The influence of substrate on the functional response of an avian granivore and its implications for farmland bird conservation

Abstract

Few studies to date have considered the effect of substrate on the functional response of an organism feeding on prey of varying visibility. Intake rates of lone captive canaries, Serinus canarius L., were measured at varying seed densities on patches of either earth or short grass (<1 cm). Experiment 1, using pale seeds, found intake rates were significantly higher and search times significantly lower on earth than on grass. Two measures of crypticity (contrast in light reflectance as measured using a spectrophotometer and an experiment with humans) found pale seeds to be more visible on earth. The results from experiment 1 could be explained by this difference in crypsis. Experiment 2 used identical seeds to those in experiment 1 except they were dyed to match their backgrounds. The two measures of crypticity both found that black seeds were less visible on earth than green seeds were on grass. However, intake rates were still significantly higher on earth than grass. Seed colour preference, vegetation impeding movement, and differences in vigilance rates or seed accessibility could not explain this result. We discuss three other potentially explanatory mechanisms, the most likely of which was the greater surface area needed for scanning created by the structure of grass. Crucially, regardless of the mechanism(s) involved, many vegetated substrates share similar properties with grass (structural complexity and shiny surfaces which reflect light) and so the outcome of our findings are likely to extend to many natural situations. Conservationists wishing to encourage granivorous birds should consider enhancing food accessibility by providing uniform substrates, such as bare earth, for them to forage on. In addition, behaviour-based models should incorporate the effects of habitat into their equations of the functional response.

Identifying set‐aside features for bird conservation and management in northeast Iberian pseudo‐steppes

“…2002, Whittingham et al 2006 in addition to the specific species in the present study , Moreira et al 2005, Serrano & Astrain 2005. The availability of bare ground in vegetated areas give easier access to ground food for birds, allowing an increase of their intake rates (Whittingham & Markland 2002). Furthermore, low cover and height also provide good visibility, which allows early predator detection (Whittingham & Evans 2004).…”

Group size modulates time budget and foraging efficiency in captive Skylarks, Alauda arvensis

“…A bird was considered to be vigilant when its head was above a horizontal line made by its body, and not orientated towards the ground. Conversely, birds were considered to be foraging when head was below the horizontal and actively scanning the ground or pecking (Whittingham and Markland 2002). The handling time, which represents the time needed to consume one prey item, and vigilance could not be separated given that Skylarks did not manipulate millet seeds and often adopted a vigilant posture when handling seeds.…”

Creating ephemeral resources: how long do the beneficial effects of grass cutting last for birds?

“…For example, a patch containing some prey items which are relatively exposed is likely to be chosen over a patch containing a greater number of prey items which are difficult to access (e.g. due to dense vegetation: Whittingham & Markland 2002). However, for one group in our study, prey abundance is likely to be the key factor determining intake rate.…”