Action: Provide supplementary food for songbirds to increase reproductive success
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- Two studies from the USA found evidence for higher population densities of magpies and American blackbirds in areas provided with supplementary food, whilst two studies from the UK and Canada found that population densities did not appear to be affected by feeding.
- Twelve studies from across the world found that breeding productivity was higher for fed birds than controls. The increases were through higher hatching or fledging rates, or higher chick survival or recruitment rates. One study from the USA found that these increases were only found in dry years.
- Eleven studies from Europe and the USA found that fed birds had no higher, or even lower breeding productivity or chick survival than control birds.
- Nine studies from Europe and North America found that the eggs of fed birds were larger or heavier, or that the chicks of fed birds were in better physical condition: being larger, heavier, faster growing, more symmetrical or having a better immune response. In one study this was only true in a heavily polluted site. However, eight studies from across the world found no evidence for better condition or increased size in the eggs or chicks of fed birds.
- Six studies from across the world found that food-supplemented pairs laid larger clutches than unfed birds, whilst 14 studies from Europe and North America found that fed birds did not lay larger clutches, or even laid smaller ones.
- Fifteen studies from across the world found that birds supplied with supplementary food began nesting or laying earlier than controls, although in two studies this was only true for young females or in one of two habitats. In one study, a high fat, high protein diet had a greater effect on laying date than a high fat, low protein diet. One study found that fed birds had shorter incubations than controls whilst another found that fed birds re-nested quicker than controls and had shorter second incubations.
- Four studies from the USA and Europe found that fed birds did not lay any earlier than controls.
- Seven studies from across the world found that fed parent birds showed positive behavioural responses to feeding, such as being more likely to re-nest, less likely to be parasitized or showing better anti-predator responses, spending more time incubating or building larger nests.
- Three studies from across the world found neutral or negative responses to feeding, including being more likely to be invaded by conspecifics, making no more breeding attempts or showing no preference for fed nest boxes compared to controls.
Supporting evidence from individual studies
A controlled cross-over study in southern Sweden in 1972-3 (Källander 1974) found that great tits Parus major nesting in an 8 ha (1972) or 6 ha (1973) area of oak-hazel woodland supplied with supplementary food began laying eggs significantly earlier than great tits in an adjacent 16 ha or 18 ha control (unfed) area. The difference was significant irrespective of whether all females, one year-old females or older females were examined (average laying date of 1st-5th May for 44 nests in fed areas vs. 6th-11th May for 75 control pairs). Supplementary food consisted of 32 or 33 trays positioned throughout the wood and provided daily with 20-35 g of mealworms from 11th-27th April 1972 and between 10th April and 14th May 1973. On average there were 2 trays/territory in 1972 and 1.5 trays/territory in 1973. The authors note that other species including wood nuthatches Sitta europea and chaffinches Fringilla coelebs also took food from the trays.
A replicated and controlled study in mixed farmland in north-east Scotland between 1971 and 1973 (Yom-Tov 1974) found that carrion crow Corvus corone nestlings in nests provided with supplementary food had significantly higher hatching, survival and fledging rates than those in control (unfed) nests (with human ‘predation’ included: 79% of 11 fed nests hatching at least one chick, 71% of ten having at least one chick surviving for ten days and 71% fledging at least one chick vs. 61% of 28, 54% of 15 and 43% of 12 for controls). Nestlings from fed nests, however, were no heavier than those from controls, when comparing first-hatched with first-hatched etc. Supplementary food consisted of one domestic hen’s egg and five dead hen chicks provided every day from when laying began and increasing to one egg and ten chicks from the seventh day after hatching until fledging. Other experiments in this study found that winter feeding (a hen’s egg and five chicks provided between January and April 1973) led to crows laying clutches earlier but did not affect clutch size (average laying date of April 13th and 4.4 eggs/clutch for ten fed territories vs. April 18th and 4.3 eggs/clutch for 21 controls). Further experiments examined the effect of moving supplementary food further from nests, but the author argues that these results are confounded by supplementary food being taken by non-target birds. Finally, additional experiments in the study found that crow nesting density did not increase following the provision of supplementary food and additional nesting sites in breeding territories, discussed in ‘Provide artificial nesting sites’.
