Assessing factors associated with changes in the numbers of birds visiting gardens in winter: Are predators partly to blame?

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.

Bird Census Data Do Not Indicate a Lack of Impact on Songbirds From the Growth of Avian Predator Populations in Britain in the Late 20th Century

“…There are also major differences with the results of Swallow et al (2016aSwallow et al ( ,b, 2019, which themselves vary substantially (Table 7). However the results emerging from these studies may be of limited value, since the model used in each case has a common intercept across all years.…”

“…This is contrary to indications emerging from Thomson et al (1998) in favor of the inclusion of annual intercepts in the linear model, which has the effect of removing predator-prey covariance attributable to long-term national trends. The existence of such correlated trends provides the original motivation for analyses of this kind, but may represent a spurious relationship arising from separate and unconnected causal mechanisms, and since this potential source of covariance contributes to the effect sizes reported by Swallow et al (2016aSwallow et al ( ,b, 2019, this limits the inferences that can be drawn. In particular the negative associations reported for Blue Tit, Starling and House Sparrow with Sparrowhawk may simply reflect temporal covariance caused by declining attendance at bird feeders, which coincided with the increase in Sparrowhawk numbers but was somewhat later than the main period of Collared Dove increase.…”

“…Cell contents indicate significant positive, negative or non-significant effects of either Sparrowhawk or Collared Dove on focal prey species. Swallow et al (2019) includes analyses using both predator density and change in predator density as explanatory variables.…”

“…Predator effect estimates for individual prey species were compared with those presented by Newson et al (2010), and from a series of more recent papers reporting predator effects estimated from GBFS data in which mean over-winter counts at bird feeding stations are modeled within a Bayesian framework, either using site-specific predator and environmental variables as linear predictors (Swallow et al, 2016a(Swallow et al, ,b, 2019, or via a joint spatio-temporal model that determines the dependency of species abundance on the probability that other species are present (Jones-Todd et al, 2018).…”

“…Support for this position has emerged from investigations that use data from the British Common Birds Census (CBC) to test for the presence of relationships between local population trends and changes in the local status of predators during the main period of decline in the late 20th century (Thomson et al, 1998;Newson et al, 2010), which found very little to suggest a significant impact of a range of increasingly abundant predator species. By contrast, studies focusing on data from the Garden Bird Feeding Survey (GBFS) have found more evidence of impact (Chamberlain et al, 2009;Bell et al, 2010;Swallow et al, 2016aSwallow et al, ,b, 2019Jones-Todd et al, 2018), but since this refers to numbers at winter feeding stations it permits less certain inference regarding impacts on breeding populations.…”