observations made in the plots, the diagram did not provide a good picture of reality. The Spotted Sandpiper only appeared in the plots of the Furo Grande, which was not reflected very well by the diagram. According to Jongmann et al. (1987), eigenvalues below 0.5 are weak, thus, the model is most likely not able to explain the situation very well. In addition, most environmental factors are indicated by relatively short arrows, i.e. their explanatory value is quite low. Finally, the CCA was also not able to find distinctions between most bird species, which are grouped together tightly. Overall, it is not quite clear if the shown diagram characterizes the situation reasonably, but all other CCAs, which resulted in far lower eigenvalues, certainly do not. Hence, the CCA also failed to reveal a clear picture of the bird distribution in relation to the benthic distribution.
Fig. 33. Diagram of a CCA calculated on bird- and benthic abundances on the plots in January 2001. For a better understanding the diagram is given twice: on top, with the bird variables (points) and the benthos variables (arrows), and at the bottom, with the sites indicated as points. Bird numbers were tide
calibrated and the benthos was included on a lower taxonomic level. The sum of all eigenvalues is 1.18.
4645 44 42
36
34 3533
32
30
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27 26
25 24
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21 20
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16 14 15
13 12
1110
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-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1
-1,5 -1 -0,5 0 0,5 1 1,5
TeRa TaPl
Pinn Orbi
Nere Neph
Neme Mage
Lumb Idot
Goni Gamm
Cope
Capi
w ill w him turn
spsa
sppl
sneg
sdsa
sbdo knot kepl
grpl
-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1
-1,5 -1 -0,5 0 0,5 1 1,5
Eigenvalue: 0.40
Eigenvalue: 0.37
copl
Chapter 3 shows that mean benthic abundances and biomasses of the intertidal at the Bragantinian peninsula were low in comparison to other areas, presumably as a result of the environmental variability of the study area. But in contrast to the Banc d’Arguin, the harvestable fraction of the biomass was at low level as well, the mean harvestable biomasses for individual bird species only reached values between <0.1 and 2.2 gAFDW/m².
However, when the range of the values is considered, there are still spots with a very high harvestable biomass for most birds, at least in the first months of the year. This might raise the need for the birds to search for profitable spots in the intertidal.
Diets
The observed diets include generally a large variety of prey items. All shorebirds feed on
“worms”, most of them prey also on bivalves and/or snails and few as well on crustaceans.
Because of this dietary variety, only a broad differentiation in a fish-eating group (herons and egrets), a “worm”-eating group, and a mixed-diet group is possible, no further specializations on certain prey groups are apparent. The results of the dropping analysis showed that also within each main prey group a variety of prey items were consumed. Recognized prey types do not differ from previously recorded prey at other wintering areas, as summarized by Poole and Gill (2000) or Skagen and Oman (1996). Since many prey taxa do not provide body parts recognizable in droppings, the diversity of the diet might be even larger. In agreement with this, the calculated optimal diet of many bird species consists of a large variety of favourable prey items. This might offer an explanation for the broad diets observed during focal observations and the analysis of droppings.
Optimal diets
Optimal diets provide only a calculated list of favourable prey items for each bird species, they do not give any information about actually ingested prey organisms. However, they might be a good estimate of what might have been eaten preferably, especially since the variability of the organisms in terms of body sizes, vertical and horizontal movements – as described in detail in chapter 3.2.4 – is taken into account. Remains of the items which are part of the optimal diet, are indeed often found in the droppings, hence, it is possible that most birds do in fact optimise their foraging.
When prey was classified as potentially attractive due to high biomasses (Uca maracoani, Tagelus plebeius and Nassarius vibrex) or high abundances (Nephytidae, Capitellidae, Gammaridae and Pinnotheirdae) (Fig. 15), only few items appeared to be of interest for the birds. But when the harvestability and the time needed to search and to handle this prey for the individual avian species is also considered, as was done with the calculation of an optimal diet, the emerging picture of attractive prey is very different. In most cases a far larger set of profitable prey becomes apparent. Thus, it is generally of large importance not only to consider abundances and biomasses of possible prey items, but also to include the accessibility and the cost of feeding on them in order to understand the prey choice of birds.
