Breeding

Phenology

Pair Formation

Figure 5. In Algonquin Park, Ontario, most females arrive on breeding grounds between the 1st and 3rd week of May, 1–2 wk after first males arrive. Pair formation begins immediately among TS male x WS female pairs, and within a few days for WS male x TS female pairs (Kopachena and Falls 1993a). Most birds pair before end of third week of May.

Nest-Building

Commences during 3rd and 4th week of May, as much as 2 wk after pair formation among some TS male x WS female pairs, or as little as a few days after pair formation among some WS male x TS female pairs. Nest-building may take 4–6 d early in season, but later nests built in as little as 2 d (JGK).

First/Later Broods

Figure 5. Although pair formation is slower among WS male x TS female pairs than among TS male x WS female pairs, there is no difference between pair types in onset of egg-laying in first nesting attempts. Between 1986 and 1990 in Algonquin Park, earliest first egg was 15 May; of 87 females initiating nests in May, 74 laid first eggs between 21 and 27 May (JGK). In Nova Scotia, first full clutch was completed May 18 (n=16 nests, Tufts 1986). In Bradley, Maine, the earliest date of the first egg was May 13; most females began laying May 20 to Jun 10 (n=145, B. Horton pers. comm.). Earliest egg date for Manitoba May 28 (n=12, Knapton 2003) and in British Columbia May 28 (n=36, Campbell et al. 2001).

First clutches are highly synchronized; synchrony decreases over the season (Tuttle 2003). Early nests often fail. Of 119 clutches initiated in May in Algonquin Park, 48 were lost to predators and 7 failed to hatch (JGK). In Bradley, Maine, Horton (pers. comm.) reported 68 failures of 164 nests before day 7. Lost clutches usually replaced. Replacement nests produced throughout Jun, with diminishing numbers through Jul. Latest egg date in Ontario is 8 Aug (Peck and James 1987), but captive pairs in outdoor aviaries laid replacement clutches as late as 24 Aug (JGK). In Algonquin Park wild pairs successfully fledging a brood prior to end of June will attempt a second brood, but such attempts are unlikely after first week in Jul (Knapton et al. 1984, Kopachena 1992). In Adirondack Park, NY, nearly all females have second broods (Tuttle 1993). One record of third brood attempt in Algonquin Park (Loncke and Falls 1973). No second broods reported from British Columbia (Campbell et al. 2001) or Maine (Brent Horton pers.comm.). Multiple broods may occur only in the southern part of the breeding range.

Nest Site

Figure 6. Little known about selection process. Because only females build nests, they are likely responsible for site selection. Nests usually on or just above ground along edges of clearings such as roadsides or power lines, in mixed or coniferous forest, cut-over areas, second growth, or edges of bogs and swamps. Nests on level ground, rarely on slopes. Location of nest within clearing depends on size and shape of clearing and presence of vertical objects in or around clearing. Preferred locations possess greater than random amount of “edge” within 3 m of nest site. Height of adjacent vertical object (stump, shrub, tree, etc.) positively correlated with distance of nest from that object (Lowther and Falls 1968). Preferred clearings are large enough to allow direct sunlight to strike ground vegetation at some point during day (JGK).

Nest usually concealed by ground vegetation. Of 42 nests described by Lowther and Falls (1968), 36 built under blueberry and 6 under mountain rice grass (Oryzopsis asperifolia), sweet fern (Comptonia peregrina), hazel, or haircap moss (Polytrichum spp.). Early nests, constructed before blueberry leaves emerge, mostly built under dead fronds of bracken fern (Pteridium aquilinum) remaining from previous year. Later nests more often constructed in blueberry patches without canopy of dead ferns. Avoids building under mature bracken fern and prefers vegetation 15–30 cm high.

