Breeding

Phenology

Pair Formation

Occurs on breeding grounds, after male and female arrive (separately). Walkinshaw (1978) observed that an unmated male will fly at, often strike and drive a female to the ground when she first appears on the territory. By the next day the pair is strongly associated with each other (see Pair Bond above). Females arrive 10–20 d after males (see Migration) and pairing occurs within 1–2 d. Females not often seen until already paired (MC).

Nest Building

Construction dates of 16 first nests in Missouri ranged from 27 Apr–18 May (DEB). In Pennsylvania, dates (n = 43) ranged from 2 May–26 May; cessation of nest building began in early Jul, latest nest start on 25 Jul (MC).

First/Later Broods

Figure 4 . In Missouri, first eggs (including renestings) laid from 29 Apr–10 Aug; earliest chick hatch on 15 May; latest fledging 7 Sep (DEB). In Michigan (1939-1948) earliest first egg dates ranged from 29 Apr – 16 May (Walkinshaw 1978).

In Pennsylvania (1987-2006), earliest first egg dates (n = 905) ranged from 3 May – 16 May; latest first egg dates ranged from 4 Jul – 27 Jul. Annual variation in these dates showed no long term trend. Earliest chick hatch on 19 May; latest fledging date 18 Aug (MC). Dependent fledglings observed in Ohio on 10 Oct (Walker 1937).

Nest several times per season; number of nesting attempts related mainly to rate of nest loss (see Demography and Populations: measures of breeding activity/clutch, reproductive success). Females fledging young begin laying in a new nest 6–20 d after fledging (Walkinshaw 1968a, DEB, MC). Egg laying in a new nest starts about 5 d after loss or desertion. (Walkinshaw 1939, MC).

Nest Site

Selection Process

Female chooses nest site after examining a number of locations (Walkinshaw 1968a). Behavior not described.

Microhabitat

Early nests on or near ground in grass clumps or at the bases of shrubs; later nests higher in crotches of shrubs or saplings (Walkinshaw 1968a, Best 1978, DEB, MC). In Illinois, nests mostly in grasses, black raspberry (Rubus occidentalis), and crab apple (Malus ioensis; Best 1978). In Missouri, early nests in grass clumps, red cedars (Juniperus virginiana), and sprouting elm stumps (Ulmus spp.); later nests often in blackberry (Rubus allegheniensis), shrubby St. Johnswort (Hypericum spathulatum), or coralberry (Symphoricarpos orbiculatus; DEB). In Pennsylvania, early nests in grass clumps or at base of honeysuckle (Lonicera spp.); later nests often higher in honeysuckle, dogwood (Cornus spp.), or hawthorne (Crataegus sp.; MC).

Site Characteristics

Most nests found in or within a short distance of shrubs or saplings; nests rarely found over 40 m from any woody vegetation (Johnston 1947, Lanyon 1981, DEB, MC); other “brushland” sparrows, e.g., Song Sparrows, do not always build ground nests so close to woody vegetation (MC). May nest in fallow fields as early as 2nd yr after cultivation (Lanyon 1981). Burning of shrub-grassland and grassland habitat in Illinois resulted in higher use of shrub-grassland after the fire (Best 1979).

Nest heights increase over breeding season. In Illinois, height (cm from ground to rim): in May—mean = 26, range 9–48, n = 53; Jun—mean = 38, range 23–77, n = 44; Jul—mean = 47, range 22–89, n = 49; Aug—mean = 48, range 30–76, n = 10 (Best 1978). In Pennsylvania, height in May—mean = 17.3 ± 23.6 SD, range 0–157, n = 133; Jun—mean = 53.3 ± 27.0 SD, range 6–210, n = 138; Jul—mean = 64.8 ± 21.3 SD, range 35–148, n = 59 (MC). In Michigan: May height mean = 7.5, n=235 (78%) on ground; June—mean = 21.4, n = 239; July—mean = 30.6, n = 240 (Walkinshaw 1978). Occasional nests in Missouri as high as 3 m (DEB). Higher nests correlate with lower predation, but in early spring, prior to the growth of leaves and groundcover, such nests would be more conspicuous to predators (Burhans et al 2002, MC).

