nest survival, phenology, and nest-site characteristics of common ...

The Wilson Journal of Ornithology 124(1):113–118, 2012

NEST SURVIVAL, PHENOLOGY, AND NEST-SITE CHARACTERISTICS OF COMMON NIGHTHAWKS IN A NEW JERSEY PINE BARRENS GRASSLAND MICHAEL C. ALLEN1 AND KIMBERLY A. PETERS1,2 ABSTRACT.—We monitored Common Nighthawk (Chordeiles minor) nests in a managed grassland in the New Jersey Pine Barrens in 2009 and 2010, and assessed habitat selection by comparing vegetation characteristics at nests with random locations. We found relatively high nest survival with an estimated 79% chance of survival through incubation (daily survival rate 5 0.987, n 5 16 nests); predation was the most common cause of failure (n 5 2). Movements of young (up to 45 m from the original nest site) were frequent, which introduced uncertainty that prevented us from estimating survival through fledging. Nest sites had significantly more open ground cover (e.g., sand, lichen) than random sites, as well as less shrub and grass cover, shallower litter, and lower mean vegetation height. Received 9 May 2011. Accepted 27 August 2011.

The breeding biology and demography of the nightjars (Caprimulgidae) has been poorly studied worldwide relative to other groups (Straight and Cooper 2000, Holyoak 2001, Cink 2002, Brigham et al. 2011). The Common Nighthawk (Chordeiles minor) is the most widely distributed and beststudied North American nightjar with a breeding range extending from Yukon, Canada to Panama (Brigham et al. 2011), but published data on reproductive rates, habitat preferences, and nesting phenology are lacking. Nesting usually occurs on the ground in a variety of open habitats including grasslands, gravel rooftops, and disturbed or open forests (Fowle 1946, Dexter 1952, Kantrud and Higgins 1992). The species is still common in many areas, but has exhibited a negative long-term population decline in the United States (Nebel et al. 2010, Sauer et al. 2011), and has been assessed as ‘threatened’ in Canada (COSEWIC 2007), and of conservation concern in several U.S. states (e.g., New Jersey; NJENSP 2008). Only two published studies to our knowledge have quantitatively examined nest survival rates of Common Nighthawks (Kantrud and Higgins 1992, Perkins and Vickery 2007), of which only one used modern techniques involving daily survival rates (Perkins and Vickery 2007). Similarly, most data on reproduction by this species are from urban rooftop nest sites (Bowles 1921; Sutton and Spencer 1949; Dexter 1952, 1956; Weller 1958; Dexter 1961; Armstrong 1965; Gramza 1967) with relatively few from natural 1 New Jersey Audubon Society, Cape May Bird Observatory, 600 Route 47 North, Cape May Court House, NJ 08210, USA. 2 Corresponding author; e-mail: [email protected]

settings (Fowle 1946, Rust 1947, Kantrud and Higgins 1992, Perkins and Vickery 2007, Lohnes 2010). The only quantitative data on vegetation characteristics at nests in natural settings are from Kantrud and Higgins (1992) and Lohnes (2010). Data on reproductive rates, timing of nesting activities, and habitat preferences are important prerequisites to effect conservation actions for any species; these are especially lacking for the Common Nighthawk. We report data from Common Nighthawk nest monitoring in 2009 and 2010 in managed grasslands in the Pine Barrens of southern New Jersey. Our objectives were to: (1) assess nest survival and predation rates for comparison with previous studies, (2) quantify nest-site characteristics and test whether they differed from those of surrounding available habitat, and (3) present location-specific information on clutch size, behavior of young, and phenology of nesting activities. METHODS Study Area.—Fieldwork was conducted on the ,3,000-ha Lakehurst section of Joint Base McGuire-Dix-Lakehurst in New Jersey, USA (40u 029 N, 74u 229 W) within the boundaries of the Pinelands National Reserve. Approximately 520 ha of the site are actively maintained as grasslands by mowing, burning, and mechanical shrub removal. All management activities at the site occur during winter or early spring which minimized disturbance to breeding grassland birds. Grasslands at the site occur in three main areas embedded within a landscape dominated by pitch pine (Pinus rigida) and oak (Quercus spp.) forests. These are: (1) Test Site–a 170-ha area surrounding a 3.5-km long runway that is rarely

