Wintering Behavior, Physiology and Site Fidelity in a Partial Migrant, the American Dipper (Cinclus Mexicanus)

Wintering Behavior, Physiology and Site Fidelity in a Partial Migrant, the American Dipper (Cinclus mexicanus) Author(s): Ivy Whitehorne Source: Waterbirds, 33(4):461-470. 2010. Published By: The Waterbird Society DOI: http://dx.doi.org/10.1675/063.033.0405 URL: http://www.bioone.org/doi/full/10.1675/063.033.0405

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BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. Wintering Behavior, Physiology and Site Fidelity in a Partial Migrant, the American Dipper (Cinclus mexicanus )

IVY WHITEHORNE

Centre for Wildlife Ecology, Department of Biological Sciences, Simon Fraser University, Burnaby, B.C., V5A 1S6, Canada

E-mail: [email protected]

Abstract.—Foraging behavior, physiological state and site fidelity of resident (sedentary) and migratory Ameri- can Dippers (Cinclus mexicanus) were compared to assess whether differences in over-wintering behavior or physiology explain the lower annual survival rate of resident individuals. Residents spent more time resting (23 vs. 14% of the time) and less time foraging (71 vs. 81% of the time) than migrants. However, there were no detectable differences in energetic intake (kJ/hr), foraging success (kJ/hr spent foraging), or physiological state (measured as size- corrected mass, hematocrit, leucocrit, total white blood cell count, and heterophil to lymphocyte ratio; immunoglobulin, triglyceride, and free glycerol levels; and total antioxidant capacity and total oxidative status of plasma). Foraging success, size-corrected mass and plasma triglyceride levels increased with date for both migrants and residents, indicating that both groups built up fat reserves as winter progressed. Resident individuals exhibited higher winter site fidelity than migrants (sighted on the same section of the river on 70 vs. 53% of weekly censuses). The lower site fidelity and, hence, greater movement of migrants may increase survival by allowing them to seek more suitable microhabitats during adverse conditions. Further work on the relationship between mortality rates, movement and winter home range size of migrant and resident dippers is needed to test the latter hypothesis. Received 6 July 2009, accepted 17 March 2010. Key words.—American Dipper, behavior, partial migrant, physiology, site fidelity, winter. Waterbirds 33(4): 461-470, 2010

Winter poses challenges for species that tures (-40°C in Montana, Bakus 1959a; -57°C remain in temperate regions year-round, as in northern Alaska, Gabrielson and Lincoln harsh conditions such as cold temperatures, 1959) and overwinter in very northerly habi- reduced food and severe weather all increase tats (64°N in Sweden, Lundberg et al. 1981; mortality risk (Newton 1998). Adaptations to 58°N in Alaska, Willson and Hocker 2008). such conditions can include changes in both However, winter is known to be a period of physiology and behavior of overwintering great energy stress (D’Amico and Hémery birds, such as the building up of fat reserves 2007), and winter conditions have been (e.g. Evans 1969; Gosler 2002), food-caching shown to inﬂuence survival and population (Brodin 2005), winter ﬂocking to improve growth rates of both American (Cinclus mexi- food intake (Beauchamp 1998; Hogstad canus) and White-throated (Cinclus cinclus) 2003), and changes in habitat use to mini- Dipper (Price and Bock 1983; Sæther et al. mize thermoregulatory costs (Jenni 1991; 2000; Loison et al. 2002; Willson and Hocker Huertas and Diaz 2001). Species that forage 2008). in water face the additional challenge of in- Long-term monitoring of a population of creased heat loss due to the thermal conduc- American Dipper in southwestern British tance of water, so compared to terrestrial for- Columbia, Canada, has revealed that >80% agers, birds foraging in water must increase of individuals are altitudinal migrants, while metabolism to stay warm (Kooyman et al. the remainder are sedentary (Morrissey et al. 1976; Stahel and Nicol 1982). Heat loss is 2004). These sedentary individuals (resi- more pronounced for smaller species (de dents) defend linear stretches of the main Vries and van Eerden 1995), such as Dippers stem of the Chilliwack River (300 m to 1 km (Cinclus spp.). Dippers are nevertheless not- in length) as breeding territories. Residents ed for their ability to persist in harsh condi- remain on these territories year-round, and tions. For example, Dippers have been re- may be constrained to remain near or de- ported to endure extremely cold tempera- fend their nesting site throughout winter. In

