Mate Retention in Caspian Terns
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The Condor87:74-18 0 The CooperOrnithological Society 1985 MATE RETENTION IN CASPIAN TERNS FRANCESCA J. CUTHBERT ABSTRACT. -Colonial seabirds that nest in stable, predictable environments tend to breed with the same mate for consecutive seasons.In some of these populations, mate retention has been shown to be correlated with previous re- productive success.Caspian Terns (Sterna cuspiu)were observed in northeastern Lake Michigan at several colony sites that vary in stability becauseof fluctuating water levels. Objectives of my study were to determine if (1) individuals tend to keep the same mate for consecutive breeding attempts, (2) mate retention is influenced by previous reproductive successor inter-year nest site stability, and (3) reproductive successis affected by mate change.Only 25% of the original pairs bred together for consecutive seasons,mate retention was independent of repro- ductive successthe previous year but not of inter-year nest site stability, and I found no significantadvantage in retaining the same mate for consecutiveseasons. These results indicate that factors other than previous reproductive success(e.g., habitat stability) influence mate selection and retention in this population of Caspian Terns. Caution should be exercised in assuming that the relationship between mate retention and previous reproductive successholds for other species of colonial seabirds. In birds that exhibit bi-parental care, selection pendent of reproductive successin the pre- of a breeding partner has great consequences vious season?(3) is mate retention indepen- for individual reproductive success.Data from dent of inter-year nest site stability? (4) is long-term studies of marked individuals in- reproductive successaffected by mate change? dicate that many colonial seabirdstend to keep and (5) if the first nesting fails and terns re- the same mate for consecutive breeding sea- nest in the same season,do they keep the same sons. Examples include Sphenisciformes mate for the second nesting attempt? (Richdale 1947, 19 5 7) Procellariiformes (Richdale 1947, Fisher 1976, Macdonald 1977, STUDY AREA AND METHODS Brooke 1978, Ollason and Dunnet 1978, Morse The study area included four islands in north- and Buchheister 1979, Boersma et al. 1980), eastern Lake Michigan where 1,100 pairs of Pelecaniformes(Nelson 1972, Fleet 1974), and Caspian Terns (30% of the Great Lakes pop- Charadriiformes (Austin 1947, Coulson and ulation) nested in 1978 and 1979. The colony White 1958, Tuck 1960, Dorward 1963, Coul- sites were: (1) the northeastern point of High son 1966, Mills 1973). While studying colony Island (45”45’N, 85”4O’W), (2) Hat Island site tenacity in Caspian Terns (Sterna cuspiu) (45”47’N, 85”18’W), (3) Shoe Island, a gravel in the Great Lakes, I discovered that mate re- bar 1 km S of Hat Island, and (4) Ile aux Galets tention was low. Only a few other investigators (45”41’N, 85”ll’W). The islandsvaried in size of colonial seabirdshave found mate changes from less than 1 ha (Shoe Island) to 3 ha (Ile to be common, and most examples are from aux Galets), 7 ha (Hat Island) and 1,430 ha pelecaniform populations (Snow 1963, Kepler (High Island). Distances between the islands 1969, Harris 1979). Previous researchon mate ranged from l-39 km and all four colonies retention in colonial seabirds has been con- were separatedby > 90 km from the only other ducted on birds that nest in stable and pre- Caspian Tern colony in Lake Michigan. Dur- dictable environments. In contrast, Caspian ing the two breedingseasons, colony sizeranged Terns from the Great Lakes population nest from 125-520 pairs (X = 278). Three other in habitat that varies in stability because of speciesof larids, Herring Gull (Lurus urgen- fluctuations in water level. I report here the tutus), Ring-billed Gull (L. dehvurensis),and results of a two-year investigation of mate re- Common Tern (Sternahirundo) nested adja- tention in Caspian Terns breeding in north- cent to the Caspian Terns (Scharf 1978). Water eastern Lake Michigan. The purpose of my level on the Great Lakes fluctuates cyclically study was to determine if habitat instability is (Cohn and Robinson 1976) and breeding Cas- an important variable affecting pairing behav- pian Terns are affecteddirectly by thesechanges ior in this population of colonial nestingbirds. (Shugart et al. 1978, Cuthbert 1981). When Five questionswere considered:(1) do Cas- water levels are above average, Shoe Island is pian Terns keep the same mate for consecutive submerged and parts of the other islands are breeding seasons?(2) is mate retention inde- vulnerable to storm damage. 1741 CASPIAN TERN MATE RETENTION 75 To determine degree of mate retention be- TABLE 1. Pair combinations in consecutive breeding tween consecutive breeding seasons,in 1978 seasons. and 1979 I attempted to follow the nest his- Pairs tories of color-marked terns. In early June Pm combinations (n = 24) Percent 1978, 24 pairs were captured with a monofila- Same mate 25 ment line snare on their nestsat Ile aux Galets New mate (previous mate present) 2 25 and High Island, and were banded with unique New mate (previous mate not seen) 9 38 color combinations of plastic leg bands and a Non-breeders U.S. Fish and Wildlife Service (USFWS) leg Both individuals 1 4 band. One individual (previous mate not seen) 1 4 Data on reproductive successof each tern Both individuals not seen 1 4 were collectedin the following manner: all nests of color-banded birds were numbered with wooden stakes and chicks were banded with USFWS bands when they were 2-3 days old. MATE RETENTION AND PREVIOUS Nests were checked periodically from early in- REPRODUCTIVE SUCCESS cubation through banding of chicks. After the The discovery that only one-fourth of the pairs chicks were banded, I watched the color- in my sample remained together for consec- marked parentsand their offspringfrom blinds utive breeding seasonsraised the following located on the edge of the colonies. Most of question:is mate retention independent of pre- my efforts were concentrated at Ile aux Galets vious success?Data were analyzed in two ways. and High Island, but I visited the other islands First, I looked at pairs regardlessof whether every 8-10 days throughout the seasonto lo- or not the original mate was present in the cate and monitor birds that were no longer study area. Of 2 1 pairs where at least one mem- nestingat their 1978 colony. Adults were con- ber was found breeding in 1978, 17 had raised sidered to have failed to reproduce only if the one or two chicks to fledging in 1979. Five entire nest contents failed to hatch or were (29%) of these successfulpairs bred together destroyed. Terns that raised one or two chicks again in 1979, but 12 pairs (7 1%) were not re- to fledging(40 + days) were recorded as having establishedthe following year. Four pairs were a successfulreproductive season.All data were unsuccessfulin 1978. One of these remained analyzedusing the Fisher Exact Test (Zar 1974). together in 1979; three individuals from the other pairs were seen breeding with different RESULTS mates in 1979. When these data were tested, I found that the null hypothesis that mate re- MATE RETENTION IN CONSECUTIVE BREEDING SEASONS tention is independent of previous reproduc- tive successheld true (P > 0.50). The identity of mates of color-marked terns For the second analysis, I considered only was recorded in 1978 and 1979 to determine the pair combinations where the original mate if Caspian Terns keep the same mate for con- was known to be present in the study area. secutive breeding seasons.Only 25% (6/24) of Birds that changedmates becausethe original the 1978 pairs were present in 1979 (Table 1). mate died or disappearedwere excluded from Of the remaining 18 pairs, 17 were no longer this analysis.The sample size was 12 pairs, of together; in one case, mate retention was un- which 10 were successfulin 1978. Of the suc- determined becauseneither member of the pair cessfulpairs, five were noted breeding together was seen at any of the active colony sites. Of again’in 1979. The other five pairs, however, the pairs that were no longer together in 1979, were no longer intact the following year and two (12%) involved situations where one or each member of the original pair bred with a both members were nonbreedersfor the entire new partner, even though its mate of the pre- 1979 season.In the remaining 15 pairs (88%), vious seasonwas observed in the study area. one or both individuals were seen breeding Two of the pairs were unsuccessfulin 1978. with a new mate in 1979. Birds seen breeding One of these remained together in 1979. The with new mates could be divided into two cat- other pair separatedand both individuals were egoriesbased on whether or not the mate from seen breeding with different mates in 1979. 1978 was present in 1979. Six pairs (40%) had These data also were tested and, again, the null separated, even though their 1978 mate was hypothesis held true (P > 0.50). present in the study area. An additional nine pairs (60%) were no longer together and one REPRODUCTIVE SUCCESS FOLLOWING member of each of these pairs was not seenin MATE CHANGE the study area for the entire 1979 breeding To determine how mate change affects sub- season. sequentproductivity of chicks, I examined the 76 FRANCESCA J. CUTHBERT TABLE 2. Reproductive successand mate retention in years; five pairs remained together and four consecutivebreeding seasons. were found nestingwith new partners. The hy- pothesisthat mate retention is independent of 1919 inter-year nest site stability was tested and re- 1978 SUCCeSSfill Unsuccessful jected (P < 0.05). These results suggestthat Successfulterns habitat stability may influence mate retention Retained mate 6 4 in this population.