Dominance Rank and Interference Competition in Foraging Among Six Species of Birds in a Park in Kaohsiung City, Taiwan

Taiwan J For Sci 26(3): 255-66, 2011 255

Research paper

Dominance Rank and Interference Competition in Foraging among Six Species of Birds in a Park in Kaohsiung City, Taiwan

Chao-Chieh Chen,") Hui-Yu Wu,2'3) Tzu-Tsen Liu,')Bao-Sen Shieh')

[ Summary

Through body size, social interaction, and foraging behavior, we investigated the dominance rank and interference competition among 6 bird species foraging at a park feeding site in Kaohsi- ung City, Taiwan. Social interactions and foraging behaviors of these birds were recorded in June to September 2009. David's scores were calculated from an interspecific interaction matrix, and the score roughly increased with the body size of birds, but some exceptions were noted. Concern- ing foraging behavior, feral pigeons (Columba livia) and Spotted-necked Doves (Streptopelia chinensis) took over the food area once they appeared even though Tree Sparrows (Passer montanus) usually arrived first. A linear regression model indicated that the number of Tree Sparrows outside the food area was positively correlated with the number of feral pigeons and Spotted-necked Doves inside the food area. Feral pigeons and Spotted-necked Doves moved away as the food was gradually consumed, and smaller species accordingly increased their foraging in the food area. Never- theless, the Tree Sparrow was also suppressed by other medium-sized birds, like the White-vented Myna (Acridotheres javanicus) and Chinese Bulbul (Pycnonotus sinensis), and they eventually occupied the food area in large numbers at a later stage. This study revealed that body size did matter and the Tree Sparrow was clearly the least dominant species among the 6. However, a discrepancy between the dominance status and interference competition in foraging was apparent. In addition to David's score, we suggest incorporating body size, group size, and interference competition to reach a more-comprehensive dominance hierarchy in bird communities. Key words: David's score, dominance hierarchy, dove, foraging behavior, Tree Sparrow. Chen CC, Wu 11Y, Liu TT, Shieh BS. 2011. Dominance rank and interference competition in foraging among six species of birds in a park in Kaohsiung City, Taiwan. Taiwan J For Sci 26(3):255-66.

°Department of Biomedical Science and Environmental Biology, Kaohsiung Medical University, 100 Shiquan 1st Rd., Kaohsiung 80708, Taiwan. ANINV'V' t_41)4NV%fniAtA.1)4V*.

2)Department of Oral Hygiene, Kaohsiung Medical University, 100 Shiquan 1st Rd., Kaohsiung 80708, Taiwan. ANINV'V' EuriRit_V*. 3) Institute of Biodiversity, National Cheng Kung University, 1 University Rd., Tainan 70101, Taiwan. ' 70101tAir'l--,4M19R. ° 4) Corresponding author, e-mail: [email protected] Received March 2011, Accepted June 2011. 2011*3)1')A, 2011*6)1-AA 256 Chen et al.Dominance rank and foraging behavior of birds in a park

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INTRODUCTION In addition to intraspecific competition, body size to be the major factor determining competition for resources among different dominance rank among birds, exceptions do animal species may also result in dominance occur. For example, the larger Black-backed hierarchies (Fisler 1977, Millikan et al. 1985, Woodpeckers (Pico ides arcticus) often stop Wallace and Temple 1987). Larger species foraging and move away when the smaller usually have a higher dominance rank and ac- Hairy Woodpeckers (P. villosus) approach cess resources earlier or take more food than within 10 m (Villard and Beninger 1993). Ap- smaller species (Hogstad 1989, Jablonski and parently, other factors such as aggressiveness Lee 1999, French and Smith 2005). Once or group size may also affect the dominance dominant species appear, they often occupy a rank (Burger and Gochfeld 1984, Chapman better position and make subordinate species and Kramer 1996, Basset 1997, Sandlin 2000, shift to other foraging sites (Alatalo 1981, Creel 2001). For example, the Grey-headed Alatalo and Moreno 1987, Jablonski and Lee Junco (Junco caniceps) won social encoun- 2002). Morse (1980, 1989) stated that an un- ters at a higher rate in larger groups (Millikan even partitioning of resources usually stems et al. 1985). On the other hand, gender, age, from different social dominance ranks among experience, and residence time at feeding birds. sites were proven to influence the dominance Although Basset (1995) considered rank among conspecific individuals (Davies Taiwan J For Sci 26(3): 255-66, 2011 257