461 462 WATERBIRDS contrast, migrants maintain separate breed- METHODS ing and wintering areas, breeding on territo- Determination of Migratory Strategy ries at higher elevation on tributaries, and overwintering at lower elevation with the res- American Dippers (hereafter Dippers) were captured throughout the year in the Chilliwack River watershed, idents (Gillis et al. 2008). Individuals rarely located in southwestern British Columbia, Canada, by switch migratory strategies (<3% of individu- mist-net, (see Morrissey et al. 2004 for detailed site de- als monitored for more than one year, Gillis scription). Individuals were marked with a numbered metal band and a unique combination of three color et al. 2008). Migrants produce fewer fledg- bands, and aged as either juvenile (hatch year or second lings each year than residents because they year) or adult (after hatch year or after second year) begin breeding later and consequently are based on feather characteristics (Pyle 1997). Individuals were sexed on the basis of breeding behavior (only fe- less likely to initiate a second brood. Howev- males incubate and brood, Kingery 1996) and/or mor- er, the lower productivity of the migrants is phology (in this population males are on average 16% partially offset because they have a higher heavier and morphological measures average 4 to 9% larger, Green et al. 2009). As there is some size overlap be- annual survival than residents (Gillis et al. tween the sexes, some individuals captured outside of the 2008). breeding season could not be sexed. Adult individuals As winter can be a period of high mortal- were classed as resident if they bred on or within 1 km of the main stem of the Chilliwack River and remained at ity for Dippers (Price and Bock 1983; Sæther that site throughout fall and winter. Individuals were et al. 2000; Loison et al. 2002; Willson and classed as migrant if they were seen in the same area for Hocker 2008), differences in annual survival two or more winters but had not bred there. Adults first banded during the fall and winter of 2006/2007 were rates between migrant and resident Ameri- classed as migrant if they consistently inhabited the same can Dippers may result from differences in area throughout winter and survived the winter (i.e. were their wintering ecology. Focal observations seen in January or later), but disappeared before the breeding season. Birds of unknown age and juveniles, were used to determine if the residents’ use which have not yet established a migratory strategy, were of multipurpose territories (i.e. used for excluded from all analyses. both breeding and overwintering) affected Time Budgets and Energetics their time budgets or reduced their energetic intake rates compared to migrants. Also, Focal observations were conducted on 19 adult Dip- the physiological state of overwintering pers (nine migrants, ten residents) between 21 Septem- ber and 23 November 2006. Each individual was American Dippers was assessed to determine observed for one session, lasting approximately one if inhabiting such a year-round territory hour (mean ± SD: 56 ± 5 min). Time budgets were ob- leads to residents being in poorer physiolog- tained by recording the instantaneous activity of the bird every 60 seconds. Activities were classified as rest- ical state than migrants. Finally, the site fidel- ing, flying, swimming, diving and shallow water foraging ity of individual American Dippers was mea- (consisting primarily of wading and pecking at the sub- sured to determine if migrants are less con- strate in shallow water, see Table 1 for details). The energetic intake and foraging success of the fo- strained to remain at a single location cal individual was determined from the number and en- through the winter, which may increase their ergetic value of all food items consumed during the ability to deal with changing conditions dur- observation period. Items consumed were classified into five categories: small, medium and large inverte- ing events such as floods. brates, salmon eggs and fish. Invertebrate size class was

Table 1. Behavioral categories used to construct time budgets for wintering migrant and resident American Dippers (Cinclus mexicanus).