According to the optimal diets some birds should be concentrating only on few items (somewhat arbitrarily determined as <5 taxa; Red Knot, Sanderling, Whimbrel and Willet), while other species should prey on many items (>5 taxa; Marbled Godwit, Ruddy Turnstone, Semipalmated Sandpiper) and in some cases they should switch from month to month between specialization and generalization (Scarlet Ibis, all plovers and Short-billed Dowitcher). If optimal diets were specialized, they always focused on bivalves and crustaceans. Only in May and June polychaetes became important, too. The attractiveness of bivalves and crustaceans is due to their high biomass values per prey item. During May and June, bivalves become rare (Fig. 14) and polychaetes therefore relatively more attractive.
A specialization of the optimal diet on bivalves and crustaceans is at least partly related to the capability to feed on large prey items. Of the seven species which feed on bivalves larger than 3 cm, five have a relatively specialized optimal diet, only two are mostly generalists. In contrast, of the 5 species which can only prey on bivalves smaller than 3 cm, only one species has a specialized optimal diet, all others have more generalized optimal diets. Bill size proves to be significant for the size of prey ingested, a relationship already identified in numerous previous studies (Holmes and Pitelka 1968; Goss-Custard et al. 1977a; Lifjeld 1984; Weber and Haig 1997). This is presumably a result of the increased ability to reach deeper burrowed prey and to handle and swallow larger organisms. The shortened handling time increases the profitability of prey. But while this holds true for bivalve- and crustacean prey at the Bragantinian peninsula, the size of “worm-“ prey did not depend on bill size of the predators. Presumably large “worms” do not raise the handling difficulties as much as hard-shelled or many-legged prey. Hence, bill size proves to be significant for the determination of the harvestable prey fraction, which is in turn responsible for the ability to prey and specialize on profitable prey items.
The benthic taxa included in the optimal diets also reflect the ability of the birds to prey on certain prey sizes. The bivalve Tellina radiata occured in the optimal diets of almost all birds.
While Protothaca pectorina was common for those birds which could feed also on larger bivalves (Grey Plover, Red Knot, Willet, Ruddy Turnstone and Whimbrel), Lucina pectinata occured mostly in the optimal diet of birds feeding on smaller bivalves (Sanderling, Semipalmated Sandpiper, Collared Plover and Semipalmated Plover). Specialization on crustaceans only occurred in the optimal diets of the Scarlet Ibis, the large sandpipers Whimbrel and Willet, and all plovers. They are split in two groups: the Scarlet Ibis and the large sandpipers, which are able to handle large crustaceans, focused on Decapoda and Uca maracoani. All plovers included additionally a variety of small crustaceans. They might have been important for them because of their mobility: they are easily detectable for the plovers hunting by visibility (Turpie and Hockey 1997). Also, the temporal and spatial variability of Callianassidae, Uca maracoani and Tagelus plebeius, as discussed in detail in chapter 3, is reflected in the calculated optimal diets.
Although specialization occurred within the optimal diet, the focus was mostly on groups of prey, and very broad optimal diets were common. This is a consequence of relatively long searching times at the Bragantinian intertidal with its sparse macrobenthic densities (chapter 3.2.4). Searching times are very time-consuming in comparison to the handling times. Thus, birds could not afford to reject even small prey items and had to maximize their energetic input through a large diet width. The conclusion of chapter 3, that opportunistically feeding birds have an advantage over highly specialized birds in highly variable environments, is supported by the observed diets, the analysis of droppings, and by the theory underlying the concept of optimal diets for the majority of birds.
Diet and the distribution of birds in relation to macrobenthos
In the last decades many studies have focused on the distribution of shorebirds in relation to their prey. Some studies compared bird distributions to the zonation of benthic organisms by sight with variable success (Wolff 1969) or with the help of dendrograms, which found clear associations (Meire and Kuyken 1984). Many attempts were made to find correlations between bird distributions and benthic densities. Although some authors found strong relationships (Bryant 1979; Colwell and Landrum 1993; Yates et al. 1993a), others failed to find a correlation (Botton et al. 1994). Many results were not clear: in some investigations only weak relationships were found (Goss-Custard 1970; Wilson Jr. 1990) and Wilson suggested that any intertidal area exceeding a critical threshold value of prey is acceptable for the birds. Some studies altered the variables to find the critical factors: Kalejta and Hockey (1994) found that one avian species is associated with prey densities while another corresponds to prey biomasses. Goss-Custard et al. (1977b) improved correlations between Redshank and prey by combining additional prey species with the most important prey item and used in some cases prey species, and in other cases much higher taxonomic groups.