Most nests are on the ground. In Ontario 29 of 270 nests were above ground (Peck and James 1987). Seven such nests described by Lowther and Falls (1968) from literature: in roots of fallen stump, a brush heap, in shrubs, in a bog, and up to 3 m in conifers. Of 197 nests found in Algonquin Park between 1986 and 1990, 6 built off ground: 23 cm above ground on lower bough of white spruce (Picea glauca), 1 m on bough of fallen white spruce, 15 cm in blueberry, 5 cm among blueberry and sweet fern, 53 cm in sweet gale (Myrica gale), and 76 cm in bracken fern. Of these 6 nests, 3 were replacements of earlier ground nests lost to predators (no prior data for other 3). Elevated nests accounted for 11.1% of 54 nests initiated after 15 June. Use of elevated nesting sites also observed among captive pairs (JGK). Although no differences were found between nests of WS and TS females in Algonquin Park, TS females built closer to ground and in brighter light than did WS females in Adirondack Park (Tuttle 1993). In Adirondack Park nests are constructed at base of grass or small shrubs. With variations in species composition of vegetation, nest sites are similar across the range, e.g. British Columbia (Campbell et al. 2001).

Nest

Construction

Only female builds nest (Lowther and Falls 1968). Nest-building occurs mainly during morning hours. Earliest stage is preparation of substrate by building up sides and bottom of natural depression with moss. Outer walls built next. Final stage is to add inner lining (JGK).

Structure And Composition

Nest is an open cup divided into bulky outer portion of course materials and inner lining of much finer materials. Nesting materials examined for 39 nests built in Algonquin Park. Outer portions of coarse grasses (36 nests), wood chips (30 nests), twigs (14 nests), pine needles (10 nests), roots (7 nests), and lesser amounts of deer hair, moss, and fine grasses (4 nests). Inner linings of fine grasses and rootlets (38 nests), deer hair (36 nests), pine needles (8 nests), and small quantities of wood chips, twigs, roots, and moss (2 nests) (Lowther and Falls 1968).

Dimensions

Outside diameter 7.0–14.0 cm (n = 48); outside height 4.6–11.4 cm (n = 44). Inside diameter 4.4–10.0 cm (n = 48); inside depth 2.5–6.4 cm (n = 46) (Peck and James 1987; see also Tuttle 1993).

Concealment

Nests well hidden from above by overlying vegetation. During incubation period, most nests visible from only 1 side. Females sit tight and maintain integrity of overlying cover by walking on and off nest during incubation period (Tuttle 1993, JGK). During nestling period, parents often enter from above. This may compromise nest cover, making nest more visible from above.

Maintenance Or Reuse Of Nest, Alternate Nests

Not observed to maintain nests. Never observed to reuse nests. Of 6 replacement nests measured in Algonquin Park, all were > 30 m from previous site (Lowther and Falls 1968).

Eggs

Shape

Subelliptical or long-elliptical (Baicich and Harrison 1997).

Size

In Algonquin Provincial Park (n = 50 eggs): mean length 21.0 mm, range 18.8–23.3; breadth 15.4 mm, range 14.2–16.8 (Lowther and Falls 1968). Calculated volume (0.507 length x breadth2, Hoyt 1979) 2.5 cm³, range 2.3–3.3.

Mass Of Fresh, Whole Egg

No direct measurements. Calculated (0.511 length x breadth², Hoyt 1979; weight coefficient based on Harris’s Sparrow, Norment and Shackleton 1993) 2.54 g, about 10% weight of adult female.

Color

Very pale blue or greenish blue; speckled, spotted, or blotched in purplish or chestnut red and paler lilac. Markings fine and profuse or concentrated about larger end (Baicich and Harrison 1997).

Surface Texture

Smooth and slightly glossy.

Eggshell Thickness

No data available. Empty shell weight (mean) 0.171 g (range 0.143–0.209, n = 20 clutches, 84 eggs; Western Foundation Vertebrate Zoology).

Clutch Size

Modal size 4 (288 nests; Peck and James 1987). For details, see Demography and Populations: clutch.