A controlled trial in coniferous forest in southwest Sweden in 1976-7 (von Brömssen & Jansson 1980), found that willow tits Parus montanus and crested tits P. cristatus in a 1.8 km2 area provided with supplementary food started laying eggs significantly earlier than individuals in a 5.5 km2 control (non-supplemented) area (willow tits: average of 2-5 days earlier, 44 pairs tested; crested tits: average 5-8 days earlier, 38 tested). There was no corresponding difference in 1978, when no food was provided. Twelve-day old crested tit nestlings were significantly heavier in the fed area (average of 12.2 g for fed area vs. 11.5 g for control, number of chicks not provided) but there was no difference in willow tit nestlings (average of 10.6 g for fed area vs. 10.7 for control, number of chicks not provided). Clutch sizes did not differ between areas for either species (willow tits: average of 8.1-8.2 eggs/clutch for fed area vs. 7.7-8.2 for controls; crested tits: average of 5.3-5.4 eggs/clutch for fed area vs. 5.0-5.2 for controls). Food was provided at feeding stations located in the approximate centre of each fed territory and contained one feeder of sunflower seeds and three coconut shells containing a mixture of tallow, soy protein, wheat germ, sun-flower seeds, vitamins and minerals. In 1976 feeding started on 20th February and in 1977 on the 20-24th March, in both years it continued until all females had started laying and resumed for the first half of June (when second clutches could be expected to be laid).
A replicated cross-over study in a pine forest site in southern Sweden in 1974-5 (Hogstedt 1981) found that female black-billed magpies Pica pica laid earlier, had larger clutches and laid larger eggs when supplied with supplementary food compared to when no food was provided (average of 3.5 days earlier, an extra 0.56 eggs/clutch and eggs weighing 0.33 g more, ten females studied). Fed pairs were less likely to lose nestlings and had higher fledging success than control (unfed) pairs (88% of 20 fed nests having one surviving nestling and 2.7 fledglings/breeding attempt vs. 48% and 1.3 fledglings/breeding attempt for 32 controls). Supplementary food consisted of 300 g of fish provided in the centre of experimental territories every other day from 15th March until eggs hatched, when it was raised to 600 g. This represents approximately 75% of a pair’s daily requirement before hatching and 65-150% after hatching. Food was also provided in seven territories without magpies, but did not attract any new pairs.
A replicated, controlled experiment on marshlands sites in Washington state, USA, in the springs of 1977-8 (Ewald & Rohwer 1982), found that red-winged blackbirds Agelaius phoeniceus from territories supplied with supplementary food laid eggs significantly earlier than blackbirds from control (unfed) territories (1977: 17 days earlier in eight territories where feeding began on 31st March, 12 days earlier in five territories fed from 5th April; 1978: 26 days earlier). However, in 1978 clutch size was significantly lower in fed territories (3.4 eggs/clutch, 122 fed clutches vs. 3.6 eggs/clutch for 126 controls), the authors suggest this is due to early-laid clutches being smaller – if clutches laid before April 10th were excluded then clutches were not significantly smaller. Fed territories provided with supplementary food also had higher rates of chick mortality than control territories (numbers not provided) and were invaded by non-territorial red-winged and yellow-headed blackbirds Xanthocephalus xanthocephalus at significantly higher rates than controls in 1978 (4.9 intruders/ha in 33 fed territories vs. 1.1/ha for 54 controls), but not in 1977, or when feeder density was reduced in 1978 (0.8 intruders/ha for 17 fed territories vs. 1.3 intruders/ha for 19 controls). Supplementary food consisted of sunflower seeds, cracked corn and puppy food on 30 cm square trays. In 1977 one tray was placed in each of 13 territories on two marshes (controls were on five other marshes); in 1978, trays were placed on a 6 m grid on a single marsh, a tray density approximately ten times that in 1977, reduced to one feeder in each territory several weeks after females arrived.
A small study from January-March in 1979-1980 in Utah, USA (Reese & Kadlec 1984), found that juvenile black-billed magpies Pica pica that established nest sites closer to supplemental food stations experienced higher rates of nestling starvation than nests further away. Very few adult magpies visited food stations. Dominant juvenile males, however, established nests significantly closer to food stations. Of seven nests within 100 m of food stations, six experienced nestling starvation while only one farther than 100 m did. Supplementary feeding did not enhance clutch (6.4 and 6.4 eggs/nest) or brood (5.0 and 4.8 nestlings / nest) size between nests closer to food stations (16 nests) and those further away (13 nests). Similarly, mean clutch initiation was not significantly different between treatments. The authors point out that supplementary feeding can serve as a proximate stimulus to commence nesting on sites that are not inherently of good quality. Food stations were provisioned with beef bones or black-tailed jack rabbit Lepus californicus carcasses.