The attempt to define the spatial relationship between birds and their prey was also made at the Bragantinian peninsula. Diverse statistical techniques were applied, but all of them were not able to display and to explain a sufficient amount of the inherent variability. Although different attributes of the prey were investigated – abundance, biomass and profitability – no clear picture emerged. After all, this result is not very surprising, since the birds do not focus on only one or two prey items, but forage mostly on a large variety of benthic taxa. This leads to a very complex pattern of relationships between birds and their prey and the descriptive abilities of Multivariate Analysis are probably too restricted to exhibit this in a sufficiently precise matter. Thus, there might be doubts if these models are applicable at all in situations where predators are not reasonably specialized. However, even for the more specialized birds of this study, no clearer picture appeared. To my knowledge, no other attempt to link environmental factors to shorebird distributions was made so far in the tropics, but there are several examples for temperate regions (Goss-Custard et al. 1991; Scheiffarth et al. 1996;
Burger et al. 1997).
Niche overlap
Niche separation can take place on different levels: through the segregation of habitats (geographical, between habitats etc.), through a vertical segregation, through a segregation in time (daily, seasonal), or through a differentiation of diet and feeding behaviours. All these levels were investigated in this study.
Temporal segregation clearly splits the avian community of the Bragantiner peninsula in the groups of migrants and residents, with migrants occurring predominantly from January until April, and raised numbers of residents in June.
A separation by habitat took place at least partly. When the use of the three sampling areas was investigated, the Spotted Sandpiper only appeared in the plots inside the mangrove (Furo Grande), while Red Knot, Sanderling, Short-billed Dowitcher and Marbled Godwit were only found in the open intertidal of the Ilha de Canelas (Appendix III, Table 38). However, most birds had a very variable distribution and clear habitat segregations based on the smaller scaled plots did not appear. Environmental factors, although known to influence prey availability (Goss-Custard 1984), cannot be linked to the distribution of the birds (Appendix III, Table 39).
However, a segregation by microhabitats was observed. The avian community is roughly split in a group of wading birds and a group of all other, mostly migratory, birds. This last group can be destinguished further in three different groups of shorebirds, according to their salinity and sediment preferences or the presence or absence of water at the feeding site.
Although the diets of the birds showed large overlaps, they can be distinguished in a fish-eating group (residential herons and egrets), a “worm”-fish-eating group and a large group with a mixed diet (both mostly migratory shorebirds). This differentiation is more or less consistent with the groups differentiated by microhabitat use. This might be due to a combination of habitat preferences of preferred prey organisms and foraging techniques which explore only a certain microhabitat. For example, the fish-prey of the wading birds is found exclusively in deeper water, and the pecking of the plovers aided by their vision can only reach prey in the upper centimetre of the sediment, mostly occupied by “worms” . Skagen and Oman showed that shorebirds in general exhibit a considerable dietary breadth and their prey choice is very flexible (1996).
Overall, the avian community is split into residential herons and egrets and migratory shorebirds. This separation is manifested by time, by the use of microhabitats and diet and it is a result of the different ecological roles these groups occupy in the system. As a consequence, residential birds do not tend to fill the niches which are occupied by the migrants during the winter time, when those leave the area in spring. The space used by them apparently remains, for the most part, abandoned during their absence. An exception to
this pattern are the Scarlet Ibise and the Collared Plover, both are (largely) residential. They use the same resources as the migratory shorebirds and do not have a special status.
Beyond this obvious segregation, less distinct niches defined by microhabitat characteristics - partly related with diet - can be distinguished among the shorebirds. The association with water and the probing depths are such attributes and they are at least partly linked to physical characteristics such as leg- and bill size. However, most birds occupy very broad niches and display a variable resource use.
To understand the lack of clear niche differentiations at the Bragantinian peninsula, the general ideas of niche formation have to be considered. Two contradictory processes are thought to affect niche differentiation (Begon et al. 1998): when only one species is considered, niche breadth is a result of resource abundance. High prey abundances are beneficial for specializations and low abundances promote broad niches (Zwarts and Wanink 1993). But under the competitive pressure of additive species – which also limits a resource – niche differentiation is enhanced (Cody 1974). The actual state of niche differentiation within an ecosystem is a result of both processes (Wiens 1992). Whether one or the other process is dominating depends on the resource abundance and the requirements of the involved species.
At the Bragantinian peninsula the food stock for the birds is very restricted as will be shown in chapter 6. It is assumed that the resource limitation is so strong, that the consequences of competition become insignificant. All birds are forced to forage opportunistically since a restriction on few prey organisms might lead to food shortages. This constraint might be less marked on large birds which are able to prey on more profitable organisms than smaller species.