Category Deﬁnition

Resting Time spent stationary, including standing, dipping, preening and vocalizing Flying Time spent in active, powered ﬂight, including aerial chases Swimming Time spent swimming (body at surface of the water but legs not touching the bottom), during which the legs are used for propulsion Diving Time spent fully submerged, during which wings are the primary means of propulsion Shallow-water foraging Time spent foraging by walking and pecking at prey items in shallow water (feet do not leave the substrate), including placing the head below the surface to look for food

WINTERING ECOLOGY OF DIPPERS 463 determined by comparing the length of the invertebrate to the length of the Dipper’s bill (bill length = 22mm, Donnelly and Sullivan 1998; see Table 2). Inver- tebrate samples (n = 50 per size class) were collected locally and air-dried at 40°C for 24 hr to determine dry mass, which was then converted into energetic value us-

ing published values for aquatic invertebrates (Table 2). 13.2 J 1.3 kJ 6.8 kJ The average size of fish consumed by Dippers was determined by comparing the fish to the length of the Dip- per’s bill in photos (average: 2.0 × bill length = 44mm; N = 11). The energetic value of fish and salmon eggs was then estimated from published data on the length-mass relationship (fish), mass (eggs), and energetic content per unit wet mass (fish and eggs; Table 2). Energetic intake is expressed as kilojoules (kJ) consumed per hour. a

Foraging success is expressed as kJ consumed per hour c e spent foraging (time spent at shallow-water foraging, swimming and diving combined). collected samples (see text for details), and DM, dry mass; WM, wet mass. Physiological State 7.8 kJ/g WM 4.4 kJ/g WM Ten measures relating to four different aspects of physiological state were assessed in wintering Dippers: general condition measures (size-corrected mass and he- ). Size of invertebrates and ﬁsh consumed was determined by matocrit), immune function (leucocrit, total white blood cell count, immunoglobulin levels, and the ratio of heterophils to lymphocytes in peripheral blood), me- b tabolism of lipid stores (levels of circulating triglycerides d and free glycerol), and oxidative stress (total antioxidant capacity and total oxidative status of plasma).

A total of 88 Dippers (28 female, 41 male, 19 un- Cinclus mexicanus known) were captured by mist-net between mid-Sep- tember 2006 and January 2007. Eleven were known residents and 24 were known migrants, with the remainder being juveniles, of unknown age, or adults whose migratory strategy could not be determined. Known residents and migrants were weighed to the nearest 0.5 g N Mass Energy content per unit mass Energetic value per item and the combined head-bill length was measured to 0.1 mm. Size-corrected mass was then calculated as the un- . (2004). standardized residual of a linear regression of mass on et al head-bill length. Due to males’ larger size and potential allometric differences between the sexes, size-corrected mass was calculated separately for each sex. Size-corrected mass could not be calculated for six migrants, as their sex could not reliably be determined. A blood sample (up to 200 μl) was collected by punc- turing the brachial vein and collecting the blood in he- parinized capillary tubes. Blood was stored over ice until

centrifugation, as soon as possible after collection Size (mean ± SD: 2.3 ± 1.5 hours). A few drops of blood were used to make an air-dried blood smear for later analysis of total and differential white blood cell counts. The re- maining blood was centrifuged in hematocrit tubes at 12 000 rpm for ﬁve minutes to separate the plasma and cells. Hematocrit and leucocrit were determined as a percentage of packed cell height to total column Bill length mm height. Plasma was then stored on ice until freezing at - . (1988).

20°C at the end of the day, and was later transferred to - et al 80°C for longer-term storage. Plasma immunoglobulin levels were assessed using an enzyme-linked immunosorbent assay (ELISA). For details of methodology, see Bourgeon et al. (2006). The intra-assay coefﬁcient of variation (CV) was 1.6%. Circu- lating free glycerol and total glycerol (free glycerol + triglycerides) were determined sequentially using a colorimetric endpoint assay (Sigma-Aldrich, St. Louis, Calculated using a length-mass relationship from Chingbu (2001). Average value from three locations within the Chilliwack River watershed (Flemming and Gross 1990). Averaged from values in Boldt and Haldorson (2004) Dempson Value taken from Brey Value taken from Hendry and Berg (1999).

