Hypotheses.

H1 If Oystercatcher foraging time depends on the tidal height, then a decreasing tidal height will cause an increase in the mean Oystercatcher abundance.

H2 If the wader species each have unique foraging techniques dependent on the tidal stage, then there will be a difference in Egret, Heron, and Oystercatcher abundances during ebb and flood tides.

 

Methods.

On 09/02/2020 from 7.32 am - 7.01 pm, time-lapse photos were taken of the foreshore at Ynys Faelog (53.2270° N, 4.1582° W). The mean abundance of wader species (Egret, Heron, Oystercatcher) identified in the foreground of each photograph was recorded. A linear regression model was used to test H1. Floodtide mean abundances p=>0.05, ebb tide mean abundances p=<0.001 subsequent Tukey post hoc used.  To test H2 a two-way Anova(p=<0.001) therefore, a Tukey post hoc was performed. The assumptions are that the data is normally distributed and has an equal variance.   

 

Results.

A positive relationship between tidal height and Oystercatcher densities H1 (R2 =0.842, p =<0.001, df =76) was identified where 84.2% of variance can be explained by the linear regression model (Figure 1). The largest and most variable abundance mean±SD in both tidal stages was Oystercatchers- Ebb 21.355±20.635 and Flood 1.487±6.621. Egret-Heron mean±SD showed little variance across each tidal stage (Figure 2). In addition, a significant Tukey post hoc (adj p=<0.001) between ebb Oystercatcher-Egret and ebb Oystercatcher-Heron abundances was confirmed, with Egret-Heron resulting in no significance (adj p=>0.05). A Tukey post hoc test between the categorical variables ebb and flood H2, displayed significant differences between Oystercatcher-Egret and Oystercatcher-Heron (adj p <0.001) abundances during ebb and flood tides. Egret-Heron abundances (adj p>0.05) did not significantly differ dependent on the tidal stage.

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Figure 1. Haematopus ostralegus mean abundance during an ebb tide at Ynys Faelog. The black solid line is the linear regression model (R2=0.842). The blue dotted lines are confidence intervals (95%).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2. Mean± SD abundance during ebb tidal stages: Oystercatcher 21.355±20.635, Egret 2.121±2.723, Heron 1.458±1.432. In comparison to flood tidal stages: Oystercatcher 1.487±6.621, Egret 0.867±1.274, Heron 2.237±2.922. Mean± SD represents wader species identified from 153 photographs taken at Ynys Faelog over 11.5 hours.

 

 

Discussion.   

This study showed that during an ebb tide, a significant relationship between decreasing tidal height and decreasing Oystercatcher abundance; this observation does not support the H1 as an increase was expected. This, however, may be explained by the area in which the camera was when the photographs were taken. Positioned relatively high up the shore of Ynys Faelog, the Oystercatchers photographed may have been resting rather than foraging. So, instead the data could show the mean abundance of Oystercatchers waiting to forage; accounting for the positive trend seen in this study. Another investigation that recorded an Oystercatcher population during tidal stages also included roosting and resting birds (Heppleston, 1971). The population observed showed a significant correlation between active birds at low tides, and roosting birds during high tides, which supports the model produced for H1. Other studies that reviewed this pattern considered prey availability and consumption instead of tidal height. For example, Oystercatchers observed out in Scottish tidal mudflats, were present during time of the expected ebb to forage on the newly exposed mussel beds (Daan & Koene, 1981; Griffiths, 1967). This suggests Oystercatcher abundance could be correlated with prey availability rather than tidal height alone.

This study additionally supports H2 as a significant adj p >0.001 was produced for the comparison of Oystercatcher-Egret/Heron means in both flood and ebb tidal stages. This can be attributed to the difference in foraging behaviours seen between short legged Oystercatchers and longer legged Egrets and Herons. Oystercatchers predate primarily on Cerastoderma edule, Patellogastropoda and Mytius edulis found within or on the substate surface (mud-rocky), so a low ebbing tide is the most optimal foraging time (Edwards, Rossiter & Nagarajan, 2021; O’Connor & Brown, 1977). Egrets and Herons, however, predate on small fish, crustaceans, and other invertebrates, this means foraging activity during both ebbing and flooding tides are successful (Willard, 1977). This supports the data for H2 as the ebb tide had the highest Oystercatcher mean density. Additional studies acknowledged tidal mudflats and marshes can also accommodate Egret and Heron populations more consistently at a range of tidal stages due to type of prey available (Trocki & Paton, 2006; Draulans & Hannon, 1988). This correlates to the relationship found in this study between Egret and Heron abundance, showing no statistical significance at either stage.

 

 

 

References.

 

Daan, S., Koene, P. (1981) On the timing of foraging flights by Oystercatchers, Haematopus ostralegus, on tidal mudflats. Netherlands Journal of Sea Research. 15:1, 1-22.

 

Draulans, D., Hannon, J. (1988) Distribution and foraging behaviour of Grey Herons Ardea cinerea in adjacent tidal and non-tidal areas. Scandinavian Journal of Ornithology. 19;4, 297-304.

 

Edwards, R., Rossiter, L., Nagarajan, R. et al. (2021) Influence of rocky shore characteristics on the foraging profitability of Eurasian Oystercatchers Haematopus ostralegus. Wetlands. 41:43.

 

Griffiths, M. (1967) Some ecological aspects of the feeding behaviour of the Oystercatcher Haematopus ostralegus on the edible mussel Mytilus edulis. 109, 412-424.

 

Heppleston, P. (1971) The feeding ecology of Oystercatchers Haematopus ostralegus in winter in Northern Scotland. Journal of Animal Ecology. 40:3, 651-772.

 

O’Connor, R., Brown, R. (1977) Prey depletion and foraging strategy in the Oystercatcher Haematopus ostralegus. Oecologia. 27, 75-92.

 

Trocki, C., Paton, P. (2006) Assessing habitat selection by foraging egrets in salt marshes at multiple spatial scales. Wetlands. 26, 307-312.

 

Willrd, D. (1977) The feeding ecology and behaviour of five species of Herons in Southeastern New Jersey. The Condor. 79;4, 462-470.