Egg-Laying

Egg-laying may commence just prior to completing nest (Lowther and Falls 1968) but usually occurs 1 or 2 d after nest complete (JGK). Eggs laid during morning hours on consecutive days until clutch complete. Female usually inattentive but may shield eggs during cold or wet weather (JGK). Not known to replace single eggs. Partial clutch loss uncommon and usually associated with nest desertion. Experimental manipulations not conducted. If entire clutch lost, replacement clutch laid in new nest at different location. Two cases of conspecific brood parasitism reported (Tuttle 1993). One report of nest with 7 eggs suggests another instance (Lowther and Falls 1968; see also Behavior: Sexual Behavior, Extra-pair Copulations).

Incubation

Onset Of Broodiness And Incubation

For clutches of ≤ 4 eggs, incubation usually starts on day last egg laid; for clutches of 5, may start on day prior to laying last egg (JGK).

Incubation Patch

Females possess single ventral brood patch, first evident during nest-building. Males do not develop brood patch, though some reduction of breast down occurs.

Incubation Period

Not well studied. Usually 12 d (JGK). Range 11–14 d (Lowther and Falls 1968).

Parental Behavior

Only females observed to incubate. No data on female attentiveness or on whether males feed incubating females.

Hardiness Of Eggs

Not studied.

Hatching

Preliminary Events And Vocalizations

Not documented.

Shell-Breaking And Emergence

Shells broken by young from inside. No information on length of time to hatch. Within a clutch, young usually hatch on same day, but individuals may hatch at different times of day. In broods of 5, last young may hatch on day after other 4 hatched.

Parental Assistance And Disposal Of Eggshells

Not studied. Eggshells disappear after each young hatches, but method of disposal unknown.

Young Birds

Condition At Hatching

Altricial, naked except for sparse patches of clove brown down on dorsal tracts of head, back, and wings (Lowther and Falls 1968). Eyes closed. Egg tooth disappears shortly after hatching. Respond to vibrations by gaping.

Growth And Development

Eyes open 3–4 d after hatching (Tuttle1993). Nestling mass follows sigmoid growth pattern. Hatch day mass 2.8 ± 0.7 g (n = 49). Growth in mass maximal between days 4 and 5 posthatching (3.0 ± 1.7 g/d, n = 45; Knapton et al. 1984). Mass of day 5 nestlings (14.6 ± 2.2 g, n = 171) is 54.3% of adult mass (26.8 ± 1.1 g, n = 37). On day 8 posthatching (just prior to nest departure), nestling mass (18.6 ± 2.2 g, n = 105) is 69.4% of adult mass (Kopachena 1992). Mass at nest departure slightly higher during cooler weather (JGK).

Tarsus length of nestlings < 5 d old not measured. At day 5, tarsus (measured ventrally from tarsus/hallux joint to posterior aspect of distal sagittal articular ridges of tibiotarsus) measures 19.7 ± 2.1 mm (n = 87) and is 80% of adult tarsus length (24.2 ± 0.8 mm, n = 31). By day 8, tarsus length (23.4 ± 1.2 mm, n = 78) is 96.7% of adult tarsus length (Kopachena 1992).

No data on growth of contour feathers or on growth of primaries and rectrices prior to day 5. Primary feathers begin to emerge at day 3–4. Rectrices begin to emerge a day later. The following measurements made of length of right seventh primary and right outer rectrix during days 5–8 posthatching (neossoptiles not included). Day 5 primary length 7.6 ± 2.7 mm (n = 83); day 5 rectrix length 0.6 ± 0.9 mm (n = 53). Between days 5 and 8, growth of these feathers linear and rapid (primaries, 4.5 ± 0.5 mm/d, n = 66; rectrices, 1.9 ± 0.5 mm/d, n = 40). Day 8 primary length 20.8 ± 2.5 mm (n = 78); day 8 rectrix length 6.1 ± 1.7 mm (n = 68). At this time, primary length is 35.6% of adult primary length (58.5 ± 2.2 mm, n = 5) and rectrix length is 8.3% of adult rectrix length (73.4 ± 4.8 mm, n = 5) (Kopachena 1992).

Development of thermoregulation not documented.

