Why do montagu's harriers disturb foraging sessions of white storks in south-east poland

Abstract. Montagu’s Harriers nest sympatrically with White Storks in SE Poland. Most of Montagu’s Harrier females are capable of modifying the foraging efficiency of White Storks by performing selective dives on foraging birds in habitat patches, which are optimal for foraging the raptor. In such habitats storks are fiercely attacked, their foraging sessions are shortened and their foraging efficiency is reduced. This phenomenon may reflect simultaneous tendency of both species for optimal habitat monopolisation during breeding. It is may be related to observed nest site fidelity of considered species.
Key words: White Stork, Ciconia ciconia, Montagu’s Harrier, Circus pygargus, behaviour, foraging.
Address: Department of Nature Conservation, Institute of Biology, Maria Curie-Sklodowska University, Akademicka 19, PL 20-033 Lublin, Poland; e-mail: [email protected].
Ïî÷åìó ëóãîâûå ëóíè ìåøàþò êîðìèòüñÿ áåëûì àèñòàì íà þãî-âîñòîêå Ïîëüøè? - È. Êèòîâ- ñêèé. - Áåðêóò. 16 (1). 2007. - Íà þãî-âîñòîêå Ïîëüøè îáà âèäà ãíåçäÿòñÿ ñèìïàòðè÷íî. Áîëüøèíñòâî ñàìîê ëóíÿ ìîãóò âëèÿòü íà ðåçóëüòàòèâíîñòü êîðìåæêè àèñòîâ, àòàêóÿ ïòèö, ñîáèðàþùèõ êîðì íà ó÷àñò- êàõ, îïòèìàëüíûõ äëÿ îõîòû ñàìèõ õèùíèêîâ.  òàêèõ ìåñòàõ ëóíè ÷àñòî íàïàäàþò íà êîðìÿùèõñÿ àèñ- òîâ, èõ êîðìîâûå ñåññèè ñîêðàùàþòñÿ, à ðåçóëüòàòèâíîñòü êîðìåæêè ñíèæàåòñÿ. Ýòî ÿâëåíèå ìîæåò îò- ðàæàòü òåíäåíöèþ ìîíîïîëèçàöèè îïòèìàëüíûõ êîðìîâûõ áèîòîïîâ â ãíåçäîâîé ïåðèîä.
prefer insects, whereas in the northern breed- ing area a basic component of their diet are The process of selecting a patch of habitat insects and small vertebrates (Lazaro, 1982; for foraging involves choosing among habitat Muzinic, Rasajski, 1992; Tsachalidis et al., patches there that differ in the probability of 2002; Kosicki et al., 2006). Moreover, it has attack of predators. Some habitat patches pro- been proven for both species in northern Eu- vide the highest rate of energetic gain but these rope the abundance of Common Vole (Micro- may be dangerous because the risk of being tus arvalis) is ultimate factor controlling a killed, hurt, or having a foraging session dis- breeding success (Creutz, 1988; Pinowska et rupted (Bryant, Grant, 1995; Goldberg et al., al., 1991; Pinowski et al., 1991; Tryjanowski, 2001; Ovadia , Dohna, 2003). In such condi- Kuzniak, 2002) c.f. (Krogulec, 1992; Butet, tion foragers to maximise energetic gain have Leroux, 1993; Salamolard et al., 2000). Also to trade off between foraging and aggression it has been proven that meadows, especially in profitable patches (Lima, Dill, 1990; Brown, when cut regularly, and pastures are basic for- aging habitats for both species (Schipper, In some areas of Europe White Stork (Ci- 1977; Clarke, 1996; Salamolard et al., 2000; conia ciconia) (hereafter stork) and Montagu’s Latus, Kujawa, 2005; Rachel, 2006). Cases of Harrier (Circus pygargus) (hereafter harrier) predation of Storks on Montagu’s Harrier nest- are sympatric. Nevertheless their social rela- lings were observed in Spain (B. Arroyo, pers.