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used, (2) Jump Circle–a 110-ha circular grassland used as an air-drop zone, and (3) Westfield–a 240-ha area encompassing two active airstrips. Fields are dominated mainly by warm-season grasses (e.g., Schizachyrium spp., Panicum spp.) with significant amounts of bare ground (e.g., sand, lichens) and early-stage shrub encroachment (mainly Pinus rigida and species in the family Ericaceae). Nest Survival and Predation Rates.—Nest searching occurred within 16 irregularly shaped plots totaling 257 ha (mean plot size 5 16 ha, range 5 9–32 ha). Plot boundaries were delineated to provide an ample amount of searchable habitat (i.e., the maximum deemed feasible to search) within each of the three grassland areas of the base. Plot distribution was: six in Test Site (76 ha), four in Jump Circle (109 ha), and six in Westfield (71 ha). We searched two to three plots each weekday for ,2 hrs per plot beginning on 15 April (2009) and 28 April (2010). Searching involved one to three observers walking parallel transects and agitating vegetation with 2-m bamboo poles to flush nesting birds. Plot visits rotated so that each plot was searched at least once every 1–2 weeks. Geographic coordinates of located nests were obtained using a global positioning system, and two small pieces of pink flagging were placed on vegetation 2–3 m from the nest (i.e., creating a line with the nest at the center). Flagging was inconspicuous among surrounding vegetation, and intended to aid in relocation at close range; we do not believe that it drew the attention of predators. Nests were generally checked every 2–3 days until fledging, failure, or until young could no longer be located. Five check intervals for three nests exceeded 3 days due to logistical constraints, four intervals were 4 days and one was 5 days. Nest searching concluded on 15 July both years, although all active nests at that time were monitored until completion. Young nighthawks are semi-precocial and tend to move from the original nest site before fledging with movements that appear to increase in distance and frequency with age (Fowle 1946; Dexter 1952; MCA, pers. obs.). We thoroughly searched the area within ,30 m of the nest (or the last location at which chicks were observed) when checks revealed empty nests. This was followed in most cases by at least one subsequent search during the next nest-check. Typically, young 1–3 days post-hatch were easy to locate as they were

invariably #1 m from the original nest site, while older young could not always be found. Thus, we calculated nest survival and predation rates through hatch-date only (i.e., success 5 hatching). This is also the approach taken by Perkins and Vickery (2007), who also worked in grassland habitat. Daily nest survival rates and confidence intervals were calculated using the logistic nest survival model within Program MARK (White and Burnham 1999, Dinsmore et al. 2002). This program was also used to evaluate whether or not daily nest survival rate varied over the course of the season. We used the likelihood ratio test (Shaffer and Thompson 2007; alpha 5 0.05) to evaluate model performance versus the null (i.e., constant survival or ‘no effect’) model. Nest failures were classified as either abandonment or predation based on timing and evidence at the nest. Daily predation rates were measured by calculating the daily survival rate based on predation failures only, and subtracting this value from one. Clutch Size and Movements of Young.—Clutch size calculations were based only on active nests that were visited at least twice prior to hatching to avoid uncertainties associated with mobile young. Observations made during checks after hatching were used to generalize pre-fledging movements of young from the original nest site. Nest-site Characteristics and Habitat Selection.— We measured maximum vegetation height (cm), after finding each nest, at which vegetation touched a meter-stick at five locations: at the nest and 0.5 m from it in each of the cardinal directions. We also measured litter depth (i.e., unrooted, dead vegetation) in 2010 at the same five locations. The mean value of the five measurements was used in subsequent calculations and analyses. We visually estimated the percent cover (65%), after nesting attempts were completed, of four vegetation categories within a 1 3 1-m2 quadrat centered on the nest: (1) grasses (including rooted live and dead grasses), (2) forbs, (3) shrubs (woody perennials), and (4) open (including sand, lichens, mosses, and litter). Only nests found prior to hatching were used in vegetation calculations, as young found after hatching may have wandered from the original nest site. We also measured vegetation characteristics in 2010 at 80 randomly-generated points within the 16 plots searched for nests to better assess habitat preferences. Points were generated in a geographic information system, and constrained to be at least