MO). Assays were run in duplicate or triplicate using 5 a b c d e μl of plasma per replicate. Intra-assay CVs were 4.3% Table 2. Categories and energetic value of prey items consumed by wintering American Dippers ( Table Small invertebrate <1/4 <6 50 0.59 mg DM 22.44 J/mg DM Medium invertebrateLarge invertebrateSalmon eggFish 1/4 to 1/2 >1/2 — 6 to 10 2.0x Average >10 50 4.04 mg DM — 44 50 25.5 mg DM 11 — 1.55 g WM 0.166 g WM 90.6 J 572 J comparison to the length of Dipper’s bill (22mm, Donnelly and Sullivan 1998). Invertebrate mass was determined locally from the literature. Mass of each type prey item was then converted derived from mass of eggs and fish were into energetic value. Prey item 464 WATERBIRDS and 4.4% for determinations of free and total glycerol, measures used in this study (e.g. Dawson and Bortolotti respectively, and inter-assay CVs were 3.7% and 0.2%. 1997; Guglielmo et al. 2002; Owen et al. 2005; Costantini Triglyceride concentrations were calculated as the dif- et al. 2008) were fitted. All nonsignificant potential con- ference between total and free glycerol concentrations. founding variables (P > 0.05) were sequentially re- Total antioxidant capacity (TAC) and total oxidative sta- moved, and terms of interest (strategy, date, and a tus (TOS) were assessed using colorimetric methods de- strategy by date interaction term) were evaluated, again veloped by Erel (2004; 2005). The TOS methodology sequentially removing nonsignificant terms, leaving was modified for smaller sample sizes (reduced to 10 μl only significant effects in the final model. Sample sizes of plasma per replicate from 35 μl). All samples were vary as not all data were available for all individuals. run in duplicate, and the intra-assay CVs were 4.3% for Finally, generalized linear models were used to ex- TAC and 6.1% for TOS. Small plasma sample volumes amine if site fidelity differed with migratory strategy. occasionally prevented all measures being determined Site fidelity was modelled as a binomial response for all individuals. (present Y/N), with number of times observed used as the response variable and the number of censuses as the Site Fidelity binomial denominator (“modelling of binomial pro- portions” function in GenStat v10, VSN International Ten census areas were established on the Chilliwack Ltd., 2007). The number of censuses varied between in- River in areas known to be used by wintering migrant dividuals as individuals did not enter the data set until Dippers. These locations have been regularly moni- the first sighting. All individuals included in the analysis tored throughout winter (November to March) and the were observed at least two times and included in four to breeding season (late March to June) every year from 13 censuses. 1999 onwards. The census areas were linear stretches of the river approximately 0.8 km in length (range 0.6 to 1.3 km), and were separated, on average, by 3.1 km of RESULTS unmonitored river (range 0.3 to 14.2 km). Census area boundaries were defined by easily identifiable riverside landmarks. All ten areas were censused weekly from Time Budgets and Energetics September 2006 to December 2006, and censused bi- weekly from January 2007 to March 2007. During each Migrants were found to spend more time census, an observer walked the length of the riverbank of each site, identifying all banded Dippers present foraging than residents, when all foraging within the boundaries of the census area with the aid of behaviors were considered together (shal- 8x binoculars or a 20-60x spotting scope. These observa- low-water foraging, diving, and swimming, tions were used to build presence/absence data over time for each banded individual at each of the ten cen- Table 3, Fig. 1a). Also, migrants were found sus areas. Most individuals (81 of 87 banded individu- to spend significantly less time resting (Table als) did not move between census areas and were only 3, Fig. 1b). Despite spending different observed at a single location over the winter. The site fidelity of each individual was then calculated as the pro- amounts of time foraging, migrants and res- portion of censuses (i.e. time) that it was present at its idents were not observed to differ in energet- typical location, from the first time it was observed until ic intake (Fig. 1c) or foraging success (Fig. the end of January. Census data from February and March were excluded as migrants may begin to move to- 1d, Table 3). Both migrants and residents wards breeding grounds in these months (Morrissey et spent less time foraging overall and less time al. 2004). To avoid confounding site fidelity with mortal- foraging in shallow water as the winter pro- ity, only individuals that were known to have survived the winter (i.e. seen in January or later, N = 15 migrants, gressed, however, the foraging success for N = 9 residents) were used. both groups increased at the same time (Ta- ble 3). As a result, energetic intake rates also Statistics increased with date, albeit non-significantly General linear models were used to examine if the (Table 3). migratory strategy of an individual influenced the There were no significant differences be- amount of time spent at various activities, including foraging, or energetic intake. A full model including strat- tween migrants and residents when shallow- egy, date (measured as days past 1 September), and a water foraging, swimming and diving were strategy by date interaction was fit. Any nonsignificant considered separately, nor did they differ in terms (P > 0.05), starting with the interaction term, were sequentially dropped to evaluate the effect of remain- the amount of time spent flying (Table 3). ing significant terms. Similarly, time spent flying, swimming or div- General linear models were also used to examine ing did not vary with date (Table 3). whether an individual’s migratory strategy affected any of the ten measures of physiological state. Initially, a full model containing migratory strategy, date, and a strate- Physiological State gy by date interaction term, as well as several potential confounding variables (sex, body mass, time since sunrise, and the time lag between capture and drawing the None of the ten physiological measures blood sample) that are known to affect the physiological differed significantly between migrant and WINTERING ECOLOGY OF DIPPERS 465 resident individuals (Table 4), after controlling for potential confounding variables (sex, body mass, time since sunrise, and the time lag between capture and drawing the blood sample). There was, however, some 0.01 0.99 seasonal variation in two of the ten mea- 0.37 0.72 1.43 0.18 -0.47 0.65

