Observations on zugunruhe in spring migrating Eared Grebes.
|Abstract:||About 200 North American Eared Grebes (Podiceps nigricollis californicus) at Tule Lake Refuge in northern California were observed engaging in successive waves of mass pattering and pattering flights on 25 May 2011. Most grebes present in a part of a canal were involved in this activity. Counts of grebes on the morning of 26 May suggest an important portion of the Eared Grebes seen in pattering could have left the area over night. The behavior was characterized as zugunruhe. Directed mass pattering of Eared Grebes may contribute to synchronization of the onward migration of the birds involved.|
Migratory birds (Research)
Animal flight (Research)
|Publication:||Name: The Wilson Journal of Ornithology Publisher: Wilson Ornithological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2012 Wilson Ornithological Society ISSN: 1559-4491|
|Issue:||Date: March, 2012 Source Volume: 124 Source Issue: 1|
|Topic:||Event Code: 310 Science & research|
|Geographic:||Geographic Scope: United States Geographic Code: 1USA United States|
North American Eared Grebes (Podiceps nigricollis californicus) are
seldom seen in flight, except when they migrate (Bent 1919, Gaunt et al.
1990). The migration of the species has been well studied (Storer and
Jehl 1985, Gaunt et al. 1990, Jehl 1997, Cullen et al. 1999, Jehl and
McKernan 2002, Jehl and Henry 2010). Cullen et al. (1999) indicate
migration flights begin around dusk and end before dawn. Jehl and Henry
(2010) note strict correspondence of departure with near-total darkness.
Grebes tend to gather as the time for departure nears (Jehl and McKernan
2002). Predeparture activities include group diving, and submerging and
surfacing in near unison. A unique call is given as grebes prepare to
depart and immediately before actual take-off (Jehl and Henry 2010).
Daytime flights are possibly observed only when grebes rebuild their flight muscles prior to migration when they may perform one or two short practice flights (Jehl and Henry 2010) or race across the surface in short practice flights, often in small groups (Jehl and McKernan 2002). I was surprised to observe a mix of pattering and flight by larger groups of Eared Grebes in Northern California during daylight conditions. I describe these common pattering flight maneuvers and discuss their possible meaning.
A study of courtship of Eared Grebes was undertaken at Upper Klamath Lake, Oregon, and Lower Klamath Refuge and Tule Lake Refuge, both in northern California, from 14 to 27 May 2011. This region is known to support thousands of Eared Grebes each year for nesting, water levels permitting. The California refuges hosted 7,397 and 3,700 nests, respectively, in 2003 and 2004 (Shuford et al. 2006). Fieldwork was from 0700 to 1700 hrs each day using a car as a blind. The car was parked at suitable places along roads near bodies of water and remained immobile for up to 3 hrs. The behavior and displays of grebes were documented either by photograph, video film or immediate voice recording. All observations of pattering flights are from Tule Lake Refuge, part of the Klamath Basin National Wildlife Refuges, an artificial water impoundment of mostly open water covering ~5,200 ha at an altitude of 1,200 m and surrounded by croplands. The observations were in an area called the English Channel (41[degrees] 51' 202 N, 121[degrees] 29' 727 W) in the central part of the wildlife tour into the refuge. This is an L-shaped canal, <50 m in width. It opens at its northern end into large sump lA, an open and shallow area of the lake. It takes a left turn after ~1.6 km in a straight line from north to south (NS canal or NS part of the English Channel) and continues east for another 0.5 km (EW canal or EW part of the English Channel) until ending at a dam-levee that separates it from the adjacent larger sump 1B (Fig. 1). The entire canal is devoid of emerging vegetation.
I differentiate between pattering (a grebe with flapping wings runs with paddling feet or even partially glides over the water surface, but remains in constant contact with the water), pattering flight (after an initial pattering, a grebe is airborne for a distance limited to a few meters during which it does not touch the water surface), and real flight (the distance covered while airborne exceeds 10 m). It is well established that Eared Grebes use pattering in the retreat display and during escape/pursuit or more generally during aggression (McAllister 1958, Cullen et al. 1999); these occurrences are not included. My objectives in this paper are to provide a full description of pattering and pattering flights by larger numbers of grebes, and to discuss possible reasons for their occurrences.