Nest

Construction Process

Only females build nests; usually accompanied by mates, who may offer nest material. Most nest building activity between 0600 and 1100 hr (Walkinshaw 1968a, DEB). Early nests take 5–8 d to build; later 2–3 d (Walkinshaw 1968a, DEB, MC). In Missouri, females made an average of 8.3 ± 5.1 SD trips/30 min to the nest (range 2–17.4, n = 9; DEB).

Structure And Composition

Nest an open cup made of large grass pieces interwoven with finer grasses. Lining of fine grasses, rootlets, and hair (Walkinshaw 1968a). Nests typically start with a framework of criss-crossed grass stems on the supporting plant (DEB).

Dimensions

Michigan nests (n = 90) (mm): mean outside diameter 124.3, range 85–210; mean outside depth 63.8, range 47–110; mean inside diameter 50.2, range 43–55; mean inside depth 39.3, range 25–62 (Walkinshaw 1968a, 1978).

Microclimate

Skowron and Kern (1980) rated Field Sparrow nests as poorly insulated, having higher heat flux and thermal conductance compared to 9 other open nesting species. Percent of sunlight reaching nest very high compared to 8 other old field songbird species (Lanyon 1981).

Maintenance Or Reuse Of Nests

Nests normally not maintained or repaired, though MC observed a few females repairing sagging nests. With one known exception (Allaire 1972), nests not reused.

Eggs

Shape

Short subelliptical.

Size

Length and breadth (mm): 1,308 eggs from 446 clutches in Michigan, mean = 17.9 (range 16.1–21.0) x 13.5 (range 12.3–14.7; Walkinshaw 1968a). In Pennsylvania, 1,187 eggs from 341 clutches, mean = 17.8 ± 0.83 SD (range 15.3–21.4) x 13.4 ± 0.44 SD (range 11.8–14.7; MC). Walkinshaw (1968a) found no discernible trends in egg size. MC found significant increase in egg length from first to last egg in clutches: egg 1, mean = 17.60 x 13.29; egg 4, mean = 17.88 x 13.36. Length and breadth increased significantly with laying date. May eggs: mean = 17.65 x 13.28, Jul eggs: 17.86 x 13.57.

Mass Of Fresh, Whole Egg (G)

Mean = 1.67, n = 68 (Walkinshaw 1939); mean = 1.69, range 1.4–2.1, n =18 (Holcomb 1966). Egg mass is 12.8–13.0% of mean female mass.

Color And Markings

Eggs smooth and slightly glossy. White or creamy background, sometimes tinted very pale bluish or greenish. Eggs spotted over much of the surface; spots tend to concentrate more highly at the large end, often forming a loose ring. Spots usually medium-, reddish- or purplish-brown, ranging from pale purple to gray (Walkinshaw 1968a). Little variation in egg color and markings of a single female (MC).

Eggshell Thickness

No data.

Egg Laying

When started, in relation to nest completion. In Pennsylvania in a sample of 705 nests discovered prior to completion, a delay of 1 d between completion and start of egg laying in 587 (83%). Longer delays (up to 4 d) usually occurred with first nests of the year, typically during unusually cold and/or wet weather (MC).

Time of day and rate of deposition: Morning arrival times measured in 3 separate populations by Burhans et al. (2001): In ne. Illinois 23.1 ± 2.4 SD min before sunrise (n = 14); in Missouri 13.5 ± 1.6 SD min before sunrise (n = 36); in Pennsylvania 1.8 ± 6.6 SD min before sunrise (n = 18) (Burhans et al 2001). Earlier arrival times correlate with higher Brown-headed Cowbird parasitism rates (see Brood Parasitism below) (Burhans 2000). One egg/d (Walkinshaw 1968a, MC).

Parental behavior during this period: Adults inattentive to nest. Usually no response to observers near the nest. Male closely accompanies female (MC).

Replacement of individual eggs or clutches: Individual eggs not replaced. If predator takes all eggs, female begins building a new nest elsewhere within 1–2 d. If all eggs are taken before the end of laying, the next egg is often deposited in or near the old nest (MC).

Incidence of intra-specific egg dumping: In general, very low. In 469 nests found before end of laying, 2 nests received 2 eggs on the same day (MC). Using protein comparisons, Petter et al. (1990) found that 4 of 52 nestlings (7.7%) had proteins mismatched with the attending female. Using DNA fingerprinting, MC found that only 2 of 1,051 nestlings had DNA mismatched with the attending female.