sures. Size-corrected mass increased with ). Least-squared means date (t25 = 2.12, P = 0.04, N = 27), as did triglyceride levels (t25 = 3.04, P < 0.01, N = 27). Neither of these date effects varied with strategy (strategy by date interaction, P > 0.1 for

both). In some cases, missing values for po- Cinclus mexicanus

tential confounding variables were replaced t P t P with the mean value to maintain the maxi- ippers (

mum sample size. Results were the same if in bold. See text for details of the final model are the individuals with missing data were excluded. 0.72 ± 0.23 3.11 <0.01 Parameter -0.65 ± 0.20 -3.25 <0.01 Site Fidelity mean ± S.D All ten census areas were observed to be used by wintering migrants, while six of the ten were used by wintering residents. Eighty- one of 87 banded birds were observed at only one of the ten census areas throughout t P the winter. Of the six individuals observed at two different locations over the winter, four were migrants, and travelled at least 0.6, 3.3, 4.3 and 6.9 km between sightings. The re- maining two were residents, and travelled at Resident Migratory strategy Date Interaction least 1.0 and 2.1 km between sightings. mean ± S.D Of the adults known to have survived the winter, migrant individuals exhibited significantly lower site fidelity than residents (t22 = 3.32, P < 0.001, N = 24, Fig. 2). Resident indi- 14 ± 3.7 23 ± 11 2.16 0.05 0.36 ± 0.13 2.82 0.01 81 ± 4.6 71 ± 13 -2.25 0.04 -0.40 ± 0.15 -2.69 0.02 Migrant

viduals were observed at their typical winter- mean ± S.D. ing location (census area) during 70 ± 16% (mean ± SD) of censuses (N = 9), while migrants were observed at their typical location only 53 ± 24% of the time (N = 15).