A replicated and controlled study on Mandarte Island, British Colombia, Canada (Arcese & Smith 1988), found that song sparrows Melospiza melodia provided with supplementary food in 1985 laid earlier, had larger clutches than control (unfed) birds, were more likely to re-nest following breeding failures and were less likely to be parasitized by brown-headed cowbirds Molothrus ater (average laying date of 10th April, 3.3 eggs/clutch, 2.8 breeding attempts and 18% of nests parasitized for 15 fed pairs vs. 28th April, 2.8 eggs/clutch, 2.2 breeding attempts and 45% for 34 controls). This led to fed females raising approximately 3.8 young/female to independence compared to 1.0 young/female for controls. However, eggs and nestlings from fed birds were no larger than those from controls nor more likely to hatch or survive to independence (averages of 3.1 g/egg, 16.8 g/nestling at six days old, 78% hatching success and 83% survival to independence vs. 3.0 g/egg, 15.6g, 73% and 70%) and young from fed territories did not have enhanced reproduction (8.6% of 58 fed young reproducing vs. 12.5% for 40 controls). Supplementary food consisted of dog food, vitamins, mealworms Tenerbrio sp. and millet seed provided continuously in feeders in the centre of each territory from five weeks before laying started (28th February) until the last young had left the nest (24th July). The authors note that 1985 was a year of peak song sparrow density. This study also described the impact of feeding on adult survival, discussed in ‘Provide supplementary food to increase adult survival’.
A controlled before-and-after study in Washington, USA, in March-June 1979-82 (Knight 1988), found that twice as many black-billed magpies Pica pica nested, and that they began laying significantly earlier, in a 0.3 ha area of willow Salix spp. in 1981 when a dead bullock was cut open to locally increase invertebrate numbers, compared to 1979-80 and 1982 when there was no bullock present (eight nests in 1981, average laying date if 24th March vs. three or four nests and a laying date of 2nd-4th April in other years). There were no corresponding changes in two areas that were not supplied with a carcass (six and nine nests and 1st-6th April for all years studied). There were no differences in clutch size or the number of young fledged from nests with the carcass present. The bullock weighed approximately 450 kg and died between 1st and 7th March 1981 and attracted large numbers of blow flies (Calliphoridae), most of which had left by the 22nd March.
A replicated, paired site study in 1984-1985 in 17 experimental and 17 control pairs of blue tit Parus caeruleus nests in Lund, Sweden (Nilsson & Smith 1988), found that blue tit pairs that were artificially provisioned with food exhibited significantly greater reproductive success. Supplementary feeding significantly shortened the length of the incubation period. The probability that an egg would hatch was higher in provisioned nests than in control nests (98 and 90% of 198 and 199 eggs respectively). However, nestling survival up to 13 days did not differ between the pairs. Each pair was selected so that the last egg in both clutches was laid on the same date and the pairs nested in similar habitats. Four days after the last egg was laid, a container with 7 g of mealworms was placed into the experimental nest, while an empty container was placed into the control nest. Age and wing length did not differ between dyad pairs.
A randomised, replicated and controlled experiment around two lakes in Washington state, USA, in 1981 (Wimberger 1988), found that red-winged blackbirds Agelaius phoeniceus from territories supplied with supplementary food laid eggs 8-13 days earlier than blackbirds from control (unfed) territories, for first, second, third and fourth nests of the year (20 territories studied). Each lake was divided in half, with one half provided with sunflower seeds from the 4th April. The average number of females in each territory was increased by feeding on one lake (average of 9.0 females/territory on fed territories vs. 6.5 females/territory on controls, eight territories studied) but not the other (average of 5.2 females/territory on fed territories vs. 4.7 females/territory on controls, 12 studied).
A cross-over study in a coppiced oak forest in southern France in 1986-7 (Clamens & Isenmann 1989) found that blue tit Parus caeruleus and great tit P. major pairs in an area provided with supplementary food started laying clutches significantly earlier than pairs in a control (unfed) area (1986: 9 pairs in a 10 ha fed area laid six days earlier than 26 pairs in a 50 ha control area; 1987: 15 pairs in a 30 ha fed area laid five days earlier than 18 pairs in a 30 ha control area). However there were no significant differences in clutch size or the number of young hatched between treatments (average of 9.2 eggs/clutch for 24 fed clutchea vs. 8.3-9.1 eggs/clutch for 44 controls) and, in 1987, significantly fewer chicks fledged from blue tit nests in fed areas than control areas (average of 3.0 chicks/nest for 11 fed pairs vs. 6.3 chicks/nest for 15 controls). There was no significant difference in productivity in 1986 (7.1 chicks/nest for seven from fed areas vs. 7.3 chicks/nest for 23 controls). The authors suggest this could be due to fed clutches hatching before the peak in natural food in 1987.
A study in Alberta, Canada, between 1986 and 1988 (Dhind & Boag 1990) found that black-billed magpies Pica pica that used supplementary food provided in a single feeder in an urban area laid eggs 6-7 days earlier than magpies that did not use the feeder (seven pairs used the feeder in 1987, six in 1988; 22 pairs did not use the feeder in 1987, 11 in 1988). Nestlings from parents that used feeders were heavier and had higher survival and fledging rates than those from parents that didn’t use the feeder, (fed nests: average weight of 184 g, ten nests, 71% survival of seven through a snow storm in 1987, 4.1-4.5 nestlings fledged/nest for 13 nests; unfed nests: 163-168 g for 13, 15% survival of 20, 1.1-1.8 nestlings fledged/nest for 31) although there were no differences in clutch size or breeding density (fed nests: 7.4-7.8 eggs/clutch for ten clutches, 126-134 m between 13 nests; unfed nests: 6.8-6.9 eggs/clutch of 23, 134-153 m between 57 nests). Supplementary food consisted of approximately 1 kg of dog food pellets provided each week from August 1986 until June 1988.