Incubation

Onset of broodiness and incubation

In two-egg clutches female begins incubating with the last egg; 3-4 eggs - incubation begins the night before the last egg is laid; 5 eggs – 2 nights before last egg (Walkinshaw 1978). During cold early spring conditions, some females delay start of incubation up to 4 d after laying the last egg (MC).

Incubation patch

Females have a ventral brood patch. Patch begins to form during building of the first nest, and is fully formed by start of incubation.

Incubation period

Defined as interval between laying of last egg and hatching of first egg: 11- 12 d; occasionally 10; as long as 17 d in cool wet springs when onset of incubation behavior may be delayed (Walkinshaw 1936, 1968a, 1978; Best 1978, MC).

Parental behavior

Only females incubate; typically, males show no interest in eggs or nest. Male accompanies female when she is off the nest and both chip when an observer is nearby; male usually does not chip when female on nest (MC). Walkinshaw (1968a) reported that males, on occasion, will feed incubating females, but MC observed no such feeding in 36 h of nest watches from a blind.

During daylight hours females spend an average of 70% of their time on the nest (Crooks and Hendrickson 1953, Walkinshaw 1968a). During 1 h observations from a blind (n = 36), females spent an average of 40 min (67%) on the nest during morning hours with no rain (MC). Females incubated eggs for periods ranging from 20–63 min (mean 33 min); periods away from the nest ranged from 6–25 min (mean 14 min; Crooks and Hendrickson 1953).

Hardiness of eggs

No data.

Hatching

Preliminary events and vocalizations

Egg pipped from inside by nestling. No vocalizations.

Shell breaking and emergence

Brood usually hatches on same day. In Pennsylvania (nests checked once daily) 226 of 369 nests hatched the same day, 137 hatched over 2 d, 6 over 3 d (MC). No data on time of day. In Michigan, typically eggs pipped ~12 hr before hatching; 1 nest had 2 eggs pipped 28 hr before hatching (Walkinshaw 1978). No data on duration of hatching, though Walkinshaw (1939) noted that an already cracked egg required “several minutes” before complete emergence; the hatching bird periodically pushed with head, neck, feet, and wings to split the eggshell nearer the larger end.

Parental assistance and disposal of eggshells

No direct parental assistance. Eggshells rapidly removed from nest; disposal method not reported.

Young Birds

Condition at hatching

Mass and linear measurements: See below; growth and development.

Amount and distribution of feathers: Naked with sparse downy neossoptiles ranging from 1–7 mm in length (Walkinshaw 1939, Wetherbee 1957). Distribution in regions (number of tufts 1st [Walkinshaw 1939], counts of neossoptiles 2nd [Wetherbee 1957]): coronal 4–6,11; occipital 5–7,4; middorsal 10–12,3; scapular 4–5,7; femoral 6,9; crural 2,2; abdominal 10–12,11. The following are neossoptile counts only (Wetherbee 1957): pelvic (upper) 2, pelvic (lower) 3, rectrix 10, greater secondary covert 9, middle secondary covert 6.

Color of plumage and bare parts: Natal down “mouse-gray” (Dwight 1900). Color of skin, legs, and bill pinkish; bill grayed near the tomia. Gape red; lining of the gape yellowish, pinker near the side. Eyes closed, but show in large gray areas under the skin. A small white egg tooth near tip of maxilla disappears in ~2 d . (Walkinshaw 1939, 1978).

Degree of coordinated movement or ability to find food: Young are helpless and depend on adults for food.

Responses, body attitude: Body usually prone on bottom of nest; responds to touch or agitation of the nest by stretching neck and head upward and opening gape. No visual responses (Dawson and Evans 1957, MC).

Growth and development

Mass increase: See Table 2 . Nestling masses of Best (1977a) and Carey (1990) are significantly lower than those of Dawson and Evans (1957); also appear lower than Walkinshaw (1939), but significance could not be determined. Mass increase is best approximated by the logistic growth equation with a weight asymptote of 11.0 g.; last hatched nestlings have significantly lower weights than siblings (Best 1977a). Brood size had no significant effect on mass increase of young (Best 1977a, Carey 1990).

Growth of body parts: Nestling tarsal length increases linearly from 0.71 cm on day of hatching (day 0) to 1.76 cm on day 5. Tarsal lengths of last hatched nestlings significantly shorter; no significant effect of brood size (Best 1977a).

