Demography and Populations

Measures Of Breeding Activity

Age At First Breeding

First year of life, annually thereafter (Middleton 1979).

Clutch Size

Mean clutch size for first nests at Guelph, ON, 5.2 eggs (SD = 0.35; n = 77; range 2–7), but influenced by age, habitat, date, and season (Middleton 1979, Jacobsen 1990). Clutch sizes comparable across range (Stokes 1950, Tyler 1968, Harrison 1978, Peck and James 1987). Renesting common in pairs that lose first nest; up to 3 nesting attempts recorded (ALAM). Clutch size smaller in renests and for second brood attempts (Middleton 1979).

Annual And Lifetime Reproductive Success

At Guelph, ON, reproductive success varies with age of parent and nesting habitat. Compared to first time breeders, experienced breeders tend to nest earlier, have larger clutches, and rear more young in a season (Middleton 1979, 1988). Over an 8–yr period, and based on number of nests for which complete data were available (despite banding effort precise number of breeding females in population could not be confirmed), reproductive success was 3.4 (SD = 1.10; n = 25) and 2.8 (SD = 1.15; n = 92) successfully reared chicks/nest for experienced and presumed first time nesters, respectively. Mayfield estimates for the probability of a nest fledging a least one young range from 0.31-0.54 (depending on habitat type) in Ontario and, for comparison, were 0.43 in New York, 0.47 in Wisconsin, and 0.66 in restored fields in Missouri (Dhondt et al. 2007).

At Guelph, ON, at least 66% of double broods are produced by experienced females (Middleton 1979, 1988). Success of attempted second broods was significantly lower than for first nests, but was no different from second nests that were replacement attempts (Middleton 1979). For double-brooded versus single-brooded females, the number of successfully reared chicks/season was 7.2 (SD = 1.34; n = 9) and 3.3 (SD = 1.3; n = 83), respectively (J. Wojnowski pers. comm.). Wojnowski estimated 15% of breeding males and 20% of females produced no young, whereas 35% of breeding males and 20% of females produced 50% of all young raised to fledging.

Most complete data on lifetime reproductive success provided by one color-marked female, followed for 5 successive seasons at Guelph, ON, that built 6 nests from which 24 young were produced. Largest clutch was 6 eggs and smallest was suspected 4 (nest parasitized by Brown-headed Cowbird). Two nests parasitized in successive seasons. Bird at least 6 yr old when she disappeared from population and had bred with 6 different mates during known lifetime.

Number Of Broods

Double broods first suspected by Mousley (1935). Between 6.7% (Middleton 1988) and 15% (Stokes 1950) of nesting females double-brooded. In Ontario, and possibly elsewhere (e.g., Wisconsin), double brooding made possible through polyandry (Middleton 1988). If polyandry adopted, first male assumes full responsibility for chicks from first nest.

Life Span And Survivorship

Maximum longevity about 11 yr, based on banding data to 1979 for free-living American Goldfinches > 5 yr (n = 95; Bird Banding Laboratory, Laurel, MD; Middleton and Webb 1984). The data fit those for captives (Phillips 1968) and encompass longevity record of 9 yr 3 mo reported by Klimkiewicz and Futcher (1989).

Males live longer than females, which may explain skewed sex ratio in favor of males (1.6/1; n = 6,201) in wild populations (Middleton 1979, Pengelly 1982, Middleton and Webb 1984, Prescott et al. 1989, Jacobsen 1990).

Mortality And Disease

Causes Of Death

Include predation, disease, exposure, collision, impalement on thorns (Berger 1968), and even ensnarement on burdock and in spider web (Tyler 1968). Late in nesting season some chicks deserted in nest by parents (Berger 1968, ALAM). No significant effect of leg band color on annual return rates, though there was a trend for yellow-banded birds to return less frequently (Watt 2001).

Exposure

Subject to extreme weather conditions. Heavy winds and rain cause of nest destruction and death of young (Berger 1968, Middleton 1979). Starvation a result of winter storms (Middleton 1982).

Predation

Most common mortality cause in nesting and free-flying birds (Stokes 1950, Berger 1968, Holcomb 1969a, Miller 1978, Middleton 1979). Known and suspected predators includes reptiles, birds, and mammals (see Behavior: Predation). Loss of nest contents steady throughout incubation but increases noticeably when young in nest (Holcomb 1969c).

Brood Parasitism

Early nests parasitized by Brown-headed Cowbird. Parasitism results in loss of goldfinch eggs and chicks and fails to produce cowbirds (see Breeding: Brood Parasitism, above).

Human/Research Impacts

Collision with motor vehicles known cause of mortality at Guelph, ON (ALAM). Nest desertion an infrequent occurrence early in nest cycle when nest contents checked, or adults trapped for banding. Human activity near nest may increase predation risk (Holcomb 1969a, ALAM). Wind turbine plants pose measurable threat in Washington (Erickson et al. 1999). Infrequent mortality results from capture and banding. Caution should be exercised in point counts, as goldfinches are repelled by orange vests and thus populations can be underestimated (Gutzwiller and Marcum 1993).

Diseases And Body Parasites

Diseases fairly well documented. Coccidiosis (Isospora gryphoni; Olson et al. 1998) or its complications a common cause of death of wild-caught captives in Ontario (Middleton and Julian 1983) and Alabama (McGraw and Hill 2000). Anti-coccidial drugs can be used to control this affliction; over half of wild-caught birds in Ontario also pass coccidial oocysts (Olson 1996). Infections with this parasite negatively impact carotenoid-based plumage and beak coloration, but not melanin pigmentation of the cap (McGraw and Hill 2000). Oocyst shedding is also weakly negatively correlated with reproductive success of social fathers at a nest (Olson 1996).

Host of recent outbreak of mycoplasmal conjunctivitis (M. gallisepticum; Fischer et al. 1997) in the eastern USA. Two birds from North Carolina and four in New York contracted this disease, very common in House Finches from eastern North America, in 1996 (Ley et al. 1997) and 1998 (Hartup et al. 2000), respectively; many more confirmed cases have recently been reported throughout the eastern half of the country (Hartup et al. 2001, Farmer et al. 2005). Salmonellosis (Salmonella typhimurium PT 40) outbreak killed one C. tristis in New Brunswick, Canada in May 1998 (Daoust et al. 2000). One record of “liver fluke” (identity unknown) at Guelph (pathology report; L. Sileo pers. comm.). Intermediate host of Schistosoma dermatitis, cause of swimmer’s itch (W. Dawson pers. comm.).

Ectoparasites include feather mites (Acari), encountered during banding of nestlings and adults, and louse flies (Hippoboscidae) captured from adults (ALAM).

Range

Initial Dispersal From Natal Site

Few fledglings recovered after independence. In 24–yr study at Guelph, ON, only 2 nestlings known to return to nest in natal area. Three additional nestlings recaptured on natal area in Jun, before nesting commenced, but not encountered again. Following independence, young form large nomadic flocks in vicinity of natal areas. Details of subsequent autumn and winter movements unknown, but many young spend first winter in northern part of range (Prescott and Middleton 1990).

Fidelity To Breeding Site And Winter Home Range

Some birds resident throughout year (Wiseman 1975), but in northern regions winter and summer populations discrete (Middleton 1978, Prescott and Middleton 1990). Returning females breed in immediate vicinity of first nesting (Middleton 1979). One female studied over 5 consecutive yr, Guelph, ON, built all nests within 30 m radius of first known nest. Males less site-specific but return to same breeding area (Middleton 1979). Female mentioned above changed mates each year, even though 3 of previous mates were found nesting elsewhere in study area (ALAM).

Recovery of winter-banded birds during subsequent winters suggests return to same wintering sites (Wiseman 1975, H. Richards pers. comm.), but analysis of banding data (Bird Banding Laboratory, ALAM) suggests considerable individual variation between years.

Home Range

During breeding season, home range limited by proximity of food supplies. Birds move freely throughout nesting habitat and make sorties to food sources, frequently > 1 km from nesting habitat (ALAM). Once incubation begun, females seldom leave immediate nest vicinity, whereas males move considerable distances between nest and feeding sites.

Winter birds show great freedom of movement. During daylight hours birds range widely, e.g., between feeders separated by as much as 7 km (ALAM). Within a season some individuals known to travel > 50 km between cities (H. Richards pers. comm., ALAM).

Population Status

Breeding Density

As estimated from Breeding Bird Survey data from 1992-2005, populations most abundant in northern and eastern quadrants of the United States, with highest densities in Wisconsin (25 birds per Breeding Bird Survey Route) and Nova Scotia, Canada (24 birds/route) (Fig. 2).

Breeding density difficult to determine because of clumped distribution of nests (Miller 1978). At Guelph, ON, density estimated at 0.78 pairs/ha (SD = 0.073; study area 68 ha; n = 8 seasons; ALAM). Miller (1978) and Dhondt et al. (2007) found maximum nest densities of 12 and 14 nests/ha, respectively, in Illinois and on willow plantations in New York.

Winter Density

See Figure 3; highest early winter densities in central Texas, Oklahoma, and parts of southeast, as well as pockets father north. Northern pockets likely represent feeder-dependent populations.

Trend analysis of Christmas Bird Count (CBC) data for Ontario found no significant change from 1968-1991 in winter populations (van Twest 1991), following the steady increase noted by Middleton (1977a) between 1920 and mid-1970s. Similar steady state (ca. 2.2 birds/party hour), with minor annual oscillations (from 1.9 in 1994 to 2.6 in 2005), in CBC data for the USA from 1992-2006 (http://www.mbr-pwrc.usgs.gov/bbs/cbccomp/h5290cp.html). Slight, gradual increase in total population size estimate in the USA since 1992, from 200,000 to currently 230,000.

Trends

Evidence from Breeding Bird Surveys (BBS) from 1985-1991 suggested populations declining throughout e. North America but remaining stable in western regions (see Fig. 7; Robbins et al. 1986, Erskine 1978, Erskine et al. 1992, Droege and Sauer 1989). No sign of breeding population decline at Guelph, ON through 1991. From 1992-2005, BBS data indicate small annual increases in population size in all but 10 of the US states in which they breed (http://www.mbr-pwrc.usgs.gov/cgi-bin/atlasa99.pl?05290&1&07). Median per-state annual increase was 2.6%. Declines were only observed in western states (median = -1.8%). Canadian populations experienced declines in half of all provinces (mostly western also), though most of these areas still support healthy populations.

Population Regulation

Predation major cause of mortality and nest failure in breeding populations (Stokes 1950, Berger 1968, Holcomb 1969a, c , Middleton 1979). Post-nestling mortality poorly understood, but most young birds probably die in first year (see Mead 1985 ). Skew in adult sex ratio apparently due to differential mortality between sexes (Jacobsen 1990). Greatest mortality probably occurs in winter (Prescott and Middleton 1990), and may be physiologically related (Dawson and Carey 1976, Carey et al. 1978, Dawson and Marsh 1986, Dawson et al. 1983, Prescott and Middleton 1990). Under stressful conditions, disease (e.g., coccidiosis) may contribute to mortality (Middleton and Julian 1983).