tion on foraging areas have not been studied com.). On the other hand attacks Montagu’s in detail. It has been known that both species Harrier on White Storks have been reported display similar preferences for open habitats (Kitowski, 1994; Kitowski, 2003a). In the and their feeding preferences are also much southeastern Poland, at major breeding sites alike (Schipper, 1977; Clarke, 1996; Salamo- Harriers occur with Storks ( Piotrowska, 2000; lard et al, 2000; Latus, Kujawa, 2005; Rachel, Kitowski, 2002; Tomialojc, Stawarczyk, 2006). Both species on southern breeding sites 2003). This paper attempts reveal pattern of Montagu’s Harriers and White Storks in SE Poland Habitats of 378 foraging sessions of White Storks (first seen peck of foraging individual) Áèîòîïû 378 ñëó÷àåâ êîðìåæêè áåëîãî àèñòà (ïî ïåðâîìó êëåâêó êîðìÿùåéñÿ îñîáè) the impact of Montagu’s Harriers on the for- al., 2006). The following habitat types were aging tactic of White Storks, especially regard- defined: a) meadows; b) pastures, areas grazed by cattle; c) arable lands; d) wetlands involv- ing marshes, water filled ground excavations and drainage ditches. The distribution of these habitats in the study area was mapped and cal- In 2000–2003 I studied 8–12 nesting pairs culated their surface areas with a digital of Storks in the villages Plawanice, Kroczyn, planimeter from a high-resolution aerial photo.
trict, SE Poland). They are close Roskosz Re- if they were cut two or more times per year, serve (51° 08´ N, 23° 37´ E, SE Poland) where non intensively cut meadows were cut once a in 2000–2003 nested approximately 9–12 pairs year. Behavioural events were timed with an of Harriers (Kitowski, 2002; Kitowski, unpubl. electronic stop-watch. Averages were com- data). Observations were performed every year pared using the Student’s t test and ANOVA from 15 April – 15 August. Harriers were re- (parametric data, given in the text as mean ± cognised as foraging if when the first observed SD) and Mann-Whitney U-test and Kruskal- they were crusing or hovering (Clarke, 1996; Wallis ANOVA (non-parametric data given in Kitowski, Wojtak, 2001; Kitowski, 2003b). the text as mean ± SE) (Sokal, Rohlf, 1981; Habitat use of foraging Storks was measured Fowler, Cohen, 1992).
by recording the amount time spent foraging in a particular place. Prey availability was not assessed. Areas less than 2.5 km from semico- lonies of Harriers were classed as “nearby for- aging patches” of Storks (Johst et al., 2001).
The hunting areas of 2.5–5 km from Harrier Totally 378 Stork’s foraging session were semicolonies were classed as “remote forag- observed. Storks when foraging near the ing patches” for Storks (Johst et al., 2001). semicolonies of harriers (n = 161) preferred Analyses of Stork foraging efficiency were meadows and pasture (Table 1). Far from only calculated for bouts of predation involv- semicolony (n = 217) storks also tended to for- ing capture vertebrate. This because the bio- age on meadows and pastures. Wetlands were mass of vertebrates is so much greater and their seldom exploited, but were used in proportion energy value is so much higher the those in- to their availability (Table 1). In 12 cases vertebrates (Antczak et al., 2002; Kosicki et (63.1 % of n = 19 bouts) used drainage ditches.
Number caught vertebrates prey (when swallowing was observed) by White Storks Êîëè÷åñòâî ïîéìàííûõ áåëûìè àèñòàìè ïîçâîíî÷íûõ (íàáëþäàëîñü ïðîãëàòûâàíèå) In case of use of Mann-Whitney U-test data are given as mean ± SE, for Student’s t-test was * p < 0.05, ** p < 0.01, *** p < 0.001.