Nestlings mostly inactive during all but last 1–2 d of nestling period. Gape response weak on hatch day but well developed by day 4. Begin to vocalize when parents visit nest at day 7. Competition and dominance hierarchies among nestlings not documented. Show fear of human intruders at day 5 (Lowther and Falls 1968). By day 8 (occasionally day 7), most nestlings will not stay in nest if temporarily removed by investigator (JGK).

Locomotory abilities poorly developed until just prior to fledging (day 9). Incapable of grasping objects with feet until day 6. Nestlings 7–8 d old stand on smooth surfaces by resting tarsus on substrate. Older nestlings or fledglings stand on feet. Incapable of flight at time of nest departure.

Causes Of Death

The following figures obtained from 171 nests found during incubation in Algonquin Park between 1986 and 1990 (JGK). Predators were main source of mortality among eggs and nestlings (see Behavior: Predation). Annual whole brood losses to predators were 44.8% (range = 22–70%). Of nests surviving through hatching, predators responsible for partial brood loss at 10% of nests (range 5.3–17.1%). Among 9 clutches of 5 where 1 young hatched a day later than nest mates, 4 lost 1 young to starvation. No evidence of young dying from exposure. No evidence of mortality caused by human disturbance during nestling period. In Adirondack Park, Tuttle (1993) reported approximately 40% nest losses and attributed these to predation (87.5% of losses) and exposure (12.5%), mainly by flooding.

Parental Care

Brooding

Young brooded throughout nestling period, less frequently prior to nest departure. During hot weather, parents shade young by standing over nest with wings slightly spread. Little information on these activities during first 4 d of nestling period. Brooding and shading primarily by females. Between 1986 and 1989, of 39 pairs observed during days 5–8 (13 h of observation/pair), all 39 females and 8 males observed brooding or shading. These males brooded or shaded young 0.18 ± 0.11 times/h, each period being 12.9 ± 13.1 min long. For 10 females, brooding or shading periods occurred 1.03 ± 0.49 times/h and were 9.05 ± 7.78 min long (JGK). Whillans and Falls (1990) measured mean time/trip during last 4 d of nestling period (measure combines brooding and shading with time spent at nest during provisioning trips). For males, this was about 20 s and remained constant from day to day. For females, mean time/trip declined from about 220 s on day 5 to about 120 s on day 8, indicating reduction in brooding with nestling age.

Brooding rhythm not documented. Females brood young at night. During day, brood more during cool, wet weather. Shading occurs during hot weather. Brooding by females often interrupted by male provisioning trips. When displaced by male, brooding female may wait for male to feed young, then return to brooding. More frequently, female leaves nest area and returns with food before brooding resumes.

Feeding

Provisioning behavior of parents not documented during first 4 d of nestling period. Likely commences soon after first egg hatches. Both parents feed nestlings at rates increasing from day to day (Whillans and Falls 1990). From day 5 to nest departure, there are morph- and sex-specific differences in provisioning rates (Knapton and Falls 1983, Kopachena and Falls 1993b, Horton 2007). Mean trips/h ±SD: WS males, 3.46 ± 0.92 (n = 19 birds); TS males, 4.09 ± 0.67 (n = 20); WS females, 4.21 ± 1.07 (n = 20); TS females, 4.87 ± 1.18 (n = 19) (Kopachena and Falls 1993b).

Despite these differences, provisioning rates of WS male x TS female pairs do not differ from those of TS male x WS female pairs, averaging 8-9 trips/h. Two WS male x WS female pairs had provisioning rates much lower than those of heteromorphic pairs (Knapton and Falls 1983). Manipulation of baseline corticosterone levels by Horton reversed the provisioning rates of WS and TS males. WS females lowered their rates in response to reduced rates of TS mates but TS females showed no change when provisioning by their WS mates increased. For details see Horton (2007) and Endocrine Correlates of Polymorphism.