Age when feathers appear: Molt into Juvenal plumage not recorded in detail. Papilae of feather tracts visible on day 0 but not pigmented. Nestlings 1 d old have clearly pigmented feather tracts. In day 2 nestlings, 3rd primary feather sheath has emerged 1.5 mm. Primary sheaths lengthen to day 7 when 3rd is 27.0 mm. Feather barbs begin to emerge from the sheath on day 6. By day 8, when birds are ready to leave the nest, feathers cover entire body surface except mid-ventral area (Dawson and Evans 1957). An 8-d-old specimen collected 1 h after leaving the nest had rectrices about 6 mm long, and feathers over much of the head were not yet free of their sheaths (Sutton 1935).

Control of body temperature: No internal control of body temperature in nestlings 0–3 d old; O₂consumption patterns indicate endothermy appears in young > 4 d old. Nestlings 0–3 d old are unable to maintain body temperature > 3° C above ambient. At 4 d young start to maintain normal body temperatures at ambient temperatures > 25° C. By 7 d, normal body temperature can be maintained at ambient temperatures as low as 15° C (Dawson and Evans 1957).

Behavior: Eyes open in 4-d-old nestlings. Nestlings < 5 d old respond to touch or agitation of the nest by raising the head and gaping. Older nestlings gape and beg only if an adult is present; human presence or agitation causes them to crouch low and huddle in the nest (Dawson and Evans 1957, MC). Muscles, especially in pectoral region, develop rapidly. Nestlings at 4 d can support themselves in an upright position and attempt to stand. After 5 d can move around the nest in a coordinated fashion (Dawson and Evans 1957). Can grasp a stick by day 6–7; balance unsteadily on a perch by day 8; attain good balance on a perch after leaving the nest (Holcomb 1966). Typically young remain inactive unless an adult is present (MC).

Locomotion-learning to walk/swim/fly: See Fledgling Stage, below.

Parental Care

Brooding

Typically, only females brood. Brooding begins on hatching day. Females at 3 nests spent 83% of daylight hours brooding on day 0, the proportion decreasing daily to zero on day 6 (Crooks and Hendrickson 1953). In Pennsylvania, mean female attentive time (all time spent at nest during 30 min blind observation) day 0 = 19.1 ± 5.0 SD min (65%), n = 52; decreased steadily to day 8 mean = 4.3 ± 5.9 SD min (14%), n = 27. Attentive time decreased significantly with increasing brood size; all nestling ages pooled, mean female attentive time at broods of 1 = 15.1 ± 8.7 SD min, n = 7; at broods of 4 mean = 10.9 ± 7.9 SD min, n = 18 (Carey 1990). Females also brood significantly more during cold and/or wet weather (Best 1977a, MC). Best (1977a) found that brooding of day 6 nestlings was highest at sunrise and sunset; a secondary peak near midday.

Females brood young overnight through day 4, but were found on the nest only 50% of the time on the night of day 5, 25% day 6 (Walkinshaw 1968a, 1978). No data on brooding rhythms or durations.

Feeding

Feeding begins on hatching day; hatched young are fed before all eggs hatch. Time of first feeding relative to hatching not reported. Feeding continues through nestling and fledgling stages. Independence from parental feeding attained between day 26 and 34 (Crooks 1948).

Roles of the parents: Overall, feeding of young is shared roughly equally by parents (Walkinshaw 1968a, Best 1977a, Carey 1990). However, parent roles vary relative to brood size, age of young, time of day, and date. At low brood sizes (1–2), males make significantly fewer trips to nests, bring significantly fewer items, and bring items of a size equal to that of their mates. At larger broods (3–4), males make more trips, bring significantly more food items, and bring items of a significantly larger size than do their mates (Carey 1990). With young nestlings (0–2 d old), males make significantly fewer trips than their mates (Carey 1990).

Thus, when less food is required by nestlings, males contribute disproportionately less. Such males may benefit by reallocating time to searching for extra-pair copulations (see Behavior: sexual); in May and early Jun, when fertilizable females are most numerous, males make significantly fewer trips to the nest than their mates do (Carey 1990).

Throughout the day, male feeding trips/h remain relatively constant, females have higher trip rates shortly after sunrise and before sunset, when brooding times are minimum (Best 1977a).