More vertebrate prey were captures per ses- sion by Storks on foraging areas near Harrier near Montagu’s Harrier’s semicolonies semicolonies (Table 2). However, for both for- aging area the number of prey caught depended on habitat class with a significant preference semicolonies 97 (60.2 %) sessions did not in- of meadows. This was there near harrier volved interaction with harriers. And its fin- semicolonies (nearby foraging patches) ished by intrinsic patch-leaving decision of the cated foraging area (remote foraging patches) (39.7 %). Stork foraging sessions were dis- turbed by animals and people (no – intrinsic patch – leaving decision of individual). Among Table 3 the disturbed sessions, a number of 58 (90.6 %, n = 64) involved harriers (Table 3). The re- Frequency of attacks on 58 individuals of maining disturbances were by: Red Foxes (Vul- White Stork in relation to habitat of foraging pes vulpes) (n = 2), other storks (n = 1), people ×àñòîòà àòàê íà 58 îñîáåé áåëîãî àèñòà â (n = 2) and cattle (n = 1). When compared to çàâèñèìîñòè îò êîðìîâîãî áèîòîïà attacks by males 6 bouts (10.3 %, n = 58), har- rier females (52 bouts, 89.7 %, n = 58) attack more frequently foraging storks. Differences were statistically significant (χ2 = 36.5, df =1, p < 0.0001). Female harriers harassed forag- ing storks more severely, performing 3.3 ± 1.4 dives (range: 1–5 dives) also lasting 32.6 ± 6.9 sec (range: 3– 40 sec.). While males per- formed 1.8 ± 1.6 dives (range 1–5 dives last- ing 30.7 ± 15.2 sec. (range: 3– 49 sec.). The differences in the number of performed dives Montagu’s Harriers and White Storks in SE Poland were found significant (Mann-Whitney U test: Z = –2.11, n = 52, n = 6, p < 0.035), but dif- ferences in time of these harassments (Mann- Dives of Montagu’s Harrier addressed into Whitney U test: Z = –1.27, n = 52, n = 6, n.s.) White Stork individuals close semicolony Females harriers were more likely to inter- Íàïàäåíèÿ ëóãîâûõ ëóíåé íà áåëûõ àèñòîâ rupt stork foraging sessions in late nestling and âîçëå ïîëóêîëîíèè early post-fledging periods (from 15 June till 15 July) compared to other phases of breed- ing of the raptor (37 events vs. 15 events: χ2 = 9.3, p < 0.002). During disturbed foraging ses- likely to occur on intensive cut meadows 17 (73.9 %) vs 6 (26.1 %), χ2 = 5.2, p < 0.02. For other foraging sessions on meadows near Har- rier semicolonies, storks preferred regularly cut meadows: 27(65.9 %) vs 14 (34,1 %), χ2 = 4.12, p < 0.04). Habitat types exploited by harrier dives at storks foraging differed sig- storks foraging near harriers’ colonies was re- nificantly for particular patches. The highest lated to the number of caught vertebrates if number of dives was performed on storks for- the sessions were not disturbed by Harrier aging on meadows and pastures (Kruskal- (ANOVA: F = 24.51, p < 0.001). Similar Wallis ANOVA: H = 14.56, df = 3, p < 0.001) relation were not found when foraging was (Table 4). Storks, during interrupted foraging interrupted by no-intrinsic patch-leaving de- sessions near Harrier colonies (nearby forag- cision of an individual: Kruskal-Wallis ing areas), tended to forage in shorter bouts ANOVA: H = 0.81, df = 3, n.s. (data for calcu- and catch fewer vertebrate prey when com- lation included in Table 2). The number of pared to uninterrupted sessions performed Comparison of foraging time sessions of White Storks in two contexts Ñðàâíåíèå âðåìåíè êîðìåæêè áåëûõ àèñòîâ â äâóõ êîíòåêñòàõ Data are given as mean ± SE for Mann-Whitney U-test and mean ± SD for Student’s t-test.
Comparison of number of vertebrate prey caught during foraging sessions of White Storks Ñðàâíåíèå êîëè÷åñòâà äîáûòûõ ïîçâîíî÷íûõ âî âðåìÿ êîðìåæêè áåëûìè àèñòàìè âîçëå ïîëóêîëîíèè ëóãîâûõ ëóíåé Data are given as mean ± SE for Mann-Whitney U-test and mean ± SD for Student’s t-test.