A randomised, replicated and controlled paired study in a Sonoran desert site in Arizona, USA, in the breeding seasons of 1986 and 1987 (Simons & Martin 1990) found that 28 cactus wren Campylorhynchus brunneicapillus broods were 10.5-20.0 g heavier from territories provided with supplementary food, compared with control broods. Parents from fed territories were also more likely to have second clutches than controls (12 of 14 fed pairs attempted second broods vs. seven of 14 control pairs). Fed nestlings were also larger and had higher post-fledging survival rates in 1986 but not 1987 (1986: 14 pairs, linear measurements 1.0-2.7 mm larger than controls, 15 fed fledglings surviving for four to six weeks after fledging vs. seven control fledglings; 1987: 14 pairs, linear measurements 0.2-0.6 mm larger than controls, 19 fed fledglings surviving vs. 16 controls). The authors suggest that differences between years were due to higher levels of natural food occurring in 1987. Supplementary food consisted of 35 g of mealworm Tenebrio molitor larvae and noctuid moth caterpillars supplied every other day from one or two days after hatching until nestlings fledged (approximately 20 days later) and comprised 75% (1986) and 80% (1987) of food delivered to nests.
A small randomised and controlled paired study in parkland and mixed woodland in southern Scotland in spring and summer 1990 (Johnston 1993) found that great tit Parus major nestlings in eight artificially enlarged broods were no larger at 15 days old when provided with supplementary food, compared to nestlings from seven control (enlarged but not fed) broods. Broods were enlarged by the addition of three nestlings (added after hatching). Supplementary food consisted of an average of 2.2 g of minced meat and nutritional supplements fed twice daily to half the experimental brood on days six through 12 after hatching. This represents most of a nestling’s daily energetic requirements. This study also investigated the impact on the condition of provisioning females, discussed in ‘Provide supplementary food to increase adult survival’.
A replicated and controlled study in grasslands in southern Sweden between 1982 and 1990 (Källander & Karlsson 1993) found that common starlings Sturnes vulgaris supplied with supplementary food began laying significantly earlier than controls (first laying date of 21st April-5th May for fed nests vs. 22nd April-10th May). There were no such differences between nests in the years when supplementary food was not supplied. However there were only occasional differences in egg weight, no differences in clutch size or nestlings weights and fledging rates were actually lower in fed nests in 1990 (4.3 young/nest for fed nests vs. 5.6 for controls). Supplementary food consisted of approximately 100g of mealworms placed in small feeders either on the outside or inside of nest boxes, supplied to different colonies in 1982 and 1985 and a subset of nests at a third colony in 1990. Feeding began approximately one month before laying started and stopped once all females began laying. Feeding represented more than the daily energetic needs of a pair of starlings.
A replicated, controlled and paired study in a mixed forest in the central Netherlands in 1987 (Verhulst 1994) found that pied flycatcher Ficedula hypoleuca chicks from pairs provided with supplementary food were significantly more likely to be recruited into the local breeding population than chicks from control (unfed) pairs (7.9% of nestlings from 12 fed clutches recaptured as breeding adults vs. 1.1% from 15 controls). However, fed pairs did not start laying earlier, produce larger clutches, hatch more young, hatch young earlier than control pairs (13 fed pairs started laying on May 18th, average of 6.3 eggs/clutch, 5.5 chicks/clutch and a hatching date of June 6th vs. May 19th, 6.2 eggs/clutch, hatching 5.4 chicks/clutch and June 6th for 16 controls), probably due to the paired nature of the study. In addition, there were no differences between treatments in terms of nestling growth, survival, tarsus length or weight (5.5 chicks/clutch fledging, average weight of 13.9 g, average tarsus length of 17.4 cm for 12 fed pairs vs. 5.5 chicks/clutch fledging, average weight of 13.8 g, average tarsus length of 17.2 cm for 15 controls). Supplementary food consisted of mealworms provided in excess beginning two days after chicks hatched. This study also examines the impact of feeding on adult survival, discussed in ‘Provide supplementary food to increase adult survival’.