1992, Stanback 1994, Emlem 1997, Martin et been feeding birds in the park every day for al. 1997, Pusey and Packer 1997). a long period of time, hundreds of birds, in- The study of urban ecosystems has be- cluding feral pigeons, Spotted-necked Doves, come quite popular in recent decades (Savard Tree Sparrows, etc, are attracted to the feed- et al. 2000, Fontana et al. 2011). Although ur- ing site almost every day. ban parks, especially forest parks, are consid- The food area was defined as the area of ered habitat islands in an ocean of buildings, ground covered by food, and it was circular they are actually important habitats for bird with a radius of about 1 m. Bread crumbs and communities living in big cities (Fernandez- bean sprouts were provided in the food area Juricic and Jokimaki 2001, Sandstrom et al. twice a day, once in the morning and once 2006). Furthermore, people may put out food in the afternoon. The quantity of food was at feeding areas of parks to provide birds with recorded whenever we began an observa- additional food (Orams 2002). According tion. The quantity of food in the food area to the US Fish and Wildlife Service (1989), was divided into 3 levels: 1) abundant, when about 82 million Americans use many dif- food had just been spread in the food area to ferent ways to feed birds, and Cowie and about 75% of food remaining; 2) medium, the Hinsley (1988) found that 75% of households remaining food covering > 50% of the food put out food during winter in Cardiff, UK. In area, and soil was clearly exposed in certain contrast, people in Taiwan seldom feed wild areas; 3) little, < 50% of the food area was birds. A feeding site in a park in Kaohsiung covered by food, and dark brown soil was ex- City provided us the opportunity to study posed in most parts of the circle. interspecific competition among some park bird species. Among them, the Tree Spar- Field methods row (Passer montanus), Chinese Bulbul From 25 June to 30 September 2009, (Pycnonotus sinensis), Spotted-necked Dove we recorded social interactions and foraging (Streptopelia chinensis), feral pigeon (Colum- behaviors of these bird species at the feeding ba livia), Grey Treepie (Dendrocitta formo- site. Concerning social interactions, we made sae), and White-vented Myna (Acridotheres note of the time, species involved, aggressive javanicus) are common bird species in city behavior, and outcome of the interaction. We parks of Kaohsiung (Chen et al. 2005) and scored a 'win' for birds that initiated aggres- frequently occur at the feeding site examined sive behavior, and a 'loss' for the bird that in this study. We investigated the dominance displayed submissive behavior. Aggressive rank and interference competition in foraging behavior included attacking others with the among these 6 bird species. beak, chasing others, and threat displays with posture or sound. Birds which displayed MATERIALS AND METHODS submissive behavior usually retreated or yielded to the bird that initiated the aggres- Study area sive behavior. For foraging behaviors, we The study site is located within the San- recorded foraging techniques applied by each Min Park (18.4 ha, 22°64'N, 120°31'E) in bird species at the feeding site. Foraging tech- Kaohsiung City, southern Taiwan. It is a well- niques included foraging within the food area, maintained park, full of trees and meadows. carrying food out of the food area, carrying Owing to an enthusiastic resident who has food up to trees, or snatching food from other 258 Chen et al.Dominance rank and foraging behavior of birds in a park