Single parents of either sex can adequately feed young, even in large broods (Fig. 5; Walkinshaw 1968a, Carey 1990). If both parents feed, fledglings are often divided between them. Male feeds fledglings unaided if female starts building a 2nd brood nest (Crooks 1948, MC). In Pennsylvania (1990–1991), some females deserted mates following fledging (see Behavior: sexual/pair bond), taking some or all of the fledglings with them. If these females paired with new males, the new males assumed sole care of the fledglings (MC).

Direct feeding; parents deliver food to young, carrying it in their beaks and placing items in the open mouths of the young.

Food of young: Almost exclusively arthropods. Types of food listed by percent of total food delivered are: of 237 food items tallied by Jones (1913), 65% Lepidoptera larvae, 19% Orthoptera (grasshoppers), 10% adult moths, 6% unknown. In Iowa, 71% Lepidoptera larvae, 16% adult flies and bees, 10% adult moths, 3% beetles (Crooks and Hendrickson 1953). In Illinois, of 255 food items, 35% Lepidoptera larvae, 20% Homoptera (cicadas), 19% spiders, 17% Orthoptera, 12% other (Best 1974a). In Pennsylvania, of 1,853 food items, 80% Lepidoptera larvae, 10% adult flies and bees, 6% Orthoptera (mostly katydids), 3% adult moths, 1% spiders (MC). Evans (1964) obtained nestling food estimates by analysis of gizzard and fecal sac contents. He found lower proportions of Lepidoptera larvae than above and a much wider variety of food types, many of which were derived from nearby forest patches.

Type of food varies with age of young and food availability. Spiders are more often the food of younger nestlings (Best 1977a). Orthoptera are common nestling food in late summer and are fed more often to older nestlings (Best 1977a, MC). In Illinois, Homoptera were common food type only in May and Jun 1972, when there was a 13-yr cicada (Magicicada sp.) emergence at the study site (Best 1977a). The Rhagionid fly Chrysopilus thoracicus was a large part of the nestling diet in Pennsylvania only during large emergences in early Jun 1987–1988 (MC).

Food size increases slightly with brood size and significantly with nestling age. Carey (1990) visually estimated food item length from a blind. Mean item length = 15.5 mm at broods of 1–3, increasing to 16.7 mm with broods of 4–5. Mean item length = 8.3 mm on day 0, increasing linearly to 20.2 mm on day 8. Best (1977a) collected food items from nestlings 1–5 d old and measured volume of each. While items < 0.05 cc were the modal size at all ages, mean volume increased steadily with age.

Rate of feeding: Frequency of feeding trips increases significantly with brood size and age of young. At broods of 1, both parents (pooled) average 4.8 feeding trips/h, 6.5 trips-broods of 2, 8.2 trips-broods of 3, and 9.0 trips-broods of 4. With nestlings aged 0–2 d, parents averaged 6.0 trips/h, 7.9 trips-age 3–5 d, 10.4 trips-age 6–8 (Carey 1990). Time intervals between successive trips range from a few seconds to 84 min. Mean interval between feedings ranges from 6–10 min (Jones 1913, Walkinshaw 1939, Crooks 1948, Best 1977a).

Typically one food item delivered/trip. Regardless of number of food items/trip, only one nestling typically fed/trip (Best 1974a, Carey 1990). Rate of feeding highest shortly after sunrise and before sunset, minimum at midday (Best 1977a).

Best (1977c) observed apportionment of food to nestlings in 6 broods. Food was delivered uniformly to young in 5 broods, the 6th contained an injured nestling that was fed less. Normal variation in nestling mass did not affect feeding frequency. No tendency for parents to preferentially feed nestlings positioned closest to them.

Nest Sanitation

Nestlings defecate after being fed; often after parents prod nestlings with their beaks. Parents typically consume fecal sacs of nestlings aged 0–4 d; those of older nestlings carried away from the nest (MC). Crooks (1948) observed a fecal sac dropped about 80 ft from the nest. No data on invertebrate associates.

Parental Carrying Of Young

Not observed.

Cooperative Breeding

Not observed.

Brood Parasitism

Identity of the parasitic species

Brown-headed Cowbird (Molothrus ater).