A replicated and controlled study in alpine meadows and pine woodlands on Honshu, Japan, in May-September 1986-9 (Nakamura 1995) found that one year-old alpine accentor Prunella collaris females in territories provided with supplementary food began laying clutches significantly earlier than those in control territories (first egg laid on average on 25th-29th June for fed 16 one year-old females vs. 5th-13th July for 17 control one year-olds). There was no such difference in older females (first egg laid on average on 17th-19th June for 18 fed females vs. 21st-23rd June for 19 controls). There were no differences in duration of the copulation period, the number of breeding attempts or the timing of settlement in territories for either one year-old or older females. Supplementary food consisted of 300 g of millet and canary seed provided two to three times a week beginning in May and continuing until September.
A small, controlled before-and-after study on Frégate Island, Seychelles, in 1989-90 (Komdeur 1996), found that Seychelles robins Copsychus sechellarum had significantly higher reproductive success in 1990 when provided with supplementary food, compared to in 1989, when food was not provided (2.0 nestlings/pair and 1.4 independent young/pair in 1990 vs. 0.8 nestlings/pair and 0.2 young/pair in 1989). There was no corresponding change in four control (unfed) territories. Food consisted of 20–25 freshly killed cockroaches, grated coconuts, boiled rice and fish provided twice a week, and an area of 4 m² of soil was broken up in an attempt to facilitate access to natural prey (e.g. worms and beetle larvae). Increases were apparently due to earlier laying dates, more time incubating and nest guarding, higher hatching and provisioning rates, higher chick fledging weight and higher levels of parental care.
A replicated and controlled trial in a dune and scrubland system in Florida, USA, in 1993 (Schosch 1996), found that female Florida scrub jays Aphelocoma coerulescens in ten groups provided with supplementary food initiated clutches significantly earlier (on average 16 days earlier) than females from 32 control (unfed) groups. There was no significant difference in clutch sizes between treatments. Non-breeders from fed groups were no less likely to become breeders than those from control groups (fed groups: five of 23 non-breeders established territories; controls: four of 23). Feeding consisted of providing dried dog food, peanuts and mealworms were provided twice daily at feeding stations in the middle of the territories from late January until females finished laying. Food was provided ‘in excess’. This study also reported on the effects on adult condition, discussed in ‘Provide supplementary food to increase adult survival’.
A replicated and controlled before-and-after trial in scrubland, gullies and caves in southern Spain in 1980-3 (Soler & Soler 1996) found that European jackdaws Corvus monedula laid earlier, laid more eggs and had higher breeding success when given supplementary food, compared to either the same colonies in previous years, or to control colonies (average laying date of 25th April, 6.0 eggs/clutch and 48% of all eggs fledging for 13 fed nests vs. 27th April, 5.4 eggs/clutch and 20% of all eggs fledgingfor 18 experimental colonies in previous years vs. vs. 27th April, 5.4 eggs/clutch and 29% of all eggs fledging for 18 control colonies). Starvation rates were also lower in fed colonies (2.5 nestlings/nest starving in 12 fed nests vs. 3.4 nestlings/nest in 12 controls). Predation rates were lower in fed colonies (0% of 20 fed nests predated vs. 22% of 27 control nests) and the authors argue that this was due to better group defence in fed colonies due to higher breeding densities, although nesting densities were only significantly higher in one colony, comparing 1983 and 1982, other comparisons were non-significant (colony A when not fed: 56%, 63% and 56% of nests occupied in 1980-2 vs. 88% in 1983 when fed; colony B: when not fed: 50%, 67% and 33% of nests occupied in 1980-2 vs. 100% in 1983). Supplementary food consisted of four hens’ eggs for each jackdaw pair supplied twice a week and 4-7 kg of bread, continuously supplied.
A randomised, replicated and controlled trial in a forest in Illinois, USA, in 1998 (Styrsky et al. 2000), found that ‘early-season’ house wren Troglodytes aedon nestlings (i.e. from clutches laid in mid-May) in nest boxes provided with supplementary food were significantly heavier than nestlings from early-season control broods (fed nestlings approximately 2% heavier than controls, 62 tested). There was no difference in weight between fed and control late-season (laid in late June to early July) broods (53 tested). There were no differences between treatments for tarsus lengths, growth rates or survival until fledgling for either early or late season broods. Food consisted of 30 g of mealworms Tenebrio molitor supplied each day within the nest boxes.
A replicated, controlled trial on an island in Lake Rotorua, North Island, New Zealand, between September 1995 and February 2001 (Castro et al. 2003), found that stitchbirds (hihis) Notiomystis cincta in territories provided with supplementary food laid significantly more eggs and had higher fledging and recruitment success than control (unsupplemented) birds (average of 4.4 eggs/clutch for 17 fed nests vs. 3.9 eggs/clutch for 18 controls; 70% fledging success for 22 fed nests vs. 32% for 14 controls and 35% recruitment success for 16 fed nests vs. 13% for 13 controls). In addition, fed females began laying second clutches significantly sooner than controls and incubated second clutches for significantly less time (15.2 days of incubation for eight fed clutches, and a 4.0 day interval before four second clutches vs. 16.8 days incubation for eight controls and a 13.3 day interval for six). There were no significant differences in hatching success or the incubation period of first clutches, between fed and control birds. Supplementary food consisted of either commercial honeyeater food (provided every day) or a solution of sugar or jam (provided every third day) in hummingbird feeders. Territories were considered ‘fed’ if nests were within 50 m of a feeder. The authors suggest that population viability on Mokoia Island may be dependent on supplementary food.