Allen and Peters N COMMON NIGHTHAWK NESTING BIOLOGY

50 m apart to minimize spatial autocorrelation. Points were not used if they were on a road or other airfield infrastructure, and were substituted with the next point on the list so there were five points completed for each plot. All random vegetation measurements were performed between 15 and 23 June to coincide with the approximate midpoint of the grassland bird nesting season. Vegetation measurements were compared between years (2009 vs. 2010 nests), and between nests and random locations (2010 only) using nonparametric Wilcoxon rank sum tests (alpha 5 0.05). Nesting Phenology.—We estimated the date of nest initiation (first egg laid) to assess nesting phenology for: (1) nests found during egg-laying (assuming 1 egg laid/day; Rust 1947), (2) nests at which the hatch date was known or could be estimated as the mid-point between two checks (assuming an 18-day incubation period; Brigham et al. 2011), and (3) nests found with young (estimated age based on photographs and field descriptions of known-age young; Brigham et al. 2011; MCA, unpubl. data). These methods provide adequate accuracy for estimating nest initiation (Nur et al. 2004). We acknowledge that our sample is biased as it excludes nests that were found and failed during incubation (i.e., eggcandling or floatation were not used to assign eggs to age classes), and therefore represents a disproportionate number of successful nests. We explicitly tested whether daily nest survival rates varied over the course of the season to address this concern. RESULTS Nest Survival and Predation Rates.—We found 20 nests during the 2 years of the study: nine in 2009, and 11 in 2010. Four of the 20 nests (all from 2010) were excluded from nest survival analyses, including three found during the young stage (i.e., post-hatching), and one found during the incubation stage that was inadvertently damaged by an observer during a check. We logged a total of 98 check intervals (median interval 5 2 days) at the remaining 16 nests, yielding 224.5 exposure-days. Thirteen of 16 nests included in nest survival analyses survived to hatching, while three failed, two due to predation, and one was abandoned. All three nest failures occurred in 2010. The abandoned nest apparently had infertile eggs as incubation was undertaken for at least 27 days. The daily nest survival rate for the 16 nests was

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0.987 (95% CI 5 0.960–0.996); thus there was ,79% chance (95% CI 5 48–93%) of surviving an 18-day incubation period (calculated as [daily survival rate]18). There was no evidence that daily nest survival rate varied over the course of the season (x2 5 1.1, df 5 1, P 5 0.29). The daily predation rate was 0.009, indicating there was a 15% chance of a nest being depredated during the 18-day incubation period (95% CI 5 4–47%). Eggs in 16 nests successfully hatched (including those found after hatching), and fledging was confirmed at only three. It is unknown whether young from the remaining 13 nests were predated or we lost track of them due to movements of young. The young at one nest were likely depredated by a northern pine snake (Pituophis melanoleucus melanoleucus) that was observed ,1 m from 0–3 day old young that were not re-located. Clutch Size and Movements of Young.—Clutch size was two at 16 of the 17 nests found prior to hatching with the other nest containing a single egg. All three nests found after hatching contained two young. We recorded movements of young at 11 of 12 nests visited more than once. The one pair of young not observed to change locations was ,5 days of age when last seen. Twenty-six movement events were observed in 42 posthatching checks. Exact distances traveled were not uniformly recorded, but ranged during a single check interval (generally 2–3 days) from 0.15 to 6 m. Younger chicks tended to move less. Young that moved were generally located close together (within 0.1 m) except for those close to fledging, which were at times ,1–2 m apart. One of four broods visited on day of hatching had not moved by the following nest check (2–3 days later), and three broods had moved only 15– 100 cm. The farthest straight-line distance recorded to the original nest site over the 14day period one pair of young were monitored was ,45 m. In contrast, a pair of 11-day old young at another nest moved only 0.5 m from the original nest site. Nest-site Characteristics and Habitat Selection.— Nest sites in both 2009 and 2010 were dominated by open ground (mean 6 SD cover: 58 6 20%, range 5 5–80%, n 5 17), followed by grass (18 6 11%, range 5 5–35%), shrubs (9 6 13%, range 5 0– 40%), and forbs (9 6 15%, range 5 0–45%). Mean vegetation height at nest sites was 11 6 7 cm (range 5 0–50 cm). Vegetation characteristics at nests were