individuals within the food area. defeated by species i (David 1987, de Vries To estimate the abundance of each bird 1998). Because our data had many interacting species at the feeding site, we used scan sam- pairs with reversals, application of David's pling at 10-min intervals (Martin and Bateson score was considered appropriate (Gammell 1993) to count the number of each species et al. 2003, Bang et al. 2010). We then used in the feeding site. If birds had flown away the Spearman rank correlation coefficient to because of a disturbance before the scanning test if the dominance index was positively time, we waited for another 10 min to repeat correlated with the body size of each bird the scanning. In between the sampling points, species (SAS 1999). Body size indices were we recorded social interactions among species derived from PRIN1 through a principal com- and the foraging behaviors of each species. ponent analysis (PCA), that integrated body We made additional observations to length and weight of the 6 bird species (Table examine the distribution of Tree Sparrows 2; SAS 1999), because body length and mass inside and outside the food area and the abun- may both play a role in dominance displays dances of large-sized bird species in the food (Robinson-Wolrath and Owens 2003). We area. We scanned the food area every 5 min to used a Chi-squared test to examine whether count the number of feral pigeons and Spot- different foraging techniques were applied ted-necked Doves inside the food area, and homogeneously by the 6 bird species in the the number of Tree Sparrows staying inside food area (SAS 1999). We performed a simple and outside the food area. linear regression to examine if the number of Tree Sparrows staying outside the food area Data analysis was related to the number of feral pigeons We calculated David's score (DS = W and Spotted-necked Doves foraging inside + W2 - L - L2; David 1987) as a dominance the food area. In addition, a Chi-squared test index for each bird species based on an in- was also used to examine whether the distri- terspecific dominance matrix of the 6 bird bution of Tree Sparrows and medium-sized species (Table 1). W and L represent the sum birds (either Chinese Bulbul or White-vented of species i's wins and losses, and W2 and Myna) were homogeneous across different L2 represent the wins and losses of species food amount stages.

Table 1. Interspecific dominance matrix of 6 bird species foraging in the food area of a Kaohsiung City park. Species are arranged by body size. Winners are listed down the left column; losers are listed across the upper row Losers Winners Feral GreySpotted-neckedWhite-ventedChinese Tree Total pigeonTreepie Dove Myna BulbulSparrow

Feral pigeon 1 33 2 2 48 86

Grey Treepie 1 - 10 0 7 19 37

Spotted-necked Dove 2 0 - 6 1 53 62

White-vented Myna 1 1 25 - 3 58 88 Chinese Bulbul 0 0 3 0 22 25 Tree Sparrow 12 2 24 3 10 - 51 Total 16 4 95 11 23 200 - Taiwan J For Sci 26(3): 255-66, 2011 259

Table 2. Body weight, length, and derived body size index of the 6 bird species included in the study. Species are ranked by the body size index Species Body weight (g)1) Body length (cm) Body size index2) Feral pigeon 267.5 32.5 L81 Grey Treepie 105.0 38.0 0.95 Spotted-necked Dove 128.0 28.8 0.40 White-vented Myna 100.0 2L0 -0A4 Chinese Bulbul 29.9 19.0 -L16 Tree Sparrow 23.5 14.5 -L56 1) Body measurements were derived from Handbooks of the Birds of the World (del Hoyo et al. 1992-2010). 2) Body size index was the PRIN1 through the principal component analysis that integrated body length and weight of the 6 bird species.