A randomised, replicated and controlled study in a 10 ha Australian reed Phragmites australis wetland in southern Australia, over two breeding seasons (September to January) in 1999-2001 (Eikenaar et al. 2003), found that Australian reed warbler Acrocephalus australis females provided with supplementary food spent longer incubating eggs than control (unfed) females (12 fed females spent approximately 57% of time incubating vs. 51% for ten controls). There was also a significant difference between days when food was provided and those when it was not (on fed days, 59% of time spent incubating vs. 52% of time on unfed days, seven tested). There were no differences between other measures of incubation attendance, such as start and end times of incubation bouts or the average length of bouts. However, broods from fed territories had larger hatching asynchronies (calculated as the difference in size, at one to three days old, between the first- and last-hatched chicks, divided by the mean size of all nestlings) than control pairs (tarsus length asynchrony of approximately 0.15 and weight asynchrony of approximately 0.35 for ten fed broods vs. 0.10 and 0.20 for 19 controls). The effect of feeding on fitness of chicks or parents was not reported. Supplementary food consisted of 30 g of blowfly maggots (over 150% of daily energetic requirements of an adult reed warbler) provided every other day whilst eggs were incubated, but stopped before eggs hatched.
A randomised, replicated and controlled paired study in mixed deciduous forests in the Netherlands in 1999 (Grieco 2003), found that blue tit Parus caeruleus nestlings from eleven nest boxes provided with supplementary food had more even leg lengths than nestlings from eleven control (unfed) nest boxes. There were no differences in leg size between treatments and no data were provided on survival or reproductive success. Supplementary food consisted of mealworm Tenerbio molitor and wax moth Galleria mellonella larvae placed inside the nest boxes and making up one third of the nestlings’ total food. Food was provided from day of hatching until the chicks fledged.
A continuation of Reynolds et al. 2003 in 2001 and 2002 (Reynolds et al. 2003) found that Florida scrub jay Aphelocoma coerulescens chicks from second and third-laid eggs had significantly higher fledging rates from territories provided with high-fat, high-protein supplements compared to those from controls in 2001 but not 2002 (2001: 100% of eleven 2nd and 3rd-hatched chicks fledged from fed nests vs. 53% of seventeen 2nd-hatched and 40% of ten third-hatched chicks from controls; 2002: 63% of eight 2nd-hatched and 71% of seven 3rd-hatched vs. 81% of 16 and 60% of ten for controls). In addition, more third-hatched chicks survived to independence in fed territories in 2001 (50% of four vs. 0% of ten) but not 2002 (43% of seven vs. 30% of ten). Fed chicks also grew flight feathers significantly faster, but there were no other differences in growth (body mass and leg length) or survival between the groups.
A randomised, replicated and controlled cross-over trial in a dune and scrubland system in Florida, USA, in 2000 and 2001 (Reynolds et al. 2003), found that that 21 female Florida scrub jays Aphelocoma coerulescens provided with high-fat, low-protein (HFLP) supplementary food and 21 females provided with high-fat, high-protein (HFHP) food laid significantly larger clutches than 55 control females in 2000 but not 2001 (2000: approximately 3.7 eggs/clutch for HFLP diets, 3.2 eggs/clutch for HFHP and 2.8 eggs/clutch for controls; 2001: 2.7 eggs/clutch, 3.1 eggs/clutch and 2.8 eggs/clutch respectively). This was due to earlier laying, discussed below. Clutches on the HFHP diet had a tendency to have larger third-laid eggs, but other eggs were not significantly different in size between treatments. Feeding began in early- or mid-January and continued until females began laying, usually six to eight weeks later. Food consisted of food pellets containing 19.3% fat and with a protein content of either 3.5% or 34.5%.
A replicated and controlled paired site study from March-August in 2000-2003 in 20 paired nest box groups (10 placed along wetland edges and 10 in farmlands) in Rutland, England (Field & Anderson 2004) found that tree sparrows Passer montanus showed no preference for nest boxes supplied with supplementary food (four fed boxes colonised vs. four unfed). There was no difference in the number of nesting attempts made by birds with or without supplementary food although the mean clutch size was significantly higher in nests closer to supplementary food (5.6 compared to 5.0 eggs / clutch). The authors point out that the small spatial scale of the study (1 km between pairs) may have confounded any effect of supplementary feeding. Nest box groups consisted of 5 nest boxes placed 2-20 m apart; sunflower seeds were randomly provided to one nest box group within each pair.