DISCUSSION Use of a multi-purpose territory (i.e. a territory used for both breeding and wintering) year round does not appear to impose energetic or physiological costs on resident Dipper, as migrant and resident Dippers do not exhibit any differences in energetics or physiological state that may contribute to the FlyingSwimmingDivingShallow-water foraging 67 ± 14 3.0 ± 2.5 4.8 ± 2.5 61 ± 13 13 ± 15 3.0 ± 6.4 ± 2.7 -0.80 5.6 ± 5.0 0.00 1.34 0.43 -1.38 0.99 0.20 0.18 0.04 ± 0.03 ± 0.04 0.97 0.27 ± 0.12 0.73 1.77 0.35 0.48 0.10 -1.86 2.01 0.08 -0.49 0.06 0.63 All foraging (swim + dive shallow-water) Energetic intake (kJ/hr) 19.4 ± 8.1 16.2 ± 11.4 -0.68 0.50 0.35 ± 0.17 2.06 0.06 1.11 0.29 Resting higher annual mortality of residents. During Foraging success (kJ/hr spent foraging) 23.4 ± 10.3 24.1 ± 13.8 0.12 0.91 Table 3. Effects of migratory 3. Effects strategy and date on daily time budgets, foraging success energetics for wintering American D Table for migrant and resident individuals are shown, as well as parameter estimates for date effects. Signiﬁcant effects retained in retained Signiﬁcant effects for migrant and resident individuals are shown, as well parameter estimates date effects. calculation. N = nine migrants and ten residents for time budgets, eight for energetics. Daily time budget (% time) Energetics 466 WATERBIRDS

Figure 1. Time budgets, energetic intake and foraging success of migrant and resident wintering American Dippers (Cinclus mexicanus). Boxplots show median, 1st and 3rd quartiles, minimum and maximum values. A) Proportion of total time spent foraging (shallow-water foraging, swimming, and diving combined), corrected for date effects, P = 0.04. B) Proportion of total time spent resting, corrected for date effects, P = 0.05. C) Calculated energetic intake, P = 0.50. D) Calculated foraging success, corrected for date effects, P = 0.91. See text and Table 2 for details of determination of energetic intake and foraging success. Numbers above boxplots indicate sample size for each category.

fall and early winter, residents were found to stress. However, the possibility that differenc- spend more time resting and less time forag- es in wintering behavior may lead to biologi- ing than migrants. Despite these differences, cally signiﬁcant differences in net energetic there was no detectable difference in the en- intake rates that could explain differences in ergetic intake and foraging success of the mi- survival cannot be entirely ruled out for grants and residents. There was also no evi- three reasons. First, time budgets and ener- dence that resident Dippers are in poorer getic intake were only assessed early in the physiological condition than migrants in season and not during extreme events (such terms of general condition, immune func- as winter ﬂoods and very low temperatures), tion, metabolism of lipid stores or oxidative and conditions may have been more chal-

WINTERING ECOLOGY OF DIPPERS 467 * , Mass (+) ** ** * ** s tested) are included in brackets mple. Heterophil to lymphocyte ratios are

Figure 2. Site ﬁdelity in wintering migrant and resident ed (all nonsigniﬁcant, P > 0.05). Sample sizes vary as not American Dippers (Cinclus mexicanus). Proportion of censuses during which individuals were observed at their typical wintering location (census site). See text for details of calculation. P < 0.001. Numbers above boxplots indicate the number of individuals in each category. 0.01).

≤ lenging later in the winter (i.e. December to February). Second, the power to detect sta-

). Least-square means ± SD for migrant and resident individuals are shown, with other tistically signiﬁcant differences was relatively

0.05, **P low as energetic intake rates were measured ≤ indirectly (which may introduce substantial error) and sample sizes were small. Finally, daily energy expenditure was not measured

Cinclus mexicanus in this study. Migrants and residents exhibited the same physiological responses as the winter

1.3 ± 0.2 22 1.5 ± 0.2advanced. 9 In 0.74 both 0.47 groups, size — corrected mass and plasma triglyceride levels increased with date. Size-corrected mass is an indicator of the size of a bird’s fat stores or muscle mass, while the level of triglycerides in plasma is indicative of the rate of which the individual is fattening (Williams et al. 1999; Schaub and Jenni 2001; Guglielmo et al. equivalent) 0.88 ± 0.10 23 0.86 ± 0.15 11 -0.12 0.91 — 2 2005). Fat stores are crucial for survival dur- O 2 ing winter, and their size often reflects a a trade off between starvation and predation risks (Gosler 2002; Brodin 2007). Combined 0.05) retained in the model listed. Direction of other significant effects (see text for list of potential confounding variable in the model listed. Direction of other significant effects 0.05) retained (x 1,000/μl) 4.0 ± 0.3 22 4.3 ± 0.5 9 1.86 0.59 Time (+) a ≤ with the observed increase in foraging success as the winter progressed and the positive but non-significant relationship between energetic intake and date, these results indicate that as fall progressed into winter Dip-