RESULTS 0.54, n = 6, p = 0.27, Fig. 1). David's score of the White-vented Myna was much higher Dominance rank than that predicted by body size, whereas the The Tree Sparrow was the smallest and Spotted-necked Dove's score was far lower least dominant species, and it was attacked or than expected. supplanted by other species up to 200 times (Table 1). The Spotted-necked Dove, although Foraging behavior relatively large in size, usually yielded ground Foraging techniques applied by the 6 to other species and was ranked second as a bird species in the food area significantly loser. On the other hand, the White-vented differed (x2 = 69.81, df = 18, p < 0.001, Fig. Myna was the most aggressive species and 2). Feral pigeons and Spotted-necked Doves chased other birds up to 88 times. The feral ate food completely in the food area. In con- pigeon was also aggressive. When foraging trast, the Grey Treepie carried food back to in the food area, it moved boldly around and trees 93A% of the time. The Chinese Bulbul supplanted other nearby species. The feral mainly foraged in the food area, but carried pigeon ranked second as a winner. David's a small portion of food to the trees. On the scores (dominance index) of the 6 species other hand, the White-vented Myna and Tree were as follows: White-vented Myna (9.54) Sparrow fed mainly in the food area, with > feral pigeon (8.46) > Grey Treepie (5.43) a few occasions of carrying food out of the > Chinese Bulbul (-6.38) > Spotted-necked food area. Dove (-7.87) > Tree Sparrow ( -9A9) (Fig. 1). The Tree Sparrow was usually the first Obviously, the first 3 species were dominant species to appear after food was spread into over the latter 3 species in the food area. the food area, and its number rapidly accu- We used a body size (or physique) index, mulated. Apparently, some of them had been extracted from PRIN 1 through the PCA that waiting around prior to the feeding time. integrated body length and weight (Table 2). Once the feral pigeons and Spotted-necked Although a positive correlation was found, Doves joined in, they immediately took over the relationship between the dominance in- the food area. These larger species occupied dex and body size was not significant (rs = the food area, causing Tree Sparrows to for- 260 Chen et al.Dominance rank and foraging behavior of birds in a park

10 White-vented Myna feral pigeon 0 Grey Treepie

75. 5 y = 4.19x + 7x10-6 = 0.54, n = 6, p = 0.27 0 -o E g Chinese Bulbul Spotted-necked Dove Tree Sparrow -10

-15 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Body size index Fig. 1. Relationship between body size and the dominance index was not significant, showing that the dominance rank was not solely determined by body size. Note that the David's score for the White-vented Myna was larger than predicted by body size, whereas the Spotted-necked Dove had a much lower score than expected.

Forage inside the food area N Carry food out of the food area Bring food up to trees Rob food from others 100%

90%

80%

70%

60%

50% 40%

30%

20%

10%

0% Feral pigeonSpotted-necked Grey Treepie Chinese Bulbul White-ventedTree Sparrow (n= 206) Dove (n = 292) (n= 72) (n= 189) Myna(n= 123) (n= 1935) Species Fig. 2. Foraging techniques in the food area significantly differed among the 6 bird species (x2 = 69.81, df = 18,p < 0.001). Five species foraged mainly in the food area, but the Grey Treepie carried food back to trees most of the time. age peripherally or stay outside the food area. of feral pigeons and Spotted-necked Doves Consequently, the number of Tree Sparrows in the food area increased (R2 = 03228, df = outside the food area increased as the number 195,p < 0.0001, Fig. 3). Taiwan J For Sci 26(3): 255-66, 2011 261

45 y = 0.64x + 5.64 840 R2 = 0.3228,p < 0.0001 35

cn20

E,15 <+..o ai 10 43 : '

5 10 15 20 25 30 35 Number of feral pigeons and Spotted-necked Doves inside the food area Fig. 3. Number of Tree Sparrows outside the food area was positively correlated with the total number of feral pigeons and Spotted-necked Doves inside the food area.

