favour the grassland sites and have strong seasonal fluctuations. For example, Spiny-cheeked Honeyeaters and Mistletoebirds appeared in May, robins were most abundant in May and August; and cuckoos favoured August and November. In November, White-necked Heron, Rufous Whistler, Rufous Songlark were usually recorded, but rarely at other times.” In the revegetation sites, notable migrants were Striated Pardalote, Eastern Spinebill and currawongs over the cooler months. “This survey shows that the urban area can be far more than an inhospitable bloc disrupting the habits of migrating birds. The Merri Creek corridor fragments the urban matrix and harbours patches of vegetation that can function as part of the greater regional migration patterns.” Studies elsewhere have found that pockets of riparian (riverside) vegetation in the urban area can serve as ‘convenience store’ stop-over sites for migrating native birds.

Rainfall:  At least 16 species showed a response to rainfall patterns. A number of Australian birds congregate in inland Australia to breed when there are occasional floods (and hence leave the Merri), while conversely some species might use the Merri corridor as a drought refuge.

Conclusions: Sue notes that “Researchers are increasingly acknowledging the value of ‘citizen-collected’ bird data and this has enabled analyses that, like this study, would otherwise have been too expensive to fund as formal research projects…The duration and extent of the Merri Creek corridor bird surveys has created a valuable scientific resource…” She concludes that “For community groups to record 30 species at a site in 90 minutes is surprisingly high given the surrounding urban matrix, and is comparable to numbers attained in healthy natural areas” (according to the Professor of Life Sciences at La Trobe University). “This study results suggest that bird migration patterns can penetrate and persist across urban areas given suitable habitat, and that the Merri Creek corridor forms part of the broader network of habitats beyond the cities – a substantial achievement of sustained community effort.”

Summarised by Ann McGregor from a report by Susanna Bryceson