The Effects of Supplementary Food on the Breeding Performance of Eurasian Reed Warblers Acrocephalus scirpaceus; Implications for Climate Change Impacts

Abstract

Understanding the mechanisms by which climate variation can drive population changes requires information linking climate, local conditions, trophic resources, behaviour and demography. Climate change alters the seasonal pattern of emergence and abundance of invertebrate populations, which may have important consequences for the breeding performance and population change of insectivorous birds. In this study, we examine the role of food availability in driving behavioural changes in an insectivorous migratory songbird; the Eurasian reed warbler Acrocephalus scirpaceus. We use a feeding experiment to examine the effect of increased food supply on different components of breeding behaviour and first-brood productivity, over three breeding seasons (2012–2014). Reed warblers respond to food-supplementation by advancing their laying date by up to 5.6 days. Incubation periods are shorter in supplemented groups during the warmest mean spring temperatures. Nestling growth rates are increased in nests provisioned by supplemented parents. In addition, nest predation is reduced, possibly because supplemented adults spend more time at the nest and faster nestling growth reduces the period of vulnerability of eggs and nestlings to predators (and brood parasites). The net effect of these changes is to advance the fledging completion date and to increase the overall productivity of the first brood for supplemented birds. European populations of reed warblers are currently increasing; our results suggest that advancing spring phenology, leading to increased food availability early in the breeding season, could account for this change by facilitating higher productivity. Furthermore, the earlier brood completion potentially allows multiple breeding attempts. This study identifies the likely trophic and behavioural mechanisms by which climate-driven changes in invertebrate phenology and abundance may lead to changes in breeding phenology, nest survival and net reproductive performance of insectivorous birds.

Supplemental food alters nest defence and incubation behaviour of an open‐nesting wetland songbird

“…Given that increasing spring temperatures are expected to drive increases in food availability in wetland habitats during the reed warbler breeding season, the results presented here suggest that warmer spring conditions will lead to reed warblers being more attentive to their nests and respond more quickly to threats at the nest. The lower rates of depredation observed in the supplemental food study in this study population (Vafidis et al 2016) may also be explained partially by the increased latency to respond to predators and the shorter off-bout durations which reduce the period of nest vulnerability to predation.…”

“…Furthermore, egg-cooling rates are non-linear (Turner 1985), so shortening an off-bout by as little as 7 s may be beneficial in terms of the energy expenditure required to warm the eggs back up. Higher rates of incubation constancy can reduce the incubation period and result in higher hatchling mass, reducing the overall period of vulnerability to predation (Eikenaar et al 2003, Vafidis et al 2016. This result is supported by the significantly lower maximum off-bout durations recorded in the supplemented nests compared to the unsupplemented nests.…”

“…org/products/camera-module/ >) and infrared-triggered trail cameras (Bushnell HD < http://Bushnell.co.uk >). Activity around the feeding stations was recorded between 06:00 and 18:00 BST every two days throughout the onset and duration of incubation in both studies (see also Vafidis et al 2016).…”

“…This may be due to several inter-related factors, including higher breeding productivity due to warmer spring conditions leading to higher food availability during earlier and longer breeding seasons (Halupka et al 2008). Experimental food supplementation (used to simulate more abundant food earlier in the breeding season, as expected under projected climate change scenarios (Bale et al 2002, Dell et al 2005) has been shown to induce reed warblers to nest earlier, shortening their incubation phase and nestling growth periods (Vafidis et al 2016). These food-driven changes in breeding behavior lead to earlier and more successful fledging, increasing the overall productivity of the first brood and allowing the first brood to be fledged early enough in the breeding season for second breeding attempts to be initiated (Vafidis et al 2016).…”

“…Experimental food supplementation (used to simulate more abundant food earlier in the breeding season, as expected under projected climate change scenarios (Bale et al 2002, Dell et al 2005) has been shown to induce reed warblers to nest earlier, shortening their incubation phase and nestling growth periods (Vafidis et al 2016). These food-driven changes in breeding behavior lead to earlier and more successful fledging, increasing the overall productivity of the first brood and allowing the first brood to be fledged early enough in the breeding season for second breeding attempts to be initiated (Vafidis et al 2016). Food-supplemented pairs of reed warblers also suffered substantially lower rates of nest predation compared with control (unsupplemented) pairs.…”