A replicated, controlled trial in mixed agricultural habitats in southern Spain (De Neve et al. 2004) found that, when provided with supplementary food, male black-billed magpies Pica pica built significantly larger nests than controls (average of 5.1 units for 39 nests in fed territories vs. 4.6 units for 123 nests in controls) and females laid significantly larger eggs (average egg volume of 10.0 cm3 for 36 fed clutches vs. 9.5 cm3 for 119 controls). However there were no significant differences between groups in terms of laying date or clutch size (average of 25th April for 46 fed clutches vs. 12th April for 162 controls; 6.9 eggs/clutch for 38 fed clutches vs. 6.8 for 128 controls). Supplementary food consisted of 150 g dog food mixed with bread and water supplied every two days from before nest-building began until after the end of the breeding season.
A randomised, replicated and controlled trial at a prairie site in Kansas, USA, in April-August 1997 (Pearse et al. 2004) found that female Bewick’s wrens Thyromanes bewickii and house wrens Troglodytes aedon from nest boxes provided with supplementary food spent longer incubating than females from control (unfed) nest boxes (Bewick’s wrens: average incubation bout of 52 min and 82% of time spent on the nest for seven fed females vs. 35 min and 70% for eight controls ; house wrens: averages of 30 min and 81% for nine fed females vs. 13 min and 71% for eight controls). There were no differences in date of clutch initiation, clutch size, the average length of time spent away from the nest or hatching success between treatments (Bewick’s wrens: average bout away from the nest of 10 min for seven fed females, hatching success of 78% vs. 13 min and 74% for eight controls; house wrens: average of 5 min and 96% for nine fed females vs. 5 min and 82% for eight controls). Supplementary food consisted of 15 g of mealworm Tenebrio molitor larvae supplied inside nest boxes every day during incubation, equivalent to almost twice a female wren’s daily energetic requirements.
A continuation of (Schoech et al. 2004) between 2000 and 2003 (30), found that breeding groups of Florida scrub jays Aphelocoma coerulescens provided with high-fat, low-protein (HFLP, provided in 2000-2 only) supplementary food started laying eggs significantly earlier than control (unfed) territories, whilst territories provided with high-fat, high-protein (HFHP) food laid significantly earlier still (first laying on average of approximately day 16th March for 50 HFHP territories vs. March 20th for 29 HFLP territories vs. March 28th for 115 controls).
A replicated, controlled experiment in Arusha National Park, Tanzania, in October and November 1995 and 1996 (Scheuerlein & Gwinner 2006), found that East African stonechat Saxicola axillaris (formerly S. torquata axillaris) chicks from ten pairs provided with supplementary food did not grow any faster between hatching and seven days old than chicks from 15 control (unfed) pairs. However, feeding appeared to reduce the impact of common fiscals Lanius collaris, a predator of stonechats, the presence of which reduced growth rates in chicks from control pairs (n = 7), but not fed pairs (n = 5). Supplementary food consisted of 30 g of mealworms Tenebrio molitor provided every day, from approximately a month before laying began.
A replicated, controlled trial in habitats in southern Spain in 2001 (De Neve et al. 2007), found that black-billed magpie Pica pica nestlings provided with supplementary food had significantly higher cell-mediated immune responses (CMI, the change in swelling around insect bites, a measure of immunocompetence) in arid scrub, but not in irrigated farmland and woodland (arid scrub: average CMI of approximately 1.18 mm for fed nestlings vs. 0.85 m for controls; irrigated farmland/woodland: average CMI of 1.20 mm for fed nestlings vs. 1.18 mm for controls; 58 nests studied). In addition, fed nestlings showed significantly lower levels of the ectoparasite Carnus haemapterus (Diptera: Carnidae), but only in arid habitats (arid scrub: average infestation intensity of 0.40 arbitrary units for fed nestlings vs. 0.90 arbitrary units for controls; irrigated farmland/woodland: average 1.60 arbitrary units for fed nestlings vs. 1.65 arbitrary units for controls; 28 nests). Feeding had no effect on chick growth, weight or on the prevalence of blood parasites. Supplementary feeding consisted of 0.1 ml of high-calorie, nutrient-rich paste provided every other day for a total of 14 days.