Unlike other morph/sex classes, provisioning rates of WS males vary relative to their song rates (songs/h) and length of their territory boundary shared with neighboring White-throated Sparrows. WS males with high song rates and long shared boundaries provide more food to young than do WS males with low song rates and short shared boundaries (Kopachena and Falls 1993b). These systematic variations in WS males’ provisioning rates might account for lack of morph- and sex-specific differences found in some samples (e.g., Whillans and Falls 1990), particularly if sample sizes small (Kopachena and Falls 1993b).

Removal of males between days 6 and 8 of nestling period resulted in increased provisioning rates by females of both morphs, whereas brooding time decreased. Young fledged from all such experimental nests (n = 18) but with fledging weights below normal (Whillans and Falls 1990).

Parents feed young by placing food into gaping mouths of nestlings. One or 2 young fed during each visit. Principal items delivered are Lepidoptera larvae (60.9% of items delivered) and adult Lepidoptera (5.1%). Other items include adult Diptera (4.5%), Orthoptera (3.5%), Diptera larvae (3.4%), adult Odonata (2.3%), Coleoptera (2.2%), Ephemeroptera (0.6%), and Trichoptera (0.4%). Miscellaneous winged insects and unidentified items account for 17.3% of items delivered to young (Knapton and Falls 1983).

Parents bring 1–5 items/trip (medians; males, 1.9, n = 21; females, 1.7, n = 25). Most items between 1 and 3 cm long (medians; males, 1.5 cm, n = 21; females, 1.4 cm, n = 25). No difference between morph and sex classes in number of items per trip or size of items delivered. No correlation between provisioning rate and number of items per trip or size of items delivered (Kopachena and Falls 1993b).

Nestlings often defecate after being fed. Fecal sacs removed immediately by parents. During days 5–8, parents occasionally eat fecal sacs. Usually fecal sacs carried away, presumably to be dropped away from nest area.

Cooperative Breeding

Not known to occur.

Brood Parasitism

Rare host of Brown-headed Cowbird (Molothrus ater), apparently because this species is uncommon in areas occupied by White-throated Sparrows. Of 192 nests found in Algonquin Provincial Park between 1986 and 1990, 1 found parasitized (0.5%). This nest contained 1 cowbird egg and 2 sparrow eggs. All hatched, and nestlings survived to fledge (JGK). In Ontario as a whole, Peck and James (1987) report 24 of 390 nests parasitized (6.2%). Twenty of 507 nests in Quebec found parasitized (4.0%); no nests were deserted (Terrill 1961). In British Columbia 6 of 34 nests contained cowbird eggs (18%) (Campbell et al. 2001). Other reports from Minnesota, Michigan, Wisconsin, Manitoba, Pennsylvania, Saskatchewan, and Alberta (Lowther and Falls 1968).

Fledgling Stage

Young leave nest 7–12 d after hatching, most frequently on day 8 or 9 (Lowther and Falls 1968). Most fledge during morning, and usually all young leave on same day. Following description based on observations at 4 nests (JGK). Young left 1 at a time, each dispersing in a different direction. It took >1 h for all young to depart. Fledglings began calling immediately after leaving and made their way into dense cover. Parents attentive, feeding young remaining in nest as well as dispersing young. Parents frequently uttered seeps (location calls) and chips and chinks (alarm calls), and were very sensitive to disturbance. Parents flew down to dispersing young with and without food, but difficult to determine whether this constituted “leading” young away from nest.

A single, marked fledgling was followed for 3 d after nest departure. Upon leaving nest, it moved directly to base of large white spruce about 10 m from nest. During next 2 d, fledgling remained within lower boughs of this tree, consistent with other observations of fledglings remaining within a limited area during first 4–5 d of fledgling period (JGK).

Upon nest departure, fledglings incapable of flight but have well developed ability to grasp objects with feet, to perch, and to move through ground vegetation. When approached by human observer, recently fledged young crouch close to ground and “freeze,” but move quickly if attempt is made to catch them. By 4–5 d postfledging, capable of some vertical flight, but sustained flight still limited. Fledglings ≥ 1 wk out of nest are strong fliers and sometimes seen following parents (JGK).