Measured at a commercial laboratory. Models include factor to control for the identity of technician who processed sa pers increased their foraging success to build a up fat reserves. Such fat reserves could serve signiﬁcant variables (P as (+) positive or (-) negative correlations, or as > < for categorical levels. Strategy by date interactions were also test available for all individuals. Asterisks indicate statistical signiﬁcanceall data were (*P Response variableSize-corrected mass (g)expressed as a unitless ratio. 0.2 ± Migrant 16 N -0.4 ± 0.3 Resident 11 N -0.69 t 0.50 p Date (+) Controlling for: Table 4. Physiology of wintering migrant and resident American Dippers ( Table Hematocrit (% by volume)Leucocrit (% by volume)White blood cell count Heterophil to lymphocyte ratio Glycerol (mmol/L) antioxidant capacity (mmol/L trolox equivalent)Total oxidative statu s (mmol/L H Total 0.42 ± 0.03 49.5 ± 0.6% 23 0.5 ± 0.04% 23 0.45 ± 0.04 22 50.3 ± 0.9% 11 0.6 ± 0.06% 0.6 ± 0.0 11 0.26 11 0.76 0.80 14 1.51 0.40 0.14 0.5 ± 0.1 — 9 — — -1.19 0.25 Sex (F >M) Immunoglobulins (absorbance units) (mmol/L)Triglyceride 0.36 ± 0.01 22 1.1 ± 0.1 0.32 ± 0.03 18 9 -1.19 1.1 ± 0.2 0.24 9 -0.02 — 0.99 Date (+) 468 WATERBIRDS