(Table 5, 6). Uninterrupted Storks foraging meadows, they exhibited stronger preference sessions were observed mainly in June (n = for pecking on frequently cut meadows than 21) and July (n = 23). Interrupted sessions took foraging on no- intensively managed meadows: place in April (n = 3), May (n = 4), and Au- 83 (71.6 %) vs. 33 (28.4 %) , χ2 = 21.5, df = 1, gust (n = 7). The frequency of uninterrupted p < 0.001. Here only 19 (8.7 %) sessions were foraging sessions in particular months of ob- disturbed by interactions, which 9 (4.1 %) were servations diverged from the expected one (χ2 interactions with adult males of Montagu’s = 32.0, df = 4, p < 0.001). During study n = 3 Harrier, and all occurred on frequently cut communal defences were also observed into meadows. The remaining were interactions which 4.25 ± 0.96 (range: 3–5 individuals) with a Red Fox (n =1), a Marsh Harrier (Cir- Montagu’s Harriers individuals were involved, cus aeruginosus) male (n =1), other storks (n which in the duration of 122 ± 55 sec. (range: =2), people on foot (n = 2), agricultural ma- 84–201 sec.) performed 3.7 ± 0.96 dives chinery (n = 4). One case, in which storks for- aged for at least 57 minutes followed tractor plowing was excluded from this calculation.
Female of harrier used to forage closer to Stork foraging sessions (n = 217) on areas the their semicolonies, and tended to prefer further than 2.5 km from harrier semicolonies meadows and pastures, avoiding arable lands (remote foraging areas) (Table 1) lasted longer and wetlands (Table 8). Due to hunting duties (Table 7) than sessions performed near harrier males rarely foraged near semicolonies and colonies but the number of vertebrate prey exploited habitat patches in proportion to their caught per unit time was smaller (Table 2) and presence. The exception was that wetlands in which were foraging bouts near harriers co- which were quite distinctly avoided (Table 8).
lonies on meadows and pastures 190 (87.6 %, Harrier foraging preferences in areas far from n = 217) (Table 1, 2, 7). If storks foraged on semicolonies were reported elsewhere. There Montagu’s Harriers and White Storks in SE Poland Comparison of the time lasting foraging sessions of White Storks Ñðàâíåíèå ïðîäîëæèòåëüíîñòè êîðìîâûõ ñåññèé áåëûõ àèñòîâ Data are given as mean ± SE for Mann-Whitney U-test and mean ± SD for Student’s t-test.
* p < 0.034, ** p < 0.0003, *** p < 0.0001.
was a distinct preference for regularly cut have been helpful in understanding why harri- meadows and pasture lands (Kitowski, Wojtak, ers interfere with foraging storks in habitat patches covered with short vegetation (regu- larly cut meadows). White Storks (Creutz, 1988; Alonso et al., 1991; Struwe, Tomsen, 1991; Johst et al., 2001; Moritzi et al., 2001) Many studies have demonstrated the effects and Montagu’s Harriers (Nieboer, 1973; of spatial and temporal clumping of resources Clarke, 1996; Kitowski, 2003b) prefer open on the frequency of competitive aggression areas with short vegetation. Here, prey is more (Grant, Guha 1993; Bryant, Grant, 1995; Gold- accessible for harriers (Clark, Stanley, 1976) berg et al., 2001; Plesner et al., 2005). These and storks (Moritzi et al., 2001) than in areas Foraging sessions Montagu’s Harrier females and males close semicolony Êîðìîâûå ñåññèè ñàìîê è ñàìöîâ ëóãîâûõ ëóíåé âîçëå ïîëóêîëîíèè Data based on every 30 minutes scaning first seen foraging individuals.
with tall grass, where vegetation provides hides Most of the foraging sessions interrupted for prey. Increasing food patch quality (easier storks (76 %) occurred in June and July. This prey accessibility) results in a decrease in the can be accounted to the increased rate of ver- intrinsic patch-leaving rate and an increase in tebrates in total prey biomass of storks as the the rate of aggressive interactions involving reproductive season advances. Observations the White Storks and Montagu’s Harriers. supported by other authors (Struwe, Thomsen, Storks and harriers seem to able to asses, at 1991; Antczak et al., 2002) including those and least in part, the quality of patch habitats in performed in south-eastern Poland confirm that which foraging. The quality of the foraging in this time frequency small vertebrates in- habitat impact in harriers in the clutch size and creases in stork’s diet (Kitowski, unpubl. data).
the number of young fledged (Butet, Leroux, It has been demonstrated statistically that 1993; Arroyo, 1997; Salamolard et al., 2000). foraging storks suffer more from being ha- In storks quality of habitat near the nesting site rassed harrier females than males. This results is significant factors affecting on breeding den- from females being more regular present near sity (Latus et al., 2000; Latus, Kujawa, 2005). semicolonies their in late nestling and early Other studies suggest that where food is post-fledging period (Kitowski, 2003a; more predictable in space, there is a strong Kitowski, unpubl. data). When nestlings do not tendency for monopolisation of foraging ar- need to be brooded, females are know to for- eas and defence prevalence increases abruptly age within the area of about 1.5 km their nests, with prey concentration or vulnerability (Grant, a males foraging further. This in turn results Grant, 1994; Bryant, Grant, 1995). A situa- from the spatial separation of hunting areas or tion like this occurs in the study area Common used varying habitat types related to the re- Voles populations fluctuate from season to sea- verse sexual sized dimorphism, which itself son because, in this species population explo- serves to reduce prey competition between sions (“vole years”) and depressions (“ non- individuals of opposite sexes (Newton, 1979; vole years”) are observed every 3–4 years Temeles, 1985). Surprising, this spatial sepa- (Pucek, 1984). Nevertheless patches where ration of the hunting areas of males and fe- prey is more accessible (such as regularly cut males of Montagu’s Harriers partialy deter- meadows) can persisted over many years and mines the foraging efficiency of White Storks.
the probability of finding better access for food Females of harriers, having a limited time and even in time “ non-vole years” is greater here area available for hunting due to their defen- compared to other areas. The distribution of sive duties (Kitowski, 2003a), must choose food can be partially predictable in for harri- most effective patches for hunting. These must ers and storks. Harriers probably become more also be located near nests. These patches hap- aggressive towards storks learn which habitat pen, also to be good nearby foraging patches patches are predictably offer prey. They then of storks.
monopolising these patch during future repro- ductive cycles as has been observed in some voles performed on study area (Trociuk, 1987; other birds (Grant, Kramer, 1992; Grant, Grant, Maruchniak, 1988) showed that meadows im- 1994; Goldberg et al., 2001). Undoubtedly, mediately adjacent to the harrier’s semicolo- harriers benefit by invest energy in monopo- nies were heavy depleted of voles due to the lising better quality patch habitats. The high exploitation by nesting harriers contrary of the probability of aggression of harriers to storks areas further away from semicolonies serves is also supported by observed nest site fidelity as the remote foraging patches of storks. Fe- both species (Profus, 1991; Krogulec, 1992; male harriers quickly remove competitor in- Kitowski 2000; Kitowski, unpubl. data). In cluding White Storks, from the areas which storks such fidelity is reflected by strong ten- they exploit. These factors contribute to higher dency to return to the nest of last year breed- rates of early departure of foraging White ing ( not to the nest of the birth) (Profus, 1991). Storks. This raises their foraging costs. As sug- Montagu’s Harriers and White Storks in SE Poland gested White Stork foraging involves a trade Arroyo B.E. (1997): Diet of Montagu’s Harrier Circus off between highly effective foraging (espe- pygargus in central Spain: analysis of temporal and cially of voles) and acute aggression from har- geographic variation. - Ibis. 139: 664-672.
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