Frequency of occurrence

Varies geographically: in Iowa 80% (16/20) nests parasitized (Crooks and Hendrickson 1953); in w. Illinois 11% (16/147; Best 1978); in ne. Illinois 52% (37/71; Strausberger and Burhans 2001); in Missouri 11% (49/442; Strausberger and Burhans 2001); in Ohio 32% (51/159; Hicks 1934). In Pennsylvania (1987-2006), only 2 of 946 nests parasitized (Burhans et al. 2001, MC). The lack of cowbird parasitism in Pennsylvania may be due to the preference of cowbirds in the Northeast to lay eggs in the nests of forest-breeding species, even when old fields are available nearby (Hahn and Hatfield 1995, 2000).

Timing of laying in relation to host’s laying

Most nests parasitized during egg laying. Female Field Sparrows arrive well before dawn to lay, on average before cowbird females (Burhans 2000, Burhans et al 2001). In one nest observation, a female cowbird chased a laying female Field Sparrow off the nest (DEB).

Response to parasitic mother, eggs, or nestlings

Parasitized nests commonly deserted. In Michigan 55% (100/182) deserted (Walkinshaw 1968a); in Illinois 63% (10/16; Best 1978). Strausberger and Burhans (2001) placed female cowbird models at Field Sparrow nests during construction and egg-laying periods. In their parasitized Illinois and Missouri populations, 22 of 26 females deserted their nests. MC made similar model presentations to Field Sparrow females in his unparasitized Pennsylvania population; only 2 of 33 deserted. Accepted cowbird eggs are incubated with host eggs, generally hatch on same day as host eggs, and nestlings are fed by host parents (DEB). During nest building, both sexes observed chasing cowbirds (Crooks and Hendrickson 1953, DEB). Model cowbirds placed near nests elicit rapid chips in Illinois and Missouri populations; in an unparasitized Pennsylvania population, models elicited significantly fewer chips (Burhans et al. 2001). Only in the heavily parasitized Illinois population were physical attacks on the cowbird model made (Burhans et al. 2001).

Effects of parasitism on host

Cowbird usually removes a host egg within a few days of laying its own. In Missouri mean clutch in non-abandoned parasitized nests = 2.4 ± 1.1 SD eggs (range 1–4, n = 5); in unparasitized nests mean = 3.69 ± 0.7 SD eggs (range 2–5, n = 137). Burhans et al. (2000) found other significant cowbird impacts on fitness: compared to unparasitized nests, parasitized nests had significantly lower nestling growth rates, lower hatching success, and reduced nestling survival.

Success Of Parasite With This Host

Generally a poor host. One of 29 cowbird eggs fledged in Iowa (Crooks and Hendrickson 1953); in Michigan, 27 of 234 eggs fledged (Walkinshaw 1968a).

Fledgling Stage

Period from hatching to departure

Typically 7–8 d; as early as 5 d if disturbed (Walkinshaw 1968a, MC).

Condition of development at departure

See Young Birds, below.

Manner of departure

MC observed departure at 5 nests from a blind. In all, departure occurred between 0800 and 1000 h. Young first hopped onto rim of nest, either sat there for a variable length of time or immediately fluttered to the ground while giving begging calls. Just prior to departure, parents at 4 nests were within 10 m of the nest chipping and rapidly flew to young in the vegetation. All young left the nest within 1 min.

Growth: Mass, Proportions, Structures

Few data. Young weigh about 10.5 g when ready to leave nest (Walkinshaw 1936). Four 13 d-old fledglings in Michigan: mean mass = 11.7 g; mean wing chord = 48.3 mm; mean tail length = 19.0 mm (Walkinshaw 1978). Young birds are comparable in size to adults by 5-6 weeks old (Walkinshaw 1978).

Association with parents or other young

Recent fledglings remain together near the nest in low cover for 2–3 d, where parents feed them every 5–10 min (Crooks 1948, Walkinshaw 1968a). Siblings typically remain in a group until independence, but sometimes partitioned into two groups, one with each parent (MC).

Ability to get around, feed, and care for self

Can begin to fly short distances when 13–14 d old (Walkinshaw 1939). Parents provide food and warn of danger until young reach independence (day 26–34; Crooks 1948).

Immature Stage

Immature birds birds flock together in late summer; appear primarily to be foraging. May follow adults and fledglings whose territories encompass the flock (MC). Based upon recaptures in the breeding season of birds banded as juveniles (hatch-year) in autumn in Michigan, Adams and Brewer (1981) propose that immature birds select their breeding habitat prior to autumn migration.