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FIG. 1. Percent vegetative cover at random locations (white boxplots, n 5 80) and Common Nighthawk nests (gray boxplots, n 5 8) on Joint Base McGuire-Dix-Lakehurst, New Jersey, USA, in 2010. Open ground included bare sand, lichens, and matted dead vegetation (litter). Boxplots show median, interquartile range, and range of data. Open circles show points .1.5 interquartile ranges above the median.

similar in 2009 and 2010 (Wilcoxon rank sum tests, W 5 24–43, P $ 0.26), but 2010 nests differed from vegetation at random locations (Fig. 1, Table 1). Nest sites in 2010 had more open ground, shorter vegetation, shallower litter, and less grass and shrub cover than random areas. Nesting Phenology.—We estimated initiation dates for 16 of the 20 nests. Four were found during egg-laying, two were estimated based on a known hatch date, seven from an estimated hatch date, and three from the estimated age of young. The median nest initiation date was 31 May with an interquartile range of 25 May to 11 June, and a range of 18 May to 28 June (Fig. 2). One nest for which we could not accurately estimate initiation date was established prior to 17 May, as it was discovered with a two-egg clutch on this date.

FIG. 2. Estimated initiation dates (first egg laid) for Common Nighthawk nests monitored on Joint Base McGuire-Dix-Lakehurst, New Jersey, USA, in 2009– 2010. Bins are in increments of 5 days, beginning on 15 May (Julian day 135). Boxplot above shows median, interquartile range, and range of dates. Asterisks indicate the average start and end dates for nest searches in 2009 and 2010.

DISCUSSION The lack of data on breeding biology for nightjars is a significant obstacle to effective conservation planning. This is especially relevant in North America as several species appear to be experiencing long-term population declines (Wilson 2008, Nebel et al. 2010, Sauer et al. 2011). We found nest survival through incubation to be 79% (0.987 daily survival) which is considerably higher than reported by Perkins and Vickery (2007) in Florida (28%, 0.932 daily survival, n 5 14 nests). Sample sizes for both studies were somewhat lower than those generally recommended for daily survival rate estimation

TABLE 1. Vegetation characteristics (mean 6 SE) at Common Nighthawk nest sites (2009–2010) and random locations (2010 only) within nest-search plots at Joint Base McGuire-Dix-Lakehurst, New Jersey, USA. Results of Wilcoxon rank sum tests are displayed between the columns: ns, P . 0.05; *, 0.05 . P . 0.01; **, 0.01 . P . 0.001. Nests 2009 (n 5 9)

Veg. height (cm) % Open % Grass % Forb % Shrub Litter depth (cm)

8.6 53.9 21.0 7.8 6.7

6 6 6 6 6

3.3 7.9 3.7 4.3 3.4

Nests 2010 (n 5 8)

ns ns ns ns ns

9.7 63.1 14.4 10.6 12.5 0.3

6 6 6 6 6 6

1.8 4.8 3.5 7.0 5.6 0.1

Random 2010 (n 5 80)

