Supplementary feeding restructures urban bird communities

Abstract

Food availability is a primary driver of avian population regulation. However, few studies have considered the effects of what is essentially a massive supplementary feeding experiment: the practice of wild bird feeding. Bird feeding has been posited as an important factor influencing the structure of bird communities, especially in urban areas, although experimental evidence to support this is almost entirely lacking. We carried out an 18-mo experimental feeding study at 23 residential properties to investigate the effects of bird feeding on local urban avian assemblages. Our feeding regime was based on predominant urban feeding practices in our region. We used monthly bird surveys to compare avian community composition, species richness, and the densities of local species at feeding and nonfeeding properties. Avian community structure diverged at feeding properties and five of the commonest garden bird species were affected by the experimental feeding regime. Introduced birds particularly benefitted, with dramatic increases observed in the abundances of house sparrow (Passer domesticus) and spotted dove (Streptopelia chinensis) in particular. We also found evidence of a negative effect on the abundance of a native insectivore, the grey warbler (Gerygone igata). Almost all of the observed changes did not persist once feeding had ceased. Our study directly demonstrates that the human pastime of bird feeding substantially contributes to the structure of avian community in urban areas, potentially altering the balance between native and introduced species.

Keywords: avian ecology; community composition; garden birds; human interactions; wildlife feeding.

Fig. 1.

Map of northern Auckland, New Zealand, showing the location of properties participating in an experimental bird feeding study. The urban–rural boundary is also shown, with land zoned as urban shaded in gray. Reference coordinates are expressed as latitude and longitude (WGS84).

Fig. 2.

NMDS ordinations of avian community composition (A) before, (B) during, and (C) after the experimental feeding regime at urban study properties in northern Auckland, New Zealand, grouped by experimental treatment. The dotted ellipses denote the 95% confidence intervals for each experimental (Exp.) group. The species centroids (relationships among species as defined by their relative abundance at different sites) are also presented (D) for the “during feeding” period, scaled by percentage of total abundance (square root-transformed) for that period. Species abbreviations (for scientific names see Table S1): BBGL, southern black-backed gull; BLKB, Eurasian blackbird; CHFN, chaffinch; FNTL, New Zealand fantail; GDFN, goldfinch; GRFN, greenfinch; KNGF, New Zealand kingfisher; MYNA, common myna; RSLA, eastern rosella; SEYE, silvereye; SPDV, spotted dove; SPRW, house sparrow; STRL, common starling; SWAL, welcome swallow; THSH, song thrush; TUI, tūī; WBLR, grey warbler.

Fig. 3.

Overall (A) species richness and (B) relative abundance of garden birds recorded during 10-min point counts at urban study properties in northern Auckland, New Zealand, before, during, and after the implementation of an experimental feeding regime. Experimental (Exp.) group: F, feeding properties; NF, nonfeeding properties. The vertical dotted lines indicate the start and end of the feeding regime. Error bars represent the SEM.

Fig. 4.

Relative abundance of the 12 (A–L) most commonly occurring garden bird species recorded during 10-min point counts at urban study properties in northern Auckland, New Zealand, before, during, and after the implementation of an experimental feeding regime. Experimental (Exp.) group: F, feeding properties; NF, nonfeeding properties. Within each species type (introduced/native) species are listed in order of mean abundance over all counts (n = 597). For species scientific names see Table 2. The vertical dotted lines indicate the start and end of the feeding regime. NB: y axis scale varies with species.