As food was consumed and the amount poor fit between body size and dominance in- declined, numbers of feral pigeons and Spot- dex, the feral pigeon, Grey Treepie, Chinese ted-necked Doves decreased as well (Fig. 4). Bulbul, and Tree Sparrow were located very In contrast, proportions of Chinese Bulbul close to the regression line. This indicates that (Fig. 4a), White-vented Myna (Fig. 4b), and body size may be important for some spe- Tree Sparrows (Fig. 4c) that foraged inside cies in determining the dominance rank, but the food area accordingly increased. Although definitely not for every species. It implies that total numbers of Chinese Bulbul (Fig. 4a) and the dominance rank of birds is also influenced White-vented Myna (Fig. 4b) also decreased by other factors besides body size. The domi- as food was gradually consumed, the number nance index of the White-vented Myna was of Tree Sparrows inversely increased (Fig. much higher than predicted by body size. The 4c). Numbers of individuals appearing in the White-vented Myna is the most abundant in- feeding site were not homogeneous at differ- troduced starling in Taiwan (Lin 2001). It has ent stages of food amount between the Tree white spots, is aggressive toward other native Sparrow and medium-sized birds (for Chinese species, and often scared away other species Bulbul, x2 = 112.34, df = 2, p < 0.001, Fig. by jumping and running around in the food 5a; for White-vented Myna, x2 = 50.74, df = 2, area while foraging. In contrast, the Spotted- p < 0.001, Fig. 5b). Eventually, the number of necked Dove was timid and easily intimi- Tree Sparrows greatly increased in the final dated by other bird species in the food area; it stage when little food was left. ranked second in the frequency of submissive behavior, and thus its dominance index was DISCUSSION much lower than expected from its body size. On the other hand, when food was abundant, Although the Spearman rank correlation all species fed intensively and tolerated one coefficient(I-,= 0.54, p = 0.27) indicated a another to some extent, and thus very few 262 Chen et al.Dominance rank and foraging behavior of birds in a park

Forage inside the food area Cany food out of the food area Bring food up to trees o Rob food from others Feral pigeons and Spotted-necked Doves 100 30 a. Chinese Bulbul 90 25 80

70 20 ,8 60

50 15 nu 40 10 t 30 0 z 20 5 10

0 11 0 Abundant(n =106) Medium (n = 64) Little(n =19)

100 30 b. White-vented Myna 90 25 80 70 20 .3 60 50 15 g oq 40 100 30 20

0 0 Abundant(n =60) Medium(n =42) Little(n =21) 100 30 Tree Sparrow 90 25 5 80 70 A 20 60 nu 50 15 0 40 nu 10 30 20 5 10

0 0 Abundant(n =768) Medium(n =240) Little(n =927) Amount of food Fig. 4. Proportions of individuals foraging inside the food area increased for Chinese Bulbul (a), White-vented Myna (b), and Tree Sparrow (c) when the number of feral pigeons and Spotted-necked Doves decreased in accordance with the amount of food left. clashes were observed. It is obvious that an pletely determined by body size; other factors abundant food supply lessened the intensity such as aggressiveness, submissiveness, food of conflict and consequently the frequency amount, group size, and foraging behavior of social interactions (Suhonen et al. 1992). may all influence the dominance ranking. Therefore, the dominance rank was not com- In terms of foraging behavior, larger spe- Taiwan J For Sci 26(3): 255-66, 2011 263

100-a Chinese Bulbul (n = 144) 90 Tree Sparrow(n= 1779) 80 70

eq 60 50 6) 40 30 20 10 0

100 90 White-vented Myna (n = 95) 80 Tree Sparrow (n = 1779) 70 60 50 40 30 20 10 0 AbundantI Medium LittleI Amount of food Fig. 5. Number of Tree Sparrows present at the feeding site was not homogeneous with that of Chinese Bulbul (a) or White-vented Myna (b) at different stages of food amount. cies exhibit obvious advantages over smaller occurred in this case because feral pigeons ones, and the presence of larger species does and Spotted-necked Doves usurped the space mean a considerable threat to smaller ones. In previously occupied by Tree Sparrows. On general, dominant birds often occupy a better the other hand, Tree Sparrows might increase position and have priority to use resources their use of poorer sites to minimize interfer- (Vehrencamp 1983, Hogstad 1989). Once the ence (Johnson et al. 2006). As a result, the dominant species appear, subordinate spe- number of Tree Sparrows outside the food cies will yield and change their foraging sites area was positively correlated with the total (Alatalo 1981, Alatalo and Moreno 1987). number of feral pigeons and Spotted-necked In this study, a large number of Tree Spar- Doves inside the food area. Occupation of rows often appeared in the food area soon the food area by feral pigeons and Spotted- after the food was spread. However, once necked Doves also created some obstacles for feral pigeons and Spotted-necked Doves ap- medium-sized species entering, even though peared, Tree Sparrows yielded the ground to the myna had a higher dominance rank over these larger species and foraged peripherally the feral pigeon and Spotted-necked Dove. or carried food out of the food area. Interfer- The proposition is supported by evidence that ence competition (Vahl et al. 2005, 2007) White-vented Mynas and Chinese Bulbuls 264 Chen et al.Dominance rank and foraging behavior of birds in a park