Observations in the southern English Channel on 25 May started at 0900 hrs. Over 200 Eared Grebes were scattered partially in loose groups all over the EW part of the English Channel around midday when about three quarters of them engaged in pattering. The grebes did so in consecutive waves, all into a western direction towards the connection to the NS canal. The sudden take-off by one or two grebes seemed to cause others in their immediate vicinity and on their way to move in the same direction. Groups of 10-30 birds pattered over a short distance (20-30 m), some briefly loosing contact with the water surface in a pattering flight. Grebes getting briefly airborne possibly did so to avoid collision with conspecifics that remained stationary on the water surface. Grebes landed ahead of others that started similar maneuvers in their wake, perhaps carrying along some of those that had just stopped pattering. A few additional waves of pattering were launched. Some birds dived after landing; others elevated their necks, remained alert, and looked around without changing their westward orientation. Most of the population, including subgroups closer to the NS canal which were not observed to patter, was swimming in the direction of the NS canal. The eastern and central parts of the EW canal were rather empty of Eared Grebes after some 2-3 rain, leaving only a few Westem (Aechmophorus occidentalis) and Clark's grebes (A. clarkii) and a few ducks remaining. Fewer than 100 Eared Grebes were still swimming in the western part of the EW canal towards the connection with the NS canal when they encountered about 40 birds swimming in a group to reenter the EW canal. A rough count less than 10 min later indicated that >200 Eared Grebes had again spread over this canal.
Pattering and pattering flights started anew only ~20 min after the start of the first general movement by the Eared Grebes. Take-off by one or two Eared Grebes incited others in their surroundings to join as before. The birds moved westward in several waves and continued swimming into the same direction after landing. More grebes left the EW canal where only about 30 remained, all towards its western end. A first group of swimming grebes returned ~1 min later. It was followed by other loose groups. I counted 130 grebes 5 rain later and soon >240 birds were again present inside the EW canal.
A longer period without group pattering, but with continuous calling, occasional displays and much surface feeding on phantom midges (Chaoborus crystillinus) followed until ~1400 hrs. Individual grebes performed feeding dives, but no group diving, or submerging and surfacing in near unison was observed. The general pattering in waves and westward swimming towards the connection with the NS canal started again and most Eared Grebes finally left the EW canal. The first grebes had turned and swam to return to the EW canal when a sudden simultaneous eastward pattering of >50 re-entering grebes occurred. Two or three more waves by other groups followed immediately. Five minutes later, 232 grebes were counted inside the EW canal.
Only the continuous and contiguous calls of the birds were heard for ~20 min. Ten birds then initiated a fourth round of pattering in waves. This time, the grebes had no common general direction. The grebes more in the central part of the observed area moved towards the dam, those already closer to the eastern end pattered into a more southwestward to westward direction. The population present divided into two groups. About 100 grebes were clustered near the dam and another 100 were scattered over the upper western third of the EW canal. The space in between both groups remained mostly empty. The western group started immediately to swim eastward while the eastern group slowly dispersed. The groups soon melted and spread over the empty space that had separated them.
Perhaps five additional pattering flights of up to 4-5 grebes were observed in between the different mass pattering and pattering flights. It was not known whether these were premature attempts to initiate a wave or whether they were unrelated to the mass movements.
The observations ended at ~1700 hrs and 257 grebes were counted in the EW canal (26 in the connecting corner square to the NS canal), 65 were present in the lower half of the NS canal and 347 in the upper half. Only five additional Eared Grebes were detected at the mouth to sump 1A. Other parts of the sump close to the English Channel were empty of Eared Grebes. A count of the birds at 0700 hrs on the following day totaled exactly 400 individuals, 269 less than the previous count. Only 77 grebes were observed inside the EW canal (28 in the connecting corner square) while the NS canal had 323 grebes. Three hours later, 126 Eared Grebes were recorded in the EW canal and 337 in the NS canal. The two counts on 26 May revealed quite differing numbers of grebes. The EW canal held 131 to 180 grebes less and the entire English Channel held 206 to 269 grebes less than on the afternoon of 25 May.