as a buffer against reduced food availability, 1994). Unfortunately, the relative intensity lower temperatures and shorter day lengths or frequency of these behaviors between res- that reduce foraging time and lengthen the idents, migrants and juveniles in this popula- nightly fast (Gosler 2002). Small birds gener- tion could not be assessed for this study. ally manage their fat stores to balance starva- In this population, individual Dippers tion and predation risk, and tend to increase rarely moved between census areas during their fat stores as conditions become increas- fall and winter (average separation 3.1 km), ingly variable or unpredictable (Brodin yet migrants were resighted less often, sug- 2007). gesting they are more likely to move outside Movement patterns during winter may an individual census area (average length 0.8 influence survival rates, as widely ranging in- km). Dippers preferentially feed in certain dividuals may be able to better respond to restricted areas (American Dipper, Thut variable food supply than sedentary individ- 1970; Brown Dipper Cinclus pallasii, Eguchi uals (Brown and Long 2007; Brown and 1988, cited in Eguchi 1990), and shallow rif- Sherry 2008). Winter site tenacity is variable fles appear particularly important (White- between populations of Dippers, ranging throated Dipper, Tyler and Ormerod 1994). from highly mobile throughout winter (e.g. Even small changes in water level can shift Price and Bock 1983; Willson and Hocker the location of such prime feeding areas. 2008) to sedentary, with individuals moving Similarly, deep and fast-flowing water is more only in the face of extreme events such as ic- difficult to feed in, as diving behavior is ener- ing-over of their habitual stretch of river getically expensive (Bryant and Tatner 1986). (e.g. Bakus 1959b). In this population, only In White-throated Dipper, foraging behavior 7% of individuals were observed at more has been shown to be strongly influenced by than one census area over the course of the water flow (da Prato 1981; Taylor and O’Hal- winter, which confirms earlier reports of lim- loran 2001; D’Amico and Hémery 2007). ited winter movement in this population Consequently, the increased movement ex- (Morrissey et al. 2004). Despite this limited hibited by migrants may make them better movement, migrants exhibited reduced site able to seek out quality microhabitats for for- fidelity compared to residents. Dippers can aging during high water levels. be nest site limited (Tyler and Ormerod Due to their complex seasonal move- 1994; Loegring and Anthony 2006), and as ments, detailed knowledge of population residents have a higher annual and lifetime structure and movement patterns is required productivity than migrants (Gillis et al. to accurately assess Dipper population trends 2008), their low-elevation nest sites are pre- from abundance data (Morrissey et al. 2004). sumably highly desirable for juvenile individ- The reduced site fidelity and greater move- uals who are seeking to establish their own ment of wintering migrants reported here territory. If residents are constrained to re- suggests that within- season movement pat- main near their nest site and defend it year- terns may also need to be taken into consider- round, they should exhibit territorial de- ation when monitoring Dipper populations. fence behaviors throughout winter and do As the observed differences in daily time so more than migrants or juveniles. In this budget and site fidelity between migrant and population, residents, migrants and juve- resident Dippers in this population do not niles will exhibit aggressive behavior towards correlate with any observable differences in other birds during winter (I. Whitehorne, energetic or physiological state, the mecha- pers. obs.), and similar behavior has been nisms underlying the higher annual survival observed in other populations (Bakus rates of migrants remains unknown. One 1959b; Whitney and Whitney 1972; Price and possibility is that migrants may benefit from Bock 1983). However, the degree to which increased movement during adverse condi- this is territory defence, defence of personal tons. For example, the migrants’ greater space, or establishment of a dominance hier- movement may allow them to move more archy is still unknown (Tyler and Ormerod easily seek out quality foraging microhabitats WINTERING ECOLOGY OF DIPPERS 469 as water levels change, and consequently re- Brown, D. R. and T. W. Sherry. 2008. Alternative strategies duce their probability of starving during a of space use and response to resource change in a wintering migrant songbird. Behavioral Ecology 19: flood. Such an effect would be difficult to de- 1314-1325. tect via physiological measures, as due to Bryant, D. M. and P. Tatner. 1986. Energetics of the annu- small size and rapid metabolism, starvation al cycle of dippers Cinclus cinclus. Ibis 130: 17-38. Costantini, D., G. Dell’Ariccia and H-P. Lipp. 2008. Long takes place very quickly in small passerines flights and age affect oxidative status of Homing Pi- and it is highly unlikely that a starving indi- geons (Columbia livia). Journal of Experimental Biol- vidual will be sampled in the brief interval ogy 211: 377-381. Chingbu, P. 2001. Occurrence and distribution of Salmin- between perfect health and death (Newton cola californiensis (Copepoda: Lernaeopodidae) on ju- 1998). Further work on the relationship be- venile Sockeye Salmon (Oncorhynchus nerka) in Lake tween mortality rates, water-level variability, Washington. Journal of Freshwater Ecology 16: 615- 620. movement and winter home range size of da Prato, S. 1981. The effect of spates on the feeding be- migrant and resident Dippers is needed to haviour of dippers. Bird Study 28: 60-62. test this hypothesis. D’Amico, F. and G. Hémery. 2007. Time-activity budgets and energetics of dipper Cinclus cinclus are dictated by temporal variability of river flow. Comparative Bio- chemistry and Physiology, Part A 148: 811-829. ACKNOWLEDGMENTS Dawson, R. D. and G. R. Bortolotti. 1997. Are avian hema- tocrits indicative of condition? American Kestrels as a The author thanks S. Richardson and D. J. Green for model. Journal of Wildlife Management 61: 1297- assistance in data collection, S. Bourgeon for oxidative 1306. stress and immunoglobulin assays and staff at Chilliwack de Vries, J. and M. R. van Eerden. 1995. Thermal con- River Fish Hatchery for access. The paper benefited ductance in aquatic birds in relation to the degree of from discussion with D. J. Green as well as comments water contact, body mass and body fat: energetic im- from two anonymous reviewers. Funding was provided plications of living in a strong cooling environment. by an NSERC Discovery Grant to D. J. Green and an Physiological Zoology 68: 11423-1163. NSERC Canada Graduate Scholarship-M to I. White- Dempson, J. B., C. J. Schwarz, M. Shears and G. Furey. horne. 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