** ** * ns * **

21.5 31.8 40.4 3.1 29.1 1.2

6 6 6 6 6 6

1.4 2.9 3.0 1.0 3.0 0.1

Allen and Peters N COMMON NIGHTHAWK NESTING BIOLOGY

(i.e., $20 nests; Hensler and Nichols 1981), which is reflected by the wide confidence intervals around these estimates (48–93% incubation survival for our study, and 11–68% in Perkins and Vickery 2007). We found apparent nest survival (i.e., % successful nests) of 81% (13 of 16 nests) in comparison to 93% in grasslands of the northern Great Plains (13 of 14 nests [excluding 1 humaninduced failure]; Kantrud and Higgins 1992), and 43% in Florida dry prairie (6 of 14; Perkins and Vickery 2007). Neither Kantrud and Higgins (1992) nor Perkins and Vickery (2007) reported nest abandonment and both concluded that predation was the main cause of nest failure. Published studies of Common Nighthawks have considered only nest survival through hatching. A complete picture of nest survival requires data for the period of young development. Including this stage would necessarily result in lower estimates of overall success. If we assumed, for example, the same daily survival rate for the period of young development as we found for incubation, the expected probability of survival to fledging at our site would be 62% (assuming 18 days from hatching until fledging; Brigham et al. 2011). The question of whether survival differs substantially between eggs and pre-fledged young in this species will likely require telemetry data due to the uncertainties associated with monitoring semiprecocial young (e.g., Fowle 1946, Rust 1947, Perkins and Vickery 2007). Clutch size of Common Nighthawks is similar across a broad geographic area with two-egg clutches dominant in Idaho (24 of 27 clutches; Rust 1947), the northern Great Plains (18 of 21; Kantrud and Higgins 1992), New Jersey (16 of 17; this study), and Florida (13 of 14; Perkins and Vickery 2007). All other clutches consisted of one egg. Perkins and Vickery (2007) argued that some one-egg clutches could be the result of partial depredation, and both Rust (1947) and Sutton and Spencer (1949) observed eggs rolling from nests when the female flushed, which could be another source of clutch reduction. We observed one nest (a 2-egg clutch) at which the second egg was not incubated, but was found ,1 m distant. The movements of young we recorded were similar to previous reports for both rooftop and non-rooftop sites (Fowle 1946, Rust 1947, Dexter 1952). Fowle (1946) reported single-day movements as far as 15–27 m in a burned clear-cut on Vancouver Island, British Columbia, compared to the maximum of 6 m in 2 days in our study. It is

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possible that Fowle’s handling of young for weighing (not done in our study) contributed to the farther single-day movements he observed. Few data exist on the habitat preferences of Common Nighthawks. However, our finding that open ground was preferred is not surprising based on several qualitative accounts (e.g., Fowle 1946, Rust 1947, Brigham et al. 2011) and two quantitative studies (Kantrud and Higgins 1992, Lohnes 2010). Typical nest sites in our study were in patches of ‘open’ ground (e.g., sand, lichens, litter), between warm-season grasses or ericaceous shrubs that often provided partial shade. Lohnes (2010) compared nest sites with random areas in the Konza Prairie in Kansas and also found a preference for open ground. Kantrud and Higgins (1992) noted that over half of 21 Common Nighthawk nests in the northern Great Plains had ‘no vegetation’ and they report an average vegetation height of 6 cm, considerably lower than that of other ground-nesting birds in the area (mean 5 33 cm). We found mean vegetation height at nest sites to be about half that of random areas (10 vs. 22 cm), a discrepancy at least partly driven by the higher number of zero height values at nest sites in open areas. The pattern of nest-initiation dates observed in our study appeared to be unimodal (Fig. 2), although it is possible that increased sample sizes would reveal a different pattern. Some nests also may have been initiated before or after nest searches (15–28 Apr to 15 Jul). This is not likely to be a significant proportion of nests, however, as nighthawks do not typically arrive on the study site until early to mid-May (MCA, pers. obs.) and fall migration in this species begins in mid-August (Walsh et al. 1999). The median initiation date we observed (31 May) was earlier than observed in the northern Great Plains (24 Jun, n 5 8 nests; Kantrud and Higgins 1992) and northern Idaho (30 Jun, n 5 27; Rust 1947), but our range (18 May–28 Jun) was within the range observed in these studies (7 May–15 Jul). ACKNOWLEDGMENTS This study was funded by the Department of Defense Legacy Resource Management Program and the U.S. Navy Agricultural Outlease Program. Field work was performed by Mike Allen, Ron Hutchison, Tamarra Martz, Kim Peters, Ben Sandstrom, Katie Schill, Lena Usyk, and Rachel Villani. We thank John Joyce of Joint Base McGuireDix-Lakehurst for helpful logistical support.

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