Abstract
The factors governing the recent declines observed in many songbirds have received much research interest, in particular whether increases of avian predators have had a negative effect on any of their prey species. In addition, further discussion has centered on whether or not the choice of model formulation has an effect on model inference. The study goal was to evaluate changes in the number of 10 songbird species in relation to a suite of environmental covariates, testing for any evidence in support of a predator effect using multiple model formulations to check for consistency in the results. We compare two different approaches to the analysis of long‐term garden bird monitoring data. The first approach models change in the prey species between 1970 and 2005 as a function of environmental covariates, including the abundance of an avian predator, while the second uses a change–change approach. Significant negative relationships were found between Eurasian Sparrowhawk Accipiter nisus and three of the 10 species analyzed, namely house Sparrow Passer domesticus, starling Sturnus vulgaris, and blue tit Cyanistes caeruleus. The results were consistent under both modeling approaches. It is not clear if this is a direct negative impact on the overall populations of these species or a behavioral response of the prey species to avoid feeding stations frequented by Sparrowhawks (which may in turn have population consequences, by reducing available resources). The species showing evidence of negative effects of Sparrowhawks were three of the four species most at risk to Sparrowhawk predation according to their prevalence in the predator's diet. The associations could be causal in nature, although in practical terms the reduction in the rate of change in numbers visiting gardens accredited to Sparrowhawks is relatively small, and so unlikely to be the main driver of observed population declines.
Highlights
Many species of passerine birds are exhibiting very different popula‐ tion trajectories across both local and national scales
The changes associated with agricultural intensifi‐ cation (Chamberlain, Fuller, Bunce, Duckworth, & Shrubb, 2000) are thought to be have been the main driver for the observed de‐ cline in some rural populations, with a reduction in the availability of favored seed during the winter months leading to decreased survival rates (Hole, 2001; Siriwardena, Baillie, & Wilson, 1999), most notably of first‐year birds (Crick, Robinson, Appleton, Clark, & Rickard, 2002)
Majority of Sparrowhawk observations relate to single birds, occa‐ sionally two, it seems reasonable to suggest that the Sparrowhawk effect is likely a contributory factor to the declines seen in House Sparrow and Starling rather than the main cause
Summary
Many species of passerine birds are exhibiting very different popula‐ tion trajectories across both local and national scales. Data collected through the BTO Common Birds Census (CBC) and the BTO/JNCC/RSPB Breeding Bird Survey (BBS) have charted a 70% decline in the House Sparrow breeding population within England since 1977 and the species has been placed on the Red List of Birds of Conservation Concern (Eaton et al, 2015; Robinson et al, 2015) Much of this debate has centered on the recovering breeding population of Sparrowhawk, a specialist predator of small birds whose English breeding popu‐ lation has increased by 115% since 1975 (Robinson et al, 2015). The methods used in this paper aim to avoid these two problems in re‐assessing the question of Sparrowhawk effects on songbirds by using an array of possible covariates to explain the observed changes in songbird abundance, while using two alternative model formula‐ tions to test for consistency in the results
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