A controlled, replicated study on San Clemente Island, California, USA, between 2000 and 2006 (Heath et al. 2008) found that pairs of San Clemente loggerhead shrikes Lanius ludovicianus mearnsi in territories provided with supplementary food and with rodent control during April-July produced, on average, 2.5 more fledglings each year (55 breeding attempts), compared to control pairs (62 attempts), and 1.4 more fledglings each year compared to territories with rodent control but no feeding (55 attempts). In drier-than-average years, fed pairs also raised more fledglings to independence (40 days old) than other pairs (1.8 more fledglings than control pairs, 0.7 more than rodent control only pairs). There was no effect in wetter-than-average years. Management in December-March did not increase either measure of productivity. This study also investigated the impact of just rodent control, which is discussed in ‘Control predators on islands’ and the success of captive-bred individuals, in ‘Release captive-bred individuals’.
A replicated and controlled study in a range of oak forest habitats on Corsica, France, in the springs of 2006 and 2007 (Bourgault et al. 2009) found that blue tits Parus caeruleus (also known as Cyanistes caeruleus) provided with supplementary food began laying eggs significantly earlier in a holm oak Quercus ilex dominated site (females from 30 fed pairs began to lay approximately seven days earlier than controls), but not in two broad-leaved oak Q. humilis dominated sites or a second holm oak site. The authors suggest that this difference is due to broad-leaved oak forests having up to ten times more food for blue tits than holm oak forests. Clutch size did not differ between fed and control territories in any of the sites. Supplementary feeding comprised unlimited access to food in feeders within territories, beginning in mid-January and continuing until egg laying began.
A replicated and controlled study in open pine Pinus sylvestris forests in southwest Finland in 2005 (Eeva et al. 2009) provided 87 great tit Parus major nestlings with three supplementary diets: autumnal moth Epirrita autumnata larvae (high in carotenoids – chemicals needed for coloured feathers) and mealworms (diet 1); mealworms and water-dispersed lutein (an important carotenoid, diet 2); mealworms and distilled water (diet 3). Nestlings fed on diets 1 and 2 were larger and heavier than control (unfed) nestlings and nestlings fed on diet 3, but only in areas with high levels of heavy metal pollution (polluted areas: average wing length of approximately 46 mm and body mass of 16.5 g for diet 1 nestlings vs. 46 mm and 15.8 g for diet 2 vs. 44 mm and 15.0 g for diet 3 and 43 mm and 15.0 g for controls; unpolluted areas: 48 mm and 16.5 g for diet 1 vs. 46 mm and 16 g for diet 2 vs. 46 mm and 16.5 g for diet 3 vs. 47 mm and 17 g for controls). In addition, diet 2 nestlings had higher blood lutein levels and correspondingly higher carotenoid chroma (a measure of colour intensity) in breast feathers than other diets and control chicks across both pollution levels (7.5-15.0 µg/ml blood lutein and 0.30-0.35 carotenoid chroma for diet 1 vs. 24.0-28.0 µg/ml and 0.41-0.45 for diet 2 vs. 7.5-16.0 µg/ml and 0.30-0.36 for diet 3 vs. 5.5-17.5 µg/ml and 0.30-0.36 for controls). Supplementary food did not have an effect on fledgling probability. One gram of food was provided every other day from the third day after hatching to the 13th, making approximately 20% of the required food in this time.
A randomised, replicated cross-over experiment in scrubland on South Island, New Zealand in austral spring 2000-1 (Barnett & Briskie 2010) found that on a day when they were provided with supplementary food, silvereyes Zosterops lateralis spent significantly longer incubating and had shorter periods away from the nest, compared to a day when they were not provided with food (adults on fed days spent approximately 94% of time incubating and periods off the nest averaged one minute vs. approximately 84% of time on nests and 3.5 minute periods off the nest, ten nests). However, feeding did not increase the length of individual incubation bouts, or the number of times parents left the nests each hour (longest incubation bouts averaged 39.6 mins when fed vs. 34.1 mins when unfed, ten nests; parents leaving nests an average of 2 times/min when fed vs. 2.3 times/min when unfed, seven nests). Supplementary food consisted of beef fat and sugar mixed and provided in pine cones on one of two experimental days and not provided on the other. No data is provided on the breeding success consequences of feeding.
A replicated, controlled study over 3 breeding seasons in 2006-2008 in 3 treatment blocks (2 experimental and 1 control; 96 nestboxes/block) of broadleaf, deciduous woodland in Worcestershire, UK (Harrison et al. 2010), found that blue tits Parus caeruleus (also known as Cyanistus caeruleus) and great tits Parus major began laying significantly earlier (by averages of two and three days respectively), had reduced clutch size (mean reduction: 0.4 and 0.7 eggs), shortened incubation periods (mean reduction: 0.9 and 0.7 days), lowered hatching success (in blue tits only: mean reduction: 1.4%) and reduced brood size (mean reduction: 0.6 and 0.5 chicks for blue and great tits respectively). Treatment blocks were separated by a 90 m buffer strip. In each year, one treatment block received no supplementary food and two treatment blocks received peanut cake (comprising 50% ground peanuts and 50% beef tallow). Treatments were rotated amongst plots.
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