Eared Grebes had arrived at Tule Lake Refuge in the course of the previous 2-3 weeks. I assume that shortly after arrival, their wing muscles were still in good flight condition on 25 May and intense practicing could not have explained the mass pattering. Most birds were actively courting, but the group pattering did not appear to be related to pair bonding. There is also no reason to believe the grebes tried to divert an aerial predator with common flight activity as several instances of Bald Eagles (Haliaeetus leucocephalus) appearing in flight over the grebes or even trying a catch in the canal did not trigger much reaction. Birds pattering to escape a pursuing conspecific or to flee possible danger incited alarm at the most to a handful of other Eared Grebes in their immediate vicinity. The generalized pattering by larger groups of Eared Grebes observed appeared unrelated to courtship, aggression, fear or predator presence. A similar or comparable behavior by Black-necked Grebes (P. n. nigricollis) in Europe has not been reported.
There is comparable agitation in Silvery Grebes (P. occipitalis) during migration towards breeding areas. Fjeldsa (1982) noted that Silvery Grebes show high restlessness and form long lines that move back and forth on a lake from where, in the subsequent night, at least part of the population departed. He termed this pre-migratory restlessness. Movements of a group of 70 Silvery Grebes at Laguna Las Encadenadas, Argentina, in December 2006, were not limited to swimming, but included sudden quasi-simultaneous take-offs of individuals more at the rear end of the line. Some flew up, reaching a height of ~2 m, possibly to avoid collision with the birds preceding them. They landed again in front of the group that was moving in one direction. The grebes at the rear end acted similarly. The group changed direction as it approached the shore, but continued swimming in a line, and pattering and flying from the back to the front (Konter 2009).
Eared Grebes at Tule Lake Refuge all swam actively into the same direction, although they did not form one line. They showed pattering and pattering flights in waves and repeated the directed group movements. A priori the comparison of total counts of grebes inside the English Channel on the following day strongly suggests at least a major portion of the population had left the area. Zugunruhe seems an appropriate characterization for the Eared Grebes' behavior. Additional pre-departure activities at Tule Lake Refuge including group diving, submerging and surfacing in near unison as reported by Jehl and Henry (2010) were not obvious. The grebes' diving and swimming seemed to be predominantly related to feeding, except the dives after mass pattering involved only a minority of a group. Active vocalization may have helped group cohesion, but it could not be distinguished from advertising by solitary birds or from contact calling by partners momentarily separated.
It is not known to where the departing grebes flew and whether they targeted breeding areas in the region or flew a long distance. Eared Grebes can move to other sites used for breeding, even after arrival in a breeding area, or emigrate from the region (Cullen 1998). It is also unknown whether the grebes departed in flocks from the English Channel and whether they headed in one or different directions. I assume they were migrants and the extent of their pattering flight maneuvers suggests an eagerness to move on.
The counts of 25 and 26 May show that not all Eared Grebes had left the English Channel over night. Grebes present in the NS part were not observed on 25 May and they may not have engaged in group pattering. The first count on 26 May showed that low numbers of grebes were present inside the EW canal and the higher later count suggests that new grebes were continuously settling there. Over 200 Eared Grebes left the English Channel during the night and this number corresponds as an order of magnitude to the numbers involved in the group pattering. Thus, most pattering grebes could have left over night and it is likely their zugunruhe contributed to a simultaneous departure. They were gradually replaced by conspecifics moving into the EW canal on the following day.
Eared Grebes often do not arrive within a short lapse of time inside a breeding region where numbers generally build up over several weeks. They synchronize, however, nest establishment (McAllister 1956, Boe 1994). In this context, it is of interest to further investigate how a conspicuous pre-migratory group pattering as observed at Tule Lake Refuge may contribute to a coordinated onward flight inside a breeding region that would facilitate simultaneous colony establishment by large numbers of pairs. Unfortunately, the data from Tule Lake Refuge do not permit any conclusion to be drawn.
I am grateful to Michele Nuss from the Tule Lake Refuge Headquarters who was of great help in the preparation of my fieldwork. I thank J. R. Jehl Jr and C. E. Braun for critical review and constructive comments on the first draft.
Received 13 July 2011. Accepted 19 September 2011.
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Andre Konter (1)
(1) Museum of Natural History, 25, rue Munster, Luxembourg L-2150, Luxembourg; e-mail: email@example.com
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