Growth during days 1–4 after nest departure was measured among fledglings held in open-topped enclosures that allowed parents to care for young but prevented young from escaping. Fledglings lose about 1 g of body mass during first day out of nest (at fledging on day 8 mean mass ± SD, 18.6 ± 2.2 g, n = 105; day 9 mass, 17.6 ± 1.7 g, n = 37). Thereafter, mass increases, but growth slower (0.41 ± 0.40 g/day, n = 26) than during nestling period; it takes 4 d for fledglings to recover from initial mass recession. At day 4 postfledging, fledgling mass (19.0 ± 1.58 g, n = 53) is 71.3% of adult mass (Kopachena 1992).

At nest departure, tarsus length of young 96.7% of adult tarsus length (Kopachena 1992; for additional measurements see above, Young Birds: Growth and Development).

Just prior to fledging, right outer rectrix and right seventh primary lengths (means ± SD) 6.1 ± 1.7 mm (n = 68) and 20.8 ± 2.5 mm (n = 78), respectively (Kopachena 1992). Rectrices grow faster after fledging (2.9 ± 0.6 mm/d, n = 25) than prior to fledging. Primaries grow more slowly after fledging (2.6 ± 0.4 mm/d, n = 26) than before fledging. At day 4 postfledging, rectrix 15.9 ± 2.5 mm (n = 25) and primary 32.2 ± 2.1 mm (n = 26). At this time, rectrix is 21.7% of adult rectrix length (73.4 ± 4.8 mm, n = 5) and primary 55.1% of adult primary length (58.5 ± 2.2 mm, n = 5).

Parents care for young for at least 2 weeks after fledging, but exact duration of offspring dependency unknown. In some cases this extends into incubation period of subsequent brood. At this time fledglings observed following male parents, but not known if female parents associate with or feed them. Observations of 3 pairs suggest females producing second brood feed fledglings during egg-laying period. One observation in Adirondack Park suggests female may feed fledglings while incubating second clutch (Tuttle 1993). Female care of fledglings does not appear to prolong interval between nestings (Kopachena and Falls 1993c).

During first 4 d of fledgling dependency, young receive 81% more provisioning trips than they do during last 4 d of nestling period (trips/h for broods of 3: males, median = 7.44, range = 3.03–13.00, n = 12; females, median = 7.13, range = 5.18–9.98, n = 11). Because fewer items/trip are brought to fledglings (males, median = 1.55, range = 1.14–1.97, n = 9; females, median = 1.65, range = 1.19–2.20, n = 9), increase in provisioning rate results in only 51% increase in amount of food received by young. Female parents provide more food to fledglings than do male parents. Provisioning of fledglings does not vary between early and late broods. No evidence of morphic differences when provisioning fledglings (Kopachena and Falls 1993c).

Parents feeding fledglings typically divide brood (Kopachena and Falls 1991). Among broods of 3, usual pattern was for 1 fledgling to receive most of its food from male parent, another to receive most of its food from female parent, and third fledgling to be shared about equally between parents. Brood division began on day young left nest and was stable for at least 4 d. Although brood division extends beyond this time, exact duration unknown. Initially parents may base division of brood on location of young, but as young become mobile, this process appears to be mediated by parental recognition of individual offspring. When tested 4 d after fledging, 60% (n = 20) of parents could distinguish young they were feeding from other brood members (Kopachena and Falls 1991). Young often remain in their parents’ territory beyond 2 wk despite harassment by the territorial male (Tuttle 1993).

Immature Stage

No data on habits or dispersal. Juveniles measured in Adirondack Park in Aug did not differ from adults in mass, wing length, or tarsus length, though tarsus length was more variable than in adults (Tuttle 1993).

Cranial pneumatization occurs from Nov in the first autumn into winter, with pronounced differences among individuals and years; later in larger individuals but unrelated to arrival dates in wintering area or social dominance. Half the birds show complete pneumatization by January, virtually all during the first winter (Wiley and Piper 1992) but small “windows” (areas of the skull not fully pneumatizated) have been reported in SY birds (Pyle 1997).