could only freely move into the food area apply Lanchester's laws and incorporate mul- after feral pigeons and Spotted-necked Doves tiple factors, including body size, group size, had gradually left. At an even later stage, Tree and social interactions to formulate a more- Sparrows would occupy the food area again realistic dominance ranking. after all other species had mostly gone. It is evident that body size does matter when birds ACKNOWLEDGEMENTS forage together at the same place, and that the Tree Sparrow was the least dominant species We thank the man that provided us the among the 6. chance to observe those birds he has been This study indicated that the dominance feeding for more than 5 yr. We are also in- status determined by one-on-one social inter- debted to HL Chen and SP Lai for their help actions is not consistent with those observed during the study. through foraging competition. The Spotted- necked Dove ranked fifth in one-on-one en- LITERATURE CITED counters; however, they co-occupied the food area with the feral pigeons, the most dominant Alatalo RV. 1981. Interspecific competition species. Such observations imply that subor- in tits Parus spp. and the Goldcrest Regulus dinate large species may eventually enhance regulus: foraging shifts in multispecific flocks. their competition when their group size be- Oikos 37:335-44. comes large. Nevertheless, this rule might not Alatalo RV, Moreno J. 1987. Body size, inter- be true for small species since the Tree Spar- specific interactions, and use of foraging sites row ranked the last in dominance regardless in tits (Paridae). Ecology 68:1773-7. of its high numbers. As a result, we consider Bang A, Deshpande S, Sumana A, Gada- that David's score might not fully represent gkar R. 2010. Choosing an appropriate index the dominance status of birds in a community. to construct dominance hierarchies in animal Since the scores are solely based on one-on- societies: a comparison of three indices Anim one interactions, they do not take into ac- Behav 79:631-6. count the group size of competitive species. Basset A. 1995. Body size-related coexistence: As the "square law" in Lanchester's "The- an approach through allometric constraints on ory of Combat" predicts, if all individuals home-range use. Ecology 76:1027-35. are equally vulnerable to attack, a large num- Basset A. 1997. Mechanisms relating biomass ber of subordinate individuals are better than concentration in individuals to interspecific a few dominant ones (Franks and Partridge resource competition. Oikos 78:31-6. 1993). The group size effect appeared to be Burger J, Gochfeld M. 1984. The effects of prominent in large birds such as the Spotted- relative numbers on aggressive interactions necked Dove, but seemed less apparent for the and foraging efficiency in gulls: the cost of be- smaller Tree Sparrow. Lanchester's "Theory ing outnumbered. Bird Behav 5:81-9. of Combat" was applied to behavioral studies Chapman MR, Kramer DL. 1996. Guarded (Franks and Partridge 1993, McGlynn 2000, resources: the effect of intruder number on the Shelley et al. 2004), in ant wars (Whitehouse tactics and success of defenders and intruders. and Jaffe 1996, McGlynn 2000), and inter- Anim Behav 52: 83-94. specific dominance matrix in birds (Shelley Chen TC, Shieh BS, Liang SH. 2005. As- et al. 2004). We suggest that further studies sociations of the community characteristics of Taiwan J For Sci 26(3): 255-66, 2011 265

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