Effects of population density on survival in Merlins

Auk, The, Jan 2000 by Lieske, David J, Warkentin, Ian G, James, Paul C, Oliphant, Lynn W, Espie, Richard H M

ABSTRACT.-Accurate estimation of survival probabilities is an important component of population demographics, and it permits a test of the life-history prediction that densities influence population dynamics via suppression of survival rates. As part of a long-term study of urban-nesting Merlins (Falco columbarius), we estimated survival rates and tested for the effects of density dependence based on capture histories from 1,354 individuals (43 males and 110 females caught for the first time as adult breeding birds, and 597 males and 604 females caught for the first time as locally produced nestlings). Overall capture probabilities were 0.55 /- SD of 0.039 per year for adults, 0.10 /- 0.075 per year for juvenile males, and 0.58 /- 0.23 per year for juvenile females. Mean survival rate of adults was 0.62 /- 0.11 per year and did not differ significantly between males and females. Overall juvenile survival rates were 0.23 /- 0.032 for males and 0.055 /- 0.012 for females. Band returns suggest that the discrepancy in survival rates between juvenile males and females resulted from higher natal dispersal of females rather than from lower survival. Survival of adults (but not juveniles) was negatively density dependent, suggesting that density-dependent declines in survival exerted a regulatory effect on population size. Received 27 July 1998, accepted 18 June 1999.

ESTIMATING PARAMETERS that influence the stability and persistence of animal populations is central to the understanding of population dynamics (Eberhardt 1985, Clobert and Lebreton 1991). Survival probability is an important component of population demography (Ricklefs 1972, Lebreton et al. 1992). The importance of estimating survival rates is clear when we consider that the stability of any animal population is at least partially an outcome of the balance between the establishment of new breeders and the mortality of older breeders (Lebreton et al. 1993).

Until recently, survival rates were estimated from capture-recapture studies using life tables. Using this procedure, recaptures or resightings of previously marked individuals are used only to confirm that individuals were alive in the previous sampling period (Pollock et al. 1990). Furthermore, survival probabilities derived from life-table methods really are estimating "return rates" (Loery and Nichols 1985), which are a function of the probability of survival between periods and the probability of capturing or sighting surviving individuals (Pollock et al. 1990, Lebreton et al. 1993). Estimates based solely on return rates commonly underestimate survival probabilities (Lebreton et al. 1993, Newton et al. 1997).

As part of a long-term study of an urbannesting population of Merlins (Falco columbarius), we sought to estimate survival rates and test for the effects of density dependence. In light of the findings of Lebreton et al. (1992), Blums et al. (1996) and Newton (1998), we also considered the likelihood that survival rates are lower in juveniles than in adults. Because density-dependent suppression of juvenile or adult survival is one way that population size can be regulated (Stubbs 1977), we were keenly interested in examining the effects of density dependence on survival. Our goal was to combine the recent advances in capture-recapture methodology with a long-term data set to consider the issue of density-dependent survival more closely.

METHODS

Study population.-Merlins have nested sporadically in the city of Saskatoon, Saskatchewan (52 deg 07'N, 106 deg 38'W), since the late 1960s (Houston and Schmidt 1981, Oliphant and Haug 1985). Saskatoon is particularly well suited for Merlins because of the high abundance of prey (i.e. House Sparrows [Passer domesticus]) and the surplus of old nests of American Crows (Corvus brachyrhynchos) and Black-billed Magpies (Pica pica) that provide crucial nesting sites for the birds (Houston and Schmidt 1981, Warkentin and James 1988).

Census and capture methods.-We conducted a systematic census of the city each spring and summer from 1985 to 1996, during which time we recorded the number and location of breeding pairs. The fulltime survey was initiated in early April and conducted until mid- to late May. Sites known to have been used in previous years were searched first, followed by examination of peripheral areas. Although the potential search area was large (122 km^sup 2^), the actual area that contained suitable nest sites was smaller, allowing us to concentrate our efforts in areas known to support nesting pairs. Nevertheless, efforts were made to visit seemingly unsuitable areas each year. Searches of the immediate vicinity of medium- to large-sized spruce trees (Picea spp.) were conducted on foot, by bicycle, or by vehicle and involved broadcasting a tape recording of the Merlin's "ki-ki-kee" call (Feldsine and Oliphant 1985). Males are known to respond strongly to these recordings (James et al. 1989). Other cues for the presence of breeding pairs included feces, feathers of Merlins or prey items, partially eaten prey remains, and the activity of pair members. In most years, searches were conducted by the authors themselves, or by a combination of the authors and trained assistants. City residents also volunteered information regarding the location of individual Merlins. Areas of suspected breeding activity were searched to confirm the presence of mated pairs. Because our search effort was consistent and the study area was of fixed size, we argue that changes in Merlin numbers from year-toyear reflected actual changes in density (see Arcese et al. 1992).