I used the prey energy content, and observed intake rates derived from the calibrated functional response model, including the capture efficiency parameter ), to determine the time taken to meet the daily energy requirement (DER) of 838.34 kJ day-1 using each foraging strategy, assuming that one prey item is collected per sweep (Figure 4.10).
The model output indicates that the greatest energy intake rates can be achieved by pelagic tactile foraging on energy dense prey items such as gobies (Pomatoschistus sp.) or ditch shrimp (P. varians). However, this strategy is only feasible if locally high abundance of prey is identified. To achieve the energy intake rates represented in Figure 4.10, the effective prey abundance needs to be roughly 150 times greater than the actual densities recorded by
0
119 assuming one prey item is collected per sweep.
The relatively low availability of large worms, due to their tendency to burrow to unattainable depths in the sediment, results in very low intake rates achievable by benthic tactile foraging.
However, much greater intake rates are obtainable by targeting this prey type visually.
Although, with the densities of large worms present at the study site, the encounter rate is so low that a bird could never meet its DER just searching for very large worms. The increased availability of medium-sized worms makes them a much more profitable prey type. Intake rates were 24.1% higher if medium-sized worms were targeted by visual rather than benthic tactile foraging at the study site.
According to Figure 4.10, benthic tactile foraging on the smallest prey types (small worms and Corophium) does not yield high enough intake rates to meet DER. However, it should be noted that the model has assumed that a maximum of 1 prey item is taken per sweep. This is a valid assumption for the larger prey types; however, according to a quantitative study of avocet faecal contents and prey swallowing rates, the author concluded that multiple small worms were captured and ingested in a single sweep (Moreira 1995b). The estimated number of worms ingested per swallow ranged from 2 to 24. If I take the upper limit of this estimate, and apply it to benthic tactile foraging on small worms and Corophium, the number of hours needed to meet DER drops to 13.3 hours and 4.6 hours, respectively.
1
Corophium sp Small worms Medium worms Large worms Medium worms Large worms Corophium sp Idotea chelipes Pomatoschistus sp
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4.5 Discussion
This chapter demonstrates that avocets are able to achieve the highest intake rates by targeting energy-dense pelagic prey, but only when these prey are present at high densities.
The relative profitability of foraging by sight or touch on benthic prey depends on the prey composition found at the site. If prey consists of few large individuals, then visual foraging is more profitable; although this strategy is only possible under certain environmental conditions, such as good visibility and calm conditions. Benthic tactile foraging represents a
‘safe’ strategy, which will usually provide sufficient intake rates to meet daily energy requirements, as long as the density of small prey is not unusually low, or the sediment too grainy, as this may interfere with prey capture (Quammen 1982).
These results show that prey availability is more important to avocet survival than prey abundance. This means that great care is required when drawing inference from benthic sampling as to the likely survival of avian predators. Avocets employ a range of feeding strategies based on local prey availability and by explicitly parameterising each strategy the results show that pelagic tactile foraging on large prey items was the most profitable strategy, followed by benthic tactile predation on Corophium and small worms, assuming that multiple prey items are collected in a single sweep. This study provides strong evidence for the assertion that multiple small prey items are collected in a single sweep of the bill, as benthic tactile foraging on very small prey was commonly observed at the study site, so under optimal foraging theory we must assume that it is an energetically profitable strategy.
Pelagic tactile foraging on Pomatoschistus and Palaemonetes was predicted to meet energetic requirements within 2.3 and 2.4 hours, respectively. For pelagic tactile foraging, the fact that observed intake rate was higher than predicted for this type of foraging has two possible interpretations. Firstly, that the actual density of pelagic prey is higher than that which I estimated from the sampling methods used. Secondly, that the distribution of the highly mobile pelagic prey is not homogeneous, but patchy, and pelagic tactile foraging is a response to a bird encountering a locally high abundance of prey. The abundances I recorded are unlikely to be reflective of the biologically relevant, high densities which transiently occur as the pelagic prey moves around. In the absence of these prey clusters, predicted intake rates for this foraging method are extremely low, suggesting that pelagic tactile foraging is a very inefficient means of prey capture at low prey densities. Thus, pelagic tactile foraging is likely to be an opportunistic strategy, which is profitable when locally high densities of large prey items are encountered. The idea that locally high
121 abundance of prey may be created by the birds as they form social foraging flocks, will be explored further in Chapter 5.
I found that at Brownsea Lagoon, foraging on larger worms was less profitable than foraging on medium-sized worms, despite the extra energy associated with individual prey items. This was due to the longer handling times limiting the maximum feeding rate, but also the very low availability of large worms. Even though the abundance of large worms estimated by core sampling was 363.83 worms m-2, due to decreased probability of worms being found in the top 2 cm of sediment (i.e. available for benthic tactile foraging), or at the surface (i.e.
available for visual foraging), the effective abundance of these worms was nearly ten times lower. As the abundance of medium-sized worms was roughly six times higher than the abundance of large worms, medium-sized worms were associated with higher intake rates.
Indeed there may be a threshold worm size which affords them some immunity from predation by avocets by virtue of their deeper burrowing depths (Esselink & Zwarts 1989).
Thus there is likely to be a complex set of feedback interactions between secondary production and avocet survival. However, the key point here is that predators target areas of high prey availability or “catchability” over abundance, which has been demonstrated in other predator-prey systems (Balme et al. 2007). As surveys of prey available to shorebirds generally focus on abundance, this highlights the importance of models such as the one I have developed here, to provide the link between abundance and intake rates.
In addition, the model indicated that for larger benthic prey, it was more efficient to hunt by vision than touch. While visual foraging had higher handling time associated with the approach time, in a prey landscape where very large, energy rich prey are present, even at low density, visual foraging may be a more efficient foraging strategy.
Comparing the predictions of the un-calibrated foraging model to observed feeding rates in the field (Figure 4.9), I derived a figure for an unknown component of the species’ foraging ecology – the efficiency of prey capture (‘ ’). However, the ecological meaning of this variable differs between the 3 foraging types. Also, by comparing predicted and observed intake rates for a second study site (Middlebere Creek), I showed that capture efficiency is site specific. For benthic tactile foraging the interpretation of is simply that only a small percentage of the prey that is contacted by a scything bill will be captured and ingested. For visual foraging, the capture efficiency is a measure of the percentage of prey within the visual field which is perceived and attacked. Both of these estimates are likely to be affected by environmental conditions – scything efficiency may be lower in sandier sediments (Quammen 1982), and visual foraging is only possible in non-turbulent water or on exposed mud, and the presence of wind may also decrease prey detection (Velásquez & Navarro
122 1993; Goss-Custard 1984). On the other hand, water movement may increase the tendency of larger worms to migrate to the surface in search of food, which increases their availability to predators (Esselink & Zwarts 1989). The most likely reason for the lower capture efficiency at Brownsea compared with Middlebere is that the sediment at the latter site is much finer (Chapter 3) and less likely to interfere with the delicate foraging apparatus of the bill. In addition, the high density of hydrobiid snails at Brownsea (Chapter 3) could also interfere with the prey-detecting sensitivity of the bill, in a similar way to grainy sediment. In pelagic tactile foraging, the capture efficiency actually provides a means of estimating the effective prey abundance for highly mobile prey.