Stopover behaviour and dominance: a case study of the Northern Wheatear Oenanthe oenanthe Juan Arizaga1,*, Heiko Schmaljohann1 & Franz Bairlein1 Arizaga J., Schmaljohann H. & Bairlein F. 2011. Stopover behaviour and domi- nance: a case study of the Northern Wheatear Oenanthe oenanthe. Ardea 99: 157–165. In migrants that occupy and defend territories during migration, social status can determine access to preferred resources and alter stopover behaviour. The potential variation in stopover behaviour between dominant and subordinate individuals of the Northern Wheatear Oenanthe oenanthe, a territorial migrant bird species, was examined at an offshore stopover area on the island of Helgoland during the spring of 2009. Overall, territories were occupied and defended by dominant birds (identified as those that won ≥50% of the conspe- cific aggressive encounters). Dominant birds also initiated most conspecific aggression. Unable to settle and defend a territory, subordinates tended to be vagrant and stopped over for shorter periods of time. This is likely due to the fact that subordinates were continuously displaced by dominants, and therefore had less access to food. Subordinates also tended to forage at lower rates. Northern Wheatears were primarily found in zones with a higher number of shelters, supporting the idea that avoidance of predation plays a role in site selection. Key words: asymmetric contests, body size, Helgoland, migration, social status, territoriality 1Institute of Avian Research “Vogelwarte Helgoland”, An der Vogelwarte 21, D-26386 Wilhelmshaven, Germany; *corresponding author ([email protected]) Migratory birds may divide their journey into periods of (Moore et al. 2003, Dierschke et al. 2005). In other flight, when energy stores are consumed, and stop- cases, social status is determined by body size, with overs, when fuel reserves are replenished. Because low larger birds likely to be more dominant (Lindström et rates of fuel accumulation result in lower speeds of al. 1990). Although there is no clear evidence showing migration, which can have a negative impact on that fuel load determines social status in migrants, lean survival and/or fitness (Newton 2008), prompt refu- birds foraged at a higher rate than heavier birds after elling is crucial at stopovers (Alerstam & Lindström crossing the Gulf of Mexico during the spring (Loria & 1990). In species that occupy and defend territories Moore 1990, Wang & Moore 1997). Such higher moti- during migration (Lindström et al. 1990, Dänhardt & vation to refuel before resuming migration may encour- Lindström 2001, Dierschke & Delingat 2001), social age leaner migrants to become dominant (i.e. to be status and hierarchy position can strongly determine able to expel other birds from a territory and therefore access to food and/or shelter (Moore et al. 2003). This to have priority access to resources). determines their stopover decision in terms of fuelling In this study, we tested hypotheses about differ- strategy or departure (Carpenter et al. 1993a, Dierschke ences in the stopover behaviour of dominant and et al. 2005). subordinate individuals of a territorial migrant bird An inherent question associated with any study species, the Northern Wheatear Oenanthe oenanthe. analysing the influence of social status on behaviour is This is a long-distance migratory passerine that breeds how dominance is measured. Age and/or sex often across most of the Holarctic and overwinters in determine dominance, with adults and/or males often Sahelian Africa (Cramp 1988). Two subspecies occur in being dominant over juveniles or females, respectively W Europe: O. o. oenanthe, which breeds all across 158 ARDEA 99(2), 2011 Europe and Asia and reaches Alaska and NW Canada, considered as a threat by dominant birds and could be and O. o. leucorhoa, which breeds in NE Canada, Ice- less frequently attacked (Davies & Houston 1981, land and Greenland (Conder 1989). Wheatears are Carpenter et al. 1993b). Subordinates could also show insect-eating passerines that feed on small inverte- high rates of aggressive encounters if they fight among brates found on the ground and in low vegetation and each other. ocasionally in the air (Cramp 1988). Stopover duration. Subordinate birds could adopt Territoriality and habitat use. The “good feeding two strategies in relation to their stopover duration: (1) site” hypothesis states that a territory is placed in an depart from the stopover site to look for better condi- optimal feeding area, where territorial birds increase tions in other areas, resulting in shorter stopover peri- survival likelihood by having consistent access to suffi- ods than dominants (Weber et al. 1999, Dierschke et al. cient food (Davies & Houston 1981, Snow & Snow 2005), or (2) remain at the site and adopt alternative 1984). The relationship between territorial behaviour strategies, such as vagrancy and/or the occupancy of and social status has rarely been analysed in stopping- less suitable habitats within the stopover locality, over migrants (but see Lindström et al. 1990, Carpenter resulting in similar or longer stopovers than dominants. et al. 1993a, Dierschke et al. 2005). Dominants are In the case of high food abundance, subordinates more likely to settle and establish a territory than should gain weight and depart with fuel loads similar subordinates (Carpenter et al. 1993a, Carpenter et al. to dominants (Dierschke et al. 2005). This suggests that 1993b), with such territories usually occurring in areas subordinates are able to compensate for their worse with abundant resources, such as food or shelter situation in comparison with dominants (Dierschke (Marra et al. 1993, Cuadrado 1995, 1997). Con- et al. 2005). Such compensation, however, could only sequently, subordinates can adopt three alternative be possible when food is very abundant (Carpenter et strategies: (1) depart from the stopover site, (2) estab- al. 1993b, Dierschke et al. 2005). We hypothesize that, lish a territory in a suboptimal habitat away from domi- except in case of superabundance of food, subordinates nants (Marra et al. 1993, Stutchbury 1994), or (3) stop over for shorter periods of time than dominants. become vagrant (Sherry & Holmes 1996, Catry et al. 2004). The latter possibility would allow subordinates to briefly enter the territory of dominant birds to feed. METHODS In the Northern Wheatear, we hypothesize that domi- nants defend territories (Conder 1989, Dierschke et al. Sampling area and protocol 2005), and subordinates are vagrant. Helgoland is a 1.5 km2 island in the North Sea approxi- Foraging. Subordinate birds are often forced to mately 50 km from the coast of north Germany. either utilize suboptimal habitat (Figuerola et al. 2001) Helgoland contains open grasslands and beach as chief or to have intermittent access to food in higher quality habitats. It is used as a stopover site by O. o. oenanthe habitat (i.e. entering a territory to feed; Carpenter et al. and O. o. leucorhoa during both the autumn and spring 1993b, Dierschke et al. 2005), resulting in lower forag- migration (Dierschke & Delingat 2001, 2003). During ing rates compared to dominants holding territories. spring, O. o. leucorhoa faces a longer flight from Alternatively, subordinates could reach a mean foraging Helgoland over the sea than O. o. oenanthe to reach its rate similar to that of dominants under super-abundant breeding grounds (O. o. leucorhoa to Iceland/ Green- food availability (Ellegren 1991, Carpenter et al. land/E Canada; O. o. oenanthe to Scandinavia). 1993b, Dierschke et al. 2005). This happens when Data were collected during the spring migration subordinates are able to increase their foraging rates period from 15 April to 21 May 2009. Wheatears in (Carpenter et al. 1993b, Dierschke et al. 2005). Such a Helgoland tend to concentrate along the beach, where strategy, however, could be valid only for a fraction of food availability is normally higher (Delingat & subordinates and under particular conditions Dierschke 2000), and birds are more territorial than in (Dierschke et al. 2003). We hypothesized that subordi- other areas (Dierschke & Delingat 2001). Accordingly, nate Northern Wheatears have lower foraging rates the study was carried out on the northeastern beach than dominants, indicating less access to food. (550 × 30 m). The most northeastern part of the beach Aggression. Subordinates may be involved in more was bordered by a seawall flanked by blocks of stone aggressive encounters with dominant birds (Dierschke with many cavities; such cavities were consistently used et al. 2005) since subordinate birds forage in widely by Wheatears as shelter (J. Arizaga, pers. obs.). The spread areas along which they cross the territories of opposite (northwest) extreme of the beach was dominants. Alternatively, subordinates may not be bordered by a cliff that is more than 40 m high, at the Arizaga et al.: STOPOVER BEHAVIOUR AND DOMINANCE 159 base of which there were many large rocks, also provid- 51.4 ± 7.7, n = 29 ing shelter for Wheatears. The rest of the beach was 12 50.0 (0.0-83.3) open sand, devoid of rocks and other types of shelter. 10 Behind the beach there was a narrow dune-line followed by a shrubby area. 8 Wheatears were captured with spring traps baited 6 with mealworms. Upon capture they were ringed both frequency with metal and coloured rings (code: metal-colour on 4 the left leg/colour-colour-colour on the right leg). Birds 2 were sighted with a telescope (magnification 60×). 0 Trapping mainly occurred from 12:00 to 16:00. The 0 20 40 60 80 100 island’s size allowed almost all of the migrant birds aggression won (%) landing on Helgoland to be detected upon arrival; stud- ies of colour-ringed birds showed a re-sighting likeli- Figure 1. Frequency distribution of the percentage of aggressive encounters won by Northern Wheatears stopping-over on hood of up to 83% (Schmaljohann & Dierschke 2005). Helgoland. Mean ± SE, sample size and median (together with This allows for reliable measures of stopover duration percentiles